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  • 1.
    Abril, Claudia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Steffen, Rebekka
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Local earthquake tomography and earthquake relocation in the Tjörnes Fracture Zone (North Iceland)Manuscript (preprint) (Other academic)
  • 2.
    Benediktsdottir, Asdis
    et al.
    Univ Iceland, Inst Earth Sci, Nord Volcanol Ctr, Sturlugata 7, IS-101 Reykjavik, Iceland.;Univ Iceland, Dept Earth Sci, Sturlugata 7, IS-101 Reykjavik, Iceland..
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Brandsdottir, Bryndis
    Univ Iceland, Sci Inst, Inst Earth Sci, Reykjavik, Iceland..
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ambient noise tomography of Eyjafjallajokull volcano, Iceland2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 347, p. 250-263Article in journal (Refereed)
    Abstract [en]

    We present a shear-velocity model for the Eyjafjallajokull stratovolcano, based on ambient seismic noise tomography applied to seven months of data from six permanent stations and -10 temporary seismic stations, deployed during and after the 2010 volcanic unrest. Vertical components of noise were cross correlated resulting in 30 robust phase-velocity dispersion curves between 1.6 and 6.5 s in period, displaying a +/- 20% variation in phase velocity beneath the volcano. The uneven distribution of noise sources, evaluated using signal-to-noise ratios, was estimated to cause less than 2% error in most curves. Sensitivity kernels showed resolution down to 10 km and the lateral resolution of the resulting phase-velocity maps was about 5 km. The model reveals east-west oriented high-velocity anomalies due east and west of the caldera. Between these a zone of lower velocity is identified, coinciding with the location of earthquakes that occurred during the summit eruption in April 2010. A shallow, southwest elongated low-velocity anomaly is located 5 km southwest of the caldera. The limited depth resolution of the shear-velocity model precludes detection of melt within the volcano.

  • 3. Brown, D
    et al.
    Juhlin, C
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Tryggvason, A
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Steer, D
    Ayarza, P
    Beckholmen, M
    Rybalka, A
    Bliznetsov, M
    The Crustal Architecture of the Southern and Middle Urals from the URSEIS, ESRU, and Alapaev Reflection Seismic Profiles2002In: Orogenic Processes in the Paleozoic Uralides, American Geophysical Union , 2002, p. 33-48Chapter in book (Refereed)
  • 4. Brown, D.
    et al.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Ayala, C.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Bea, F.
    Alvarez-Marron, J.
    Carbonell, R.
    Seward, D.
    Glasmacher, U.
    Puchkov, V.
    Perez-Estaun, A.
    Mountain building processes during continent-continent collision in the Uralides2008In: Earth-Science Reviews, ISSN 0012-8252, E-ISSN 1872-6828, ISSN 0012-8252, Vol. 89, no 3-4, p. 177-195Article in journal (Refereed)
    Abstract [en]

    Since the early 1990's the Paleozoic Uralide Orogen of Russia has been the target of a significant research initiative as part of EUROPROBE and GEODE, both European Science Foundation programmes. One of the main objectives of these research programmes was the determination of the tectonic processes that went into the formation of the orogen. In this review paper we focus on the Late Paleozoic continent-continent collision that took place between Laurussia and Kazakhstania. Research in the Uralides was concentrated around two deep seismic profiles crossing the orogen. These were accompanied by geological, geophysical, geochronological, geochemical, and low-temperature thermochronological studies. The seismic profiles demonstrate that the Uralides has an overall bivergent structural architecture, but with significantly different reflectivity characteristics from one tectonic zone to another. The integration of other types of data sets with the seismic data allows us to interpret what tectonic processes where responsible for the formation of the structural architecture, and when they were active. On the basis of these data, we suggest that the changes in the crustal-scale structural architecture indicate that there was significant partitioning of tectonothermal conditions and deformation from zone to zone across major fault systems, and between the lower and upper crust. Also, a number of the structural features revealed in the bivergent architecture of the orogen formed either in the Neoproterozoic or in the Paleozoic, prior to continent-continent collision. From the end of continent-continent collision to the present, low-temperature thermochronology suggests that the evolution of the Uralides has been dominated by erosion and slow exhumation. Despite some evidence for more recent topographic uplift, it has so far proven difficult to quantify it.

  • 5. Brown, Dennis
    et al.
    Juhlin, Christopher
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics.
    Friberg, Magnus
    Rybalka, Alexander
    Puchkov, Viktor
    Gosha, Petrov
    Structural architecture of the Southern and Middle Urals foreland from reflection seismic profiles2006In: Tectonics, ISSN doi:10.1029/2005TC001834, Vol. 25, p. 1002.-Article in journal (Refereed)
  • 6.
    Bödvarsson, Reynir
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Space and time variations in crustal stress using microearthquake source information from South Iceland Seismic zone2006Report (Other academic)
  • 7.
    Bödvarsson, Reynir
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics.
    Slunga, Ragnar
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics.
    Development of routine multi-event evaluation and interpret improved locations in terms of earthquake foci migration and other detailed behaviour.2003Report (Other scientific)
  • 8.
    Garcia Juanatey, Maria de los Angeles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Integrated MagnetoTelluric and seismic reflection study: Skellefte Ore District, northern Sweden2011Conference paper (Refereed)
    Abstract [en]

    The Skellefte District is a very rich mining area in northern Sweden. The main deposits consist of volcanic-hosted massive sulphides VHMS rich in zinc, copper, lead, gold and silver. Since the area has been mined and explored for over a century, today's challenge is to locate deeper deposits. The VINNOVA 4D modeling project aims to address this challenge by understanding the regional setting of the district and its evolution over time. © 2011 Society of Exploration Geophysicists.

  • 9.
    García Juanatey, María de los Angeles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bauer, Tobias E.
    Luleå University of Technology.
    Dehghannejad, Mahdieh
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    2D and 3D MT in the central Skellefte Ore District, northern Sweden2019In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 764, p. 124-138Article in journal (Refereed)
    Abstract [en]

    New broadband magnetotelluric (MT) data have been acquired along two parallel profiles in the central part of the metallogenic Skellefte district in northern Sweden. The data were recorded as part of the Swedish 4D modelling of mineral belts project and cover an area with several economical and sub-economical deposits. The dimensionality and quality of the data were carefully analyzed and new error floors were systematically determined prior to inverse modelling in 2D and 3D. The algorithms used were EMILIA and WSINV3DMT. For the 2D inversion, only the determinant of the impedance tensor was used, while for the 3D inversion all elements were considered. The obtained models fit the inverted data, and image the main regional features. A detailed comparison reveals the superiority of the 3D model, both in model structures and data fit. After assessing the main features in the model, an interpretation is proposed and refined with the support of previous geophysical studies. The most interesting features are large and medium-sized conductors associated with crustal-scale shear zones and faults within the Skellefte Group rocks. These may be depicting a network of fossil pathways for hydrothermal fluid transport and as such, provide new insight into past processes in the area.

    The full text will be freely available from 2021-04-13 15:31
  • 10.
    García Juanatey, María de los Ángeles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Dehghannejad, Mahdieh
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bauer, Tobias E
    Weihed, Par
    Skytta, Pietari
    Magnetotelluric measurements in the Skellefte ore district, northern Sweden2013Conference paper (Refereed)
    Abstract
  • 11.
    García Juanatey, María de los Ángeles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Imaging the Kristineberg mining area with two perpendicular magnetotelluric profiles in the Skellefte Ore District, northern Sweden2013In: Geophysical Prospecting, ISSN 0016-8025, E-ISSN 1365-2478, Vol. 61, no 1, p. 200-219Article in journal (Refereed)
    Abstract [en]

    New magnetotelluric (MT) data from two perpendicular profiles in the Kristineberg area, northern Sweden, were analysed and modelled. In the Skellefte Ore District, the Kristineberg volcanic hosted massive sulphide (VHMS) deposit mine is one of the largest and deepest (1250 m). Seventeen broadband magnetotelluric stations were installed along two existing seismic reflection lines. The profiles were 6 and 12 km long with 500 m and 1 km site spacing, respectively. The obtained MT transfer functions in the period range of 0.0015–200 s are of fairly good quality. Detailed strike and dimensionality analysis reveal consistent but period dependent, strike directions, indicating a change in the geoelectrical strike with depth. From the two-dimensional inversion of the determinant of the impedance tensor, two stable conductivity models with good data fit were obtained. The addition of seismic reflection information from the co-located survey, improved the data fit of one of them. Extensive sensitivity analyses helped to delineate the well resolved regions of the models and to determine the position of pronounced boundaries. The results are in good agreement with previous studies, especially regarding the presence of a deep conductor interpreted as a structural basement to the district. They also reveal with more detail the configuration of the main geological units of the Skellefte Ore District, especially of the ore bearing volcanic rocks and the embedded alteration zones.

  • 12.
    García Juanatey, María de los Ángeles
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bergström, Ulf
    Geological Survey of Sweden.
    Hübert, Juliane
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    MT and reflection seismics in northwestern Skellefte Ore District, Sweden2013In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 78, no 2, p. B65-B76Article in journal (Refereed)
    Abstract [en]

    A seismic reflection and MT survey was carried out along a 27-km long transect in northwestern Skellefte District, as part of a bigger 3D modeling project. The main motivation for the data acquisition is to elucidate the geologic relationship between the known mineralizations in the Adak mining camp to the north and in the well studied Kristineberg area south of the transect. The seismic reflection data were acquired with a VIBSIST system, and show reflectivity down to 3 s. Apart from the conventional processing for crystalline environments, the seismic data was also subject to an azimuthal binning procedure and cross-dip analysis, allowing the orientation of planar reflectors in 3D. Regarding the MT data, it is primarily of good quality along the 17 installed sites. The inversion of the determinant of the impedance tensor yielded a stable 2D resistivity model, dominated by resistors corresponding to the postorogenic intrusions along the transect. Adding the location of the analyzed seismic reflectors in the MT inversion rendered an integrated model that facilitated a preliminary joint interpretation of the data sets. Overall, the results are in good agreement with surface observations and reveal a crude configuration of the geologic units below the transect. The most prominent outcomes are the lateral and depth extent of the large postorogenic intrusions in the area reaching to 5- or 6-km depth, the dimensions of the nearly vertical Brännäs gabbro extending to 6-km depth, and the presence of enhanced conductivities along the transect at about 10 km depth. The latter is probably related to the deep conductor previously identified in the district.

  • 13. Guangnan, Huang
    et al.
    Yang, Liu
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Guangyi, Hu
    Tingen, Fan
    Jianhua, Dong
    Velocity Tomography Imaging Method with Variable Grid spacing/Interval2013In: Oil Geophysical Prospecting, ISSN 1000-7210, Vol. 48, no 3, p. 379-389Article in journal (Refereed)
    Abstract [en]

    In variable grid spacing tomography the underground velocity distribution is parameterized with model cells of different sizes. This method can simultaneously take into account the spatially varying resolution inherent in most datasets. E.g., due to experimental design or logistic constraints, the shallow and deep subsurface velocity distribution may be very differently determined by the available data. The variable grid spacing tomography and regular grid spacing tomography are similar in most other aspects. There are two main differences between the variable grid spacing method and the regular grid spacing method. First, ray segments calculated in the forward model cells should be divided into the larger cells of the inversion model. Second, the smoothness constraint equations needed to inhibit wild velocity variations need to be modified where the cells change in size. In a synthetic checkerboard reconstruction test with differently sized checkers the variable grid spacing method recovers the small and large checkers better than the regular grid spacing method in both two and three-dimensional test cases. For a real dataset, the variable grid spacing method distinguishes two low velocity zones better than the regular grid spacing method. Finally, it can be concluded that when the effort is spent to identify the regions in a model with best data coverage, the variable grid spacing method can produce velocity images with higher fidelity than when a uniform cell size is used. Especially, in many cases this method can enhance the fidelity of the shallow subsurface velocity distribution. In addition, variable grid spacing tomography can reduce the underdetermined regions in an inverse problem when the data coverage is irregular.

  • 14. Hreinsdottir, S
    et al.
    Arnadottir, T
    Decriem, J
    Geirsson, H
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bennett, A
    LaFemina, P
    A complex earthquake sequence captured by the continuous GPS network in SW Iceland2009In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 36, no 12, p. L12309-Article in journal (Refereed)
    Abstract [en]

    A complex sequence of earthquakes struck the western part of the South Iceland Seismic Zone (SISZ) on 29 May 2008. The sequence initiated with a M(w)6.3 (NEIC) earthquake in the western part of the SISZ. Aftershocks from the earthquake delineate two parallel N-S trending structures 4 km apart, in addition to activity along an E-W zone further westward. Continuous GPS measurements can best be explained by right-lateral strike-slip motion on two parallel N-S trending faults, with little slip occurring on other structures illuminated by earthquake activity. We estimate a total moment release of M(w)6.2, with M(w)6.1 on the first rupture and M(w)6.0 on the second rupture. High rate (1 Hz) CGPS data from a near-field station suggest that the main asperity on the Kross fault ruptured within 3 s of the initial mainshock on the Ingolfsfjall fault. Citation: Hreinsdottir, S., T. Amadottir, J. Decriem, H. Geirsson, A. Tryggvason, R. A. Bennett, and P. LaFemina (2009), A complex earthquake sequence captured by the continuous GPS network in SW Iceland, Geophys. Res. Lett., 36, L12309, doi: 10.1029/2009GL038391.

  • 15.
    Huang, Guangnan
    et al.
    East China Univ Technol, State Key Lab Nucl Resources & Environm, Nanchang, Jiangxi, Peoples R China;East China Univ Technol, Dept Geophys, Nanchang 330013, Jiangxi, Peoples R China;Iowa State Univ, Dept Math, Ames, IA USA.
    Luo, Songting
    Iowa State Univ, Dept Math, Ames, IA USA.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Li, Hongxing
    East China Univ Technol, State Key Lab Nucl Resources & Environm, Nanchang, Jiangxi, Peoples R China;East China Univ Technol, Dept Geophys, Nanchang 330013, Jiangxi, Peoples R China.
    Nobes, David C.
    East China Univ Technol, State Key Lab Nucl Resources & Environm, Nanchang, Jiangxi, Peoples R China;East China Univ Technol, Dept Geophys, Nanchang 330013, Jiangxi, Peoples R China.
    First-arrival tomography with fast sweeping method solving the factored eikonal equation2019In: Exploration Geophysics, ISSN 0812-3985, E-ISSN 1834-7533, Vol. 50, no 2, p. 144-158Article in journal (Refereed)
    Abstract [en]

    This paper presents a first-arrival tomography incorporating a fast sweeping method (FSM) solving the factored eikonal equation (factored FSM). The traveltime calculation method plays a significant role in velocity inversion. However, for a point source condition, all finite-difference based eikonal solvers suffer from the source singularity problem. Numerical error caused by source singularity will propagate from the source to all computational domains, and makes traveltimes inaccurate. A FSM solving the factored eikonal equation can deal with the source singularity problem very well. Therefore, a first-arrival tomography is developed by incorporating 2D and 3D factored FSMs to provide more accurate traveltimes in velocity inversion. For comparison, an open source package PStomo_eq is used to invert the same data set. It incorporates the traveltime calculation algorithms fdtime2d.c and fdtime3d.c. Traveltime accuracy tests show that factored FSM can generate more accurate traveltimes than FSM, fdtime2d.c and fdtime3d.c. Numerical and field data tests show that inversion with factored FSM can acquire much better tomograms than inversion with fdtime2d.c and fdtime3d.c. Therefore, it is worthwhile using a more accurate traveltime computation method in velocity inversion.

  • 16.
    Hübert, Juliane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    García Juanatey, María de los Ángeles
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    The upper crustal 3-D resistivity structure of the Kristineberg area, Skellefte district, northern Sweden revealed by magnetotelluric data2013In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 192, no 2, p. 500-513Article in journal (Refereed)
    Abstract [en]

    A 3-D model of the crustal electrical resistivity was constructed from the inversion of magnetotelluric data in the Kristineberg area, Skellefte district, the location of one of Sweden's most successful mining activities. Forward modelling of vertical magnetic transfer data supports our model which was derived from the magnetotelluric impedance only. The dominant features in the 3-D model are the strong conductors at various depth levels and resistive bodies of variable thickness occurring in the shallower subsurface. The deepest conductor, previously associated with the Skellefte crustal conductivity anomaly, is imaged in the southern part of the area as a north-dipping feature starting at similar to 4 km depth. Several shallow conductors are attributed to graphite in the black shales defining the contact between the metasedimentary rocks and the underlying metavolcanic rocks. Furthermore, an elongated intermediate depth conductor is possibly associated with alteration zones in the metavolcanic rocks that host the ore occurrences. The most prominent crustal resistors occur in the southern and northern part of the area, where their lateral extent on the surface coincides with the late-orogenic Revsund type intrusions. To the east, a resistive feature can be correlated to the early-orogenic Viterliden intrusion. The 3-D model is compared with two previous 2-D inversion models along two perpendicular profiles. The main electrical features are confirmed with the new model and previous uncertainties regarding 3-D effects, caused by off-profile conductors, can be better assessed in 3-D, although the resolution is lower due to a coarser model discretization. The comparison with seismic sections along two north-south profiles reveals structural correspondence between electrical features, zones of different reflectivity and geological units.

  • 17.
    Hübert, Juliane
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Smirnov, Maxim
    Oulu University, Finland.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pedersen, Laust B.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    MT measurements in the western part of the Paleoproterozoic Skellefte Ore District, Northern Sweden: a contribution to an integrated geophysical study2009In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 475, no 3-4, p. 493-502Article in journal (Refereed)
    Abstract [en]

    A 2D conductivity model of the Kristineberg area in the Skellefte Ore District, Northern Sweden, has been derived from new magnetotelluric measurements. This complements an intensive geophysical and geological study of the area, including reflection seismics, gravity and aeromagnetic data modeling as well as geological field observations. In a pilot study, 20 broadband MT stations were installed in May 2007 along a 20 km long north–south profile. Dimensionality analysis shows that a 2D interpretation of the data is justified, although the presumed geoelectric strike direction of N75°E is not consistent over the whole profile. The new conductivity model of the upper crust agrees well with the results from the seismic studies. Interpreting both independent data sets confirms the major features from the previous model, such as the thickness of the Revsund granites in the south, the existence of a structural basement with metasedimentary origin, and gives new insight into the nature of the volcanic rocks and their possible mineral content.

  • 18.
    Jeddi, Zeinab
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ambient-noise tomography of Katla volcano, south Iceland2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 347, p. 264-277Article in journal (Refereed)
    Abstract [en]

    A shear-wave velocity model of subglacial Katla volcano, southern Iceland, has been developed using ambient seismic noise tomography based on data from a temporary network operating between May 2011 and August 2013 and permanent stations around the volcano. Phase-velocity dispersion curves were obtained using cross correlations of vertical components of 136 station pairs and non-linearly inverted for phase-velocity maps between 1.7 and 7.5 s. Local dispersion curves were inverted for shear-velocity variation with depth using a grid search imposing a fixed ice layer at the top. The resulting one-dimensional (1-D) velocity models were combined to obtain a pseudo three-dimensional (3-D) model with estimated lateral resolution of 8 km and depth resolution varying from close to 1 km near the surface to about 8 km at 10 km depth. Shear wave velocities are generally higher within the Katla central volcano than in its surroundings. The most striking feature of the model is a high-velocity anomaly beneath the caldera at >6 km depth interpreted to be due to cumulates resulting from differentiation of shallower magma intrusions and remelting of subsiding upper crust. No shallow low-velocity anomaly is resolved beneath the central caldera, but a low-velocity region is found at 2-4 km depth beneath the western half of the caldera. V-p/V-s ratios, estimated from average velocity-depth profiles from surface-wave data and higher frequency P-wave data, are anomalously high (>1.9) compared to average Icelandic crust, particularly in the top 2-3 km. This is argued not to be an artifact due to lateral refraction or topography. Instead, this anomaly could be explained as an artifact caused by velocity dispersion due to attenuation and a difference in frequency content, and possibly to a degree by the compositional difference between the transalkalic Fe-Ti basalts of Katla and average tholeiitic Icelandic crust.

  • 19.
    Jeddi, Zeinab
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Sgattoni, Giulia
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    A peculiar cluster of microearthquakes on the eastern flank of Katla volcano, southern Iceland2017In: Jökull: Journal of The Glaciological and Geological Societies of Iceland, ISSN 0449-0576, Vol. 67, p. 1-16Article in journal (Refereed)
    Abstract [en]

    A peculiar cluster of seismicity near the tip of Sandfellsjokull on the eastern flank of Katla volcano in southern Iceland has been analyzed in detail using data from a temporary seismic network. A total of 300 events were detected between July 2011 and August 2013, most of them from a swarm between December 4th and 12th, 2011. The sparser permanent network detected a small fraction of these events, but also a larger swarm in November 2010. When seismic activity started in this area is uncertain because of changes in the detection capability of the network over time. The events are of low magnitude (-0.5 < ML < 0.5) and the b-value of their magnitude distribution is high (1.6 +/- 0.1). Based on their frequency content (4-25 Hz) and clear P and S arrivals, the events are classified as volcano-tectonic. Two multiplets probably with different source mechanism are identified in their population. The events locate at approximately 3.5 km depth. Most of them are tightly clustered according to double difference relative locations in a volume that is only about 400 m in diameter in all directions. Several events are scattered up to 800 m beneath this volume. There is some suggestion of elongate structure in the cluster with a NNE/SSW strike and a dip of 60 degrees. We argue that these events cannot be due to a glacial or a broad tectonic process. Possibly, a localized source of fluid pressure, e.g., a small magma body at depth may be the source of these events.

  • 20.
    Jeddi, Zeinab
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Reykjavik Univ, Sch Sci & Engn, Reykjavik, Iceland.
    The Katla volcanic system imaged using local earthquakes recorded with a temporary seismic network2016In: Journal of Geophysical Research - Solid Earth, ISSN 2169-9313, E-ISSN 2169-9356, Vol. 121, no 10, p. 7230-7251Article in journal (Refereed)
    Abstract [en]

    Katla is one of the most active subglacial volcanoes in Iceland. A temporary seismic network was operated on and around Katla for 2.5 years. A subset of similar to 800 analyzed local earthquakes clustered geographically in four regions: (1) the caldera, (2) the western region, (3) the southern rim, and (4) the eastern rim of the glacier. Based on the frequency content of recorded seismograms, each event was labeled as volcano tectonic (VT), long period (LP), or 'Mixed'. The southern cluster consists of LP events only, and the eastern cluster consists of VT events, while the western cluster is 'Mixed' although primarily LP. The caldera seismicity is confined to a subregion centered in the northeastern part of the caldera above 1 km below sea level (bsl) and gradually deepens away from its center to about 4 km depth. Deeper events are almost all VT, whereas LP events in the center of caldera locate at shallow depths. This is also where the velocities are lowest in the top 3 km of the crust of our 3-D tomographic model. A high-velocity core (similar to 6.5 km/s) is found at 4 km bsl beneath this low-velocity zone. We propose that a "subcaldera" may be developing within the present caldera and suggest a conceptual model for Katla volcano with a thin volume (similar to 1 km thick) that may host hot rhyolitic material in the shallow crust below the relocated seismic activity and above the high-velocity core. We interpret this core to consist of mafic cumulates resulting from fractionation of mafic intrusions and partial melting of subsiding hydrothermally altered rocks.

  • 21.
    Jonsdottir, Kristin
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pohjola, Veijo
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Shomali, Zaher Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lp-events at Katla volcano, Iceland, are glacial and not volcanic in origin2009In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007Article in journal (Other academic)
    Abstract [en]

    Repeating long-period (lp) earthquakes are commonly observed in volcanic regions worldwide. They are usually explained in terms of a volcanic source effect or anomalous propagation through the volcano. Recently, large lp-events have also been associated with the motion of massive ice streams. Our joint analysis of climatic and new seismic data shows that small lp-events observed at Katla volcano, Iceland, are in fact related to ice movement in a steep outlet glacier and not, as previously thought, to volcanic intrusive activity. The over 13000 lp-events recorded since 2000 are consistent in character and magnitude with seasonal changes of the glacier. As the current global warming trend could cause similar earthquake sequences at other glacier covered volcanoes, identifying them as glacial rather than eruption precursors is vital.

  • 22.
    Jónsdóttir, Kristín
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Soosalu, H.
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Habits of a glacier-covered volcano: Seismicity patterns and velocity structure of Katla volcano, Iceland2007In: Annals of Glaciology, ISSN 0260-3055, E-ISSN 1727-5644, Vol. 45, p. 169-177Article in journal (Refereed)
    Abstract [en]

    The Katla volcano, overlain by the Mýrdalsjökull glacier, is one of the most active and hazardous volcanoes in Iceland. Earthquakes show anomalous magnitude-frequency behaviour and mainly occur in two distinct areas: within the oval caldera and around Goðabunga, a bulge on its western flank. The seismicity differs between the areas; earthquakes in Goðabunga are low frequency and shallow whereas those beneath the caldera occur at greater depths and are volcano-tectonic. The seismicity shows seasonal variations but the rates peak at different times in the two areas. A snow budget model, which gives an estimate of the glacial loading, shows good correlation with seismic activity on an annual scale. Data recorded by the permanent network South Iceland Lowland (SIL), as well as by a temporary network, are used to invert for a 3D seismic velocity model underneath Eyjafjallajökull, Goðabunga and the Katla caldera. The tomography resolves a 15 km wide, aseismic, high-velocity structure at a depth of more than 4 km between the Eyjafjallajökull volcano in the west and the Katla volcano in the east. Anomalously low velocities are observed beneath the Katla caldera, which is interpreted as being a significantly fractured area of anomalously high temperature.

  • 23.
    Kashubin, Artem S.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Rybalka, A. V.
    Kashubina, T. V.
    Shkred, I. G.
    The Krasnouralsky profile in the Middle Urals, Russia: a tomographic approach to vintage DSS data2009In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 472, no 1-4, p. 249-263Article in journal (Refereed)
    Abstract [en]

    The Middle Urals region has been widely studied with geophysical methods over the past decades. An integrated program is in progress to summarize this knowledge, including modern reprocessing of controlled-source seismic data. This work is devoted to the Krasnouralsky DSS profile. We applied modern tomography inversion algorithms in 2D and 2.5D on first break traveltime picks from an archive catalogue. A number of initial models and various smoothing constraints were used to investigate the influence of starting models on the final model. Robustness and uncertainty of the recovered models were estimated with hypothesis testing and checkerboard tests. The recovered velocity structure shows a thicker crust below the contact of the West Uralian Zone and the Central Uralian Zone and below the Tagil–Magnitogorsk Zone. Deep high velocity anomalies on both sides of this zone are interpreted as crustal thinning or alteration of the crust by intrusions of mantle material. Our results suggest that it is worthwhile reinterpreting DSS traveltime data with modern inversion techniques.

  • 24.
    Li, Ka Lok
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Brandsdottir, Bryndis
    Univ Iceland, Inst Sci, Reykjavik, Iceland..
    Focusing patterns of seismicity with relocation and collapsing2016In: Journal of Seismology, ISSN 1383-4649, E-ISSN 1573-157X, Vol. 20, no 3, p. 771-786Article in journal (Refereed)
    Abstract [en]

    Seismicity is generally concentrated on faults or in fault zones of varying, sometimes complex geometry. An earthquake catalog, compiled over time, contains useful information about this geometry, which can help understanding the tectonics of a region. Interpreting the geometrical distribution of events in a catalog is often complicated by the diffuseness of the earthquake locations. Here, we explore a number of strategies to reduce this diffuseness and hence simplify the seismicity pattern of an earthquake catalog. These strategies utilize information about event locations contained in their overall catalog distribution. They apply this distribution as an a priori constraint on relocations of the events, or as an attractor for each individual event in a collapsing scheme, and thereby focus the locations. The latter strategy is not a relocation strategy in a strict sense, although event foci are moved, because the movements are not driven by data misfit. Both strategies simplify the seismicity pattern of the catalog and may help to interpret it. A synthetic example and a real-data example from an aftershock sequence in south west Iceland are presented to demonstrate application of the strategies. Entropy is used to quantify their effect.

  • 25.
    Lindblom, Eva
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Uski, Marja
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Microearthquakes illuminate the deep structure of the endglacial Parvie fault, northern Sweden2015In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 201, no 3, p. 1704-1716Article in journal (Refereed)
    Abstract [en]

    At 155 km, the Parvie fault is the world's longest known endglacial fault (EGF). It is located in northernmost Sweden in a region where several kilometre-scale EGFs have been identified. Based on studies of Quaternary deposits, landslides and liquefaction structures, these faults are inferred to have ruptured as large earthquakes when the latest ice sheet disappeared from the region, some 9500 yr ago. The EGFs still exhibit relatively high seismic activity, and here we present new earthquake data from northern Sweden in general and the Parvie fault in particular. More than 1450 earthquakes have been recorded in Sweden north of 66A degrees latitude in the years 2000-2013. There is a remarkable correlation between this seismicity and the mapped EGF scarps. We find that 71 per cent of the observed earthquakes north of 66A degrees locate within 30 km to the southeast and 10 km to the northwest of the EGFs, which is consistent with the EGFs' observed reverse faulting mechanisms, with dips to the southeast. In order to further investigate the seismicity along the Parvie fault we installed a temporary seismic network in the area between 2007 and 2010. In addition to the routine automatic detection and location algorithm, we devised a waveform cross-correlation technique which resulted in a 50 per cent increase of the catalogue and a total of 1046 events along the Parvie fault system between 2003 and 2013. The earthquakes were used to establish an improved velocity model for the area, using 3-D local earthquake tomography. The resulting 3-D velocity model shows smooth, minor velocity variations in the area. All events were relocated in this new 3-D model. A tight cluster on the central part of the Parvie fault, where the rate of seismicity is the highest, could be relocated with high precision relative location. We performed depth phase analysis on 40 of the larger events to further constrain the hypocentral locations. We find that the seismicity on the Parvie fault correlates very well with the mapped surface trace of the fault. The events do not align along a well-defined fault plane at depth but form a zone of seismicity that dips between 30A degrees and 60A degrees to the southeast of the surface fault trace, with distinct along-strike variations. The seismic zone extends to approximately 35 km depth. Using this geometry and earthquake scaling relations, we estimate that the endglacial Parvie earthquake had a magnitude of 8.0 +/- A 0.4.

  • 26.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Thunehed, Hans
    GeoVista, Luleå.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    The Paleoproterozoic Kristineberg mining area, northern Sweden: Results from integrated 3D geophysical and geologic modeling, and implications for targeting ore deposits2009In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 74, no 1, p. B9-B22Article in journal (Refereed)
    Abstract [en]

    The Kristineberg mining area in the western part of the Paleoproterozoic Skellefte Ore District, northern Sweden, is well known for its base-metal and recent gold discoveries. A pilot 3D geologic model has been constructed on a crustal scale, covering an area of 30×30  km to depths of 10  km. Constrained 3D inverse and forward gravity modeling have been performed to confirm and refine previous modeling along seismic profiles using mainly 2.5D techniques. The 3D inverse gravity modeling was geared to generating isodensity surfaces that enclose regions within the model of anomalous density contrast. The 3D forward gravity modeling was conducted to include faulting and folding systems that are difficult to include in the inversion. The 3D geologic model supports many previous interpretations but also reveals new features of the regional geology that are important for future targeting of base-metal and gold deposits. The margins of a thick granite in the south dip steeply inward, suggesting the possibility of room to accommodate another large base-metal deposit if the granitic rocks are juxtaposed with volcanic rocks at depth. Gravity modeling also suggests the observed Bouguer gravity high within the western metasediments can be explained by a large mafic intrusion that has dioritic to tonalitic composition and no significant magnetic signature. Because mafic-ultramafic intrusions within metasediments can indicate gold, this interpretation suggests the western metasediments have a high gold potential.

  • 27.
    Malehmir, Alireza
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Tryggvason, Ari
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Juhlin, Christopher
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Rodriguez-Tablante, Johiris
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Weihed, Par
    Luleå University, Ore Geology.
    Seismic imaging in the Skellfte ore district, northern Sweden: Seismic imaging in the Skellfte ore district, northern Sweden2005In: 20th World Mining Congress: 20th World Mining Congress, 2005, p. 399-404Conference paper (Refereed)
    Abstract [en]

    In the western part of the Skellefte ore district, which is the most important metallogenic zone in Northern Sweden, several world class mines, e.g., the Kristineberg VMS mine (20.1mt, Cu-Zn-Pb-Ag-Au) are situated. In order to understand why these deposits occur where they do, it is important to understand the crustal architecture of the region. One way to understand the contact relationships between the ore bearing volcanic formations and the surrounding rocks is to develop a detailed 3-D geological model of the region. To establish the structural geologic framework at depth, new seismic reflection data were acquired along two profiles in the Kristineberg area in late August and early September 2003. Data along the two seismic profiles (Profile 1 and Profile 5), each about 25 km long and running in parallel, were collected with the purpose of obtaining high resolution images of the top 10 km of the crust. Although the structural geology is very complex, preliminary stacked sections of the data have revealed numerous reflections which can be correlated with surface geology. Results along Profile 1, which passes on top of the Kristineberg mine show the mine to be located in a major synform extending down to about 2.5-3 km depth. The structure and stratigraphy of the Kristineberg area have been debated for many years. Our seismic results suggest that the deposits occur on the northern limb of a regional syncline. The results help to identify new prospective areas, both down-plunge from known ores, and on the southern limb of the ore-bearing syncline. The results for Profile 5 show that the Revsund granite can have a thickness about 3-3.5 km but not more. Ultramafic rocks are imaged clearly. Diffraction patterns can be interpreted as orienting from either a mafic-ultramafic intrusion or a mineralization zone. A detailed study has to be done in order to determine the source of this reflectivity. In this study seismic reflection profiling has been particularly effective for imaging the major structures around the ore body, demonstrating that the seismic-reflection technique can be used for delineating complex structures that are significant in mineral exploration.

  • 28.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Rodriguez-Tablante, Johiris
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Weihed, Pär
    Luleå University of Technology, Luleå, Sweden.
    Seismic imaging and potential field modeling to delineate structures hosting VHMS deposits in the Skellefte Ore District, northern Sweden2006In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 426, no 3-4, p. 319-334Article in journal (Refereed)
    Abstract [en]

    The Skellefte District in northern Sweden is a roughly 150 by 50 km2 large early Proterozoic massive sulphide belt. Based on high-resolution reflection seismic data along two parallel seismic profiles, potential field modeling has been carried out and two geologic cross sections have been constructed that are consistent with the available geophysical data as well as surface geologic observations. The combined modelling suggests that the Kristineberg deposit occurs on the northern limb of a regional E–W striking syncline. The interpretations help to identify new prospective areas, both down-plunge from known ores, and on the ore-bearing horizon on the southern limb of the syncline. The new results suggest that the post-orogenic Revsund granites can be divided into two major types of intrusives, those which are intruded as domes/stokes with a maximum present day thickness of about 3–3.5 km and those which are intruded as thin sheets, with a maximum thickness of a few hundred meters. The margins of the intrusions are generally inclined inwards, suggesting that the current erosion level is near the middle, or toward the base, of the granites. The contact between the Skellefte volcanic rocks and the Bothnian Basin has been interpreted as a thrust fault. We also suggest that crustal thickening predates the Skellefte volcanism and that the interpreted Bothnian Basin rocks are either a structural basement or a separate terrane to the Skellefte volcanism. Diffraction patterns in the reflection seismic data can be interpreted as originating from either a mafic–ultramafic intrusion or a mineralization zone, similar to observations elsewhere in the world. The results obtained in this study have greatly improved our understanding of the tectonostratigraphic framework and architecture of the poly-deformed c. 1.9 Ga Skellefte VHMS belt and is a key step towards building a 3D geological model in the area.

  • 29.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lickorish, Henry
    Weihed, Pär
    Luleå University of Technology.
    Regional structural profiles in the western part of the Palaeoproterozoic Skellefte Ore District, northern Sweden2007In: Precambrian Research, ISSN 0301-9268, E-ISSN 1872-7433, Vol. 159, no 1-2, p. 1-18Article in journal (Refereed)
    Abstract [en]

    The Kristineberg mining area is situated in the western part of the Palaeoproterozoic Skellefte Ore District, northern Sweden, and is well known for its VHMS base-metal and gold deposits. This paper presents five upper crustal geological cross sections that have been constructed and mainly constrained by seismic reflection data, potential field modeling as well as geological field observations. These profiles are visualized in 3D to highlight the three dimensionality and internal consistency of structures across the region. The resulting structural model for the Skellefte volcanics and overlying metasediments comprises two thrust-sheets that expose the Skellefte volcanics in the cores of hanging-wall anticlinal structures. A shear-zone is imaged as a band of seismic reflectivity terminated by the southern Revsund granite unit. Another shear-zone, possibly a continuation of the Skellefte Shear Zone (SSZ) runs through the centre of the region and accounts for some of the structural complexity and shearing observed between the two anticlinal exposures of the volcanics. Additional smaller scale shear-zones have been identified from geological and geophysical mapping within the main structural blocks of the Skellefte volcanics. The Mala volcanic rocks in the north are separated from the Skellefte volcanics by a fault that cuts discordant to the strike of the Mala volcanics. A structural basement has been proposed to the Skellefte volcanics, constrained by seismic reflection data. Exposures of Bothnian Basin rocks south of the Revsund granite outcrops, suggest that the domain beneath the north dipping reflectivity is associated with Bothnian Basin stratigraphy. The preferred interpretation for the contact between the Skellefte volcanics and the Bothnian Basin rocks is a thrust fault that brings the felsic volcanic rocks over the metasedimentary domain. The Revsund granites are divided into two major groups based on their present day thickness and shapes. Although parts of the Viterliden intrusion are almost undeformed, it is cut by a series of shear-zones, causing the magnetic lineations seen within these rocks. The structural profiles presented demonstrate that the Kristineberg ore is situated in the northern limb of a local synformal structure. The new crustal-scale structural model demonstrates the potential of integrating geophysical and geological data when modelling structures hosting mineralization in a complex region like the Skellefte District. The structural profiles presented in this paper, have greatly improved our understanding of the 3D tectonostratigraphy and architecture of the poly-deformed ca. 1.9 Ga the Skellefte Ore District.

  • 30.
    Malehmir, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Wijns, Chris
    First Quantum Minerals Ltd, Perth, WA, Australia.
    Koivisto, Emilia
    Univ Helsinki, Helsinki, Finland.
    Lindqvist, Teemu
    Univ Helsinki, Helsinki, Finland.
    Skyttä, Pietari
    Univ Turku, Turku, Finland.
    Montonen, Markku
    Boliden FinnEx Oy, Kevitsa, Finland.
    Why 3D seismic data are an asset for exploration and mine planning?: Velocity tomography of weakness zones in the Kevitsa Ni-Cu-PGE mine, northern Finland2018In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 83, no 2, p. B33-B46Article in journal (Refereed)
    Abstract [en]

    Kevitsa is a disseminated Ni-Cu-PGE (platinum group elements) ore body in northern Finland, hosted by an extremely high-velocity (6.5-8.5 km/s) ultramafic intrusion. It is currently being mined at a depth of approximately 100 m with open-pit mining. The estimated mine life is 20 years, with the final pit reaching a depth of 500-600 m. Based on a series of 2D seismic surveys and given the expected mine life, a high-resolution 3D seismic survey was justified and conducted in the winter of 2010. We evaluate earlier 3D reflection data processing results and complement that by the results of 3D first-arrival traveltime tomography. The combined results provide insights on the nature of some of the reflectors within the intrusion. In particular, a major discontinuity, a weakness zone, is delineated in the tomography results on the northern side of the planned pit. Supported by the reflection data, we estimate the discontinuity, likely a thrust sheet, to extend down approximately 600 m and laterally 1000 m. The weakness zone terminates prominent internal reflectivity of the Kevitsa intrusion, and it is associated with the extent of the economic mineralization. Together with other weakness zones, a couple of which are also revealed by the tomography study, the discontinuity forms a major wedge block that influences the mine bench stability on the northern side of the open pit and likely will cause more issues during the extraction of the ore in this part of the mine. We argue that 3D seismic data should routinely be acquired prior to commencement of mining activities to maximize exploration efficiency at depth and also to optimize mining as it continues toward depth. Three-dimensional seismic data over mineral exploration areas are valuable and can be revisited for different purposes but are difficult to impossible to acquire after mining has commenced.

  • 31.
    Malehmor, Alireza
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Wijns, Chris
    First Quantum Minerals Ltd., Perth, Australia.
    Koivisto, Emilia
    University of Helsinki, Finland.
    Lindqvist, Teemu
    University of Helsinki, Finland.
    Skyttä, Pietari
    University of Turku, Finland.
    Montonen, Markku
    Boliden FinnEx Oy, Finland.
    Why 3D seismic data are an asset for both exploration and mine planning? Example of Kevitsa Ni-Cu-PGE, Finland2016In: Lithosphere 2016: Ninth Symposium On Structure, Composition And Evolution Of The Lithosphere In Fennoscandia / [ed] Ilmo Kukkonen, Suvi Heinonen, Kati Oinonen, Katriina Arhe, Olav Eklund, Fredrik Karell, Elena Kozlovskaya, Arto Luttinen, Raimo Lahtinen, Juha Lunkka, Vesa Nykänen, Markku Poutanen , Eija Tanskanen and Timo Tiira, Helsinki, Finland: University of Helsinki, Institute of Seismology , 2016, p. 83-86Conference paper (Refereed)
    Abstract [en]

    Kevitsa is a disseminated Ni-Cu-PGE ore body in northern Finland, hosted by an extremely high-velocity (6-8 km/s) mafic-ultramafic intrusion. It is currently being mined at a depth of about 100 m using open-pit mining method. The mine life is expected to be about 20 years, with the final pit depth at around 400-500 m. Based on a series of 2D seismic surveys and given the expected mine life, a high-resolution 3D seismic survey was justified and acquired in winter 2010. Various researchers and teams have exploited these data because of the unique nature of geology, and the data being challenging to interpret but rich in reflectivity. In this study, we present 3D reflection data processing results and complement them with 3D first break tomography work recently carried out. The combined results allow to provide some insights about the nature of some of the reflectors. It for example shows how the tomography results can be used for rock quality studies and further planning of the pit. In particular, we observe a major fracture system, resolved by the tomography results and running in the middle of the planned pit, with the reflection data providing information about its depth extent, estimated to be at least about 500 m. We argue that 3D seismic data should be acquired prior to commencement of mining activities in order to maximize exploration efficiency at depth, but also to optimize mining as it continues towards depth.

  • 32.
    Marti, J.
    et al.
    CSIC, Inst Earth Sci Jaume Almera, ICTJA, Barcelona, Spain..
    Villasenor, A.
    CSIC, Inst Earth Sci Jaume Almera, ICTJA, Barcelona, Spain..
    Geyer, A.
    CSIC, Inst Earth Sci Jaume Almera, ICTJA, Barcelona, Spain..
    Lopez, C.
    IGN, Observ Geofis Cent, Madrid, Spain..
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Stress barriers controlling lateral migration of magma revealed by seismic tomography2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, article id 40757Article in journal (Refereed)
    Abstract [en]

    Understanding how monogenetic volcanic systems work requires full comprehension of the local and regional stresses that govern magma migration inside them and why/how they seem to change from one eruption to another. During the 2011-2012 El Hierro eruption (Canary Islands) the characteristics of unrest, including a continuous change in the location of seismicity, made the location of the future vent unpredictable, so short term hazard assessment was highly imprecise. A 3D P-wave velocity model is obtained using arrival times of the earthquakes occurred during that pre-eruptive unrest and several latter post-eruptive seismic crises not related to further eruptions. This model reveals the rheological and structural complexity of the interior of El Hierro volcanic island. It shows a number of stress barriers corresponding to regional tectonic structures and blocked pathways from previous eruptions, which controlled ascent and lateral migration of magma and, together with the existence of N-S regional compression, reduced its options to find a suitable path to reach the surface and erupt.

  • 33.
    Melchiorre, Caterina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Application of a fast and efficient algorithm to assess landslide-prone areas in sensitive clays in Sweden2015In: Natural hazards and earth system sciences, ISSN 1561-8633, E-ISSN 1684-9981, Vol. 15, no 12, p. 2703-2713Article in journal (Refereed)
    Abstract [en]

    We refine and test an algorithm for landslide susceptibility assessment in areas with sensitive clays. The algorithm uses soil data and digital elevation models to identify areas which may be prone to landslides and has been applied in Sweden for several years. The algorithm is very computationally efficient and includes an intelligent filtering procedure for identifying and removing small-scale artifacts in the hazard maps produced. Where information on bedrock depth is available, this can be included in the analysis, as can information on several soil-type-based cross-sectional angle thresholds for slip. We evaluate how processing choices such as of filtering parameters, local cross-sectional angle thresholds, and inclusion of bedrock depth information affect model performance. The specific cross-sectional angle thresholds used were derived by analyzing the relationship between landslide scarps and the quick-clay susceptibility index (QCSI). We tested the algorithm in the Göta River valley. Several different verification measures were used to compare results with observed landslides and thereby identify the optimal algorithm parameters. Our results show that even though a relationship between the cross-sectional angle threshold and the QCSI could be established, no significant improvement of the overall modeling performance could be achieved by using these geographically specific, soil-based thresholds. Our results indicate that lowering the cross-sectional angle threshold from 1 : 10 (the general value used in Sweden) to 1 : 13 improves results slightly. We also show that an application of the automatic filtering procedure that removes areas initially classified as prone to landslides not only removes artifacts and makes the maps visually more appealing, but it also improves the model performance.

  • 34.
    Reynir, Bödvarsson
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Geofysik.
    Björn, Lund
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Geofysik.
    Roland, Roberts
    Ragnar, Slunga
    Ari, Tryggvason
    Seismologisk studie relaterad till Citybanan i Stockholm2004Report (Other scientific)
  • 35.
    Rodriguez-Tablante, Johiris
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Palm, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Cross-profile acquisition and cross-dip analysis for extracting 3D information from 2D surveys, a case study from the western Skellefte District, northern Sweden2007In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 63, no 1, p. 1-12Article in journal (Refereed)
    Abstract [en]

    Two nearly parallel seismic profiles were acquired in the Kristineberg area, located in the western part of the Skellefte ore district, in northern Sweden. A novel approach in the acquisition was that all shots from each profile were recorded onto both lines, resulting in a third CDP line halfway between the two profiles. The combination of geometry, acquisition parameters and geological complexity of the area required specific processing techniques to produce a stacked section that could be correlated with the two in-line processed seismic profiles. The processing sequence included a carefully designed frequency filter, optimized stack methods and pseudo 3D processing by applying cross-dip corrections. Due to the long offsets between the profiles, the resulting stacked section does not provide any information in the upper 1.2 s. Below 1.2 s, the upper crust shows a north-dipping event that correlates well with observations on the in-line data. Sub-horizontal reflectivity is also observed in the upper 4 s of the stack, similar to what is observed on the profile west of the cross-profile. Cross-dip analysis shows that the north-dipping event observed in all three profiles has a westerly dip component. Our study shows that additional information on the sub-surface may be obtained through cross-profile acquisition and processing.

  • 36.
    Sadeghisorkhani, Hamzeh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Mapping the source distribution of microseisms using noise covariogram envelopes2016In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 205, no 3, p. 1473-1491Article in journal (Refereed)
    Abstract [en]

    We introduce a method for mapping the noise-source distribution of microseisms which uses information from the full length of covariograms (cross-correlations). We derive a forward calculation based on the plane-wave assumption in 2-D, to formulate an iterative, linearized inversion of covariogram envelopes in the time domain. The forward calculation involves bandpass filtering of the covariograms. The inversion exploits the well-known feature of noise cross-correlation, that is, an anomaly in the noise field that is oblique to the interstation direction appears as cross-correlation amplitude at a smaller time lag than the in-line, surface wave arrival. Therefore, the inversion extracts more information from the covariograms than that contained at the expected surface wave arrival, and this allows us to work with few stations to find the propagation directions of incoming energy. The inversion is naturally applied to data that retain physical units that are not amplitude normalized in any way. By dividing a network into groups of stations, we can constrain the source location by triangulation. We demonstrate results of the method with synthetic data and one year (2012) of data from the Swedish National Seismic Network and also look at the seasonal variation of source distribution around Scandinavia. After preprocessing and cross-correlation, the stations are divided into five groups of 9-12 stations. We invert the envelopes of each group in eight period ranges between 2 and 25 s. Results show that the noise sources at short periods (less than 12 s) lie predominantly in the North Atlantic Ocean and the Barents Sea, and at longer periods the energy appears to have a broader distribution. The strongly anisotropic source distribution in this area is estimated to cause significant biases of velocity measurements compared to the level of heterogeneity in the region. The amplitude of the primary microseisms varies little over the year, but secondary microseisms are much weaker in summer than in winter. Furthermore, the peak period of the secondary microseisms shifts from 5-6 s in winter to 4-5 s during the summer.

  • 37.
    Sadeghisorkhani, Hamzeh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Velocity-measurement bias of the ambient noise method due to source directivity: A case study for the Swedish National Seismic Network2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 3, p. 1648-1659Article in journal (Refereed)
    Abstract [en]

    The bias of velocity measurements from ambient-noise covariograms due to an anisotropic distribution of noise sources is studied assuming that the noise field consists of planar surface waves from large distance. First, general characteristics of the bias are described in terms of their dependence on wavelength, source-anomaly amplitude and width. Second, the expected bias of measurements in Sweden based on a noise-source model for the adjacent regions is analysed. The bias is conceptually explained and described in terms of two regimes, namely a high-frequency and a finite-frequency regime and their parameter domains quantified. Basic scaling laws are established for the bias. It is generally found to be small compared to lateral heterogeneity, except in the finite-frequency regime when inter-station distance is small compared to a wavelength and in regions of low levels of heterogeneity. The potential bias, i.e., its peak-to-peak variation, is generally higher for group-velocity than phase-velocity measurements. The strongly varying noise-source distribution as seen from Sweden results in predictions of relatively strong bias in the area at relevant frequencies and inter-station distances. Levels of heterogeneity in the Baltic shield are relatively low, rendering the potential bias significant. This highlights the need for detailed studies of source anisotropy before application of ambient-noise tomography, particularly in regions with weak velocity heterogeneity. Predicted bias only partially explains deviations of phase-velocity measurements from a regional average for individual station pairs. Restricting measurements to station pairs with inter-station distance exceeding five wavelengths limits the potential velocity bias in the area to within 1%. This rather dramatic restriction can be relaxed by directional analysis of the noise-source field and application of azimuthal restrictions to the selected station pairs for measurement.

  • 38.
    Sadeghisorkhani, Hamzeh
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    GSpecDisp: a Matlab GUI package for phase-velocity dispersion measurements from ambient-noise correlations2018In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, p. 41-53Article in journal (Refereed)
    Abstract [en]

    We present a graphical user interface (GUI) package to facilitate phase-velocity dispersion measurements of surface waves in noise-correlation traces. The package, called GSpecDisp, provides an interactive environment for the measurements and presentation of the results. The selection of a dispersion curve can be done automatically or manually within the package. The data are time-domain cross-correlations in SAC format, but GSpecDisp measures phase velocity in the spectral domain. Two types of phase-velocity dispersion measurements can be carried out with GSpecDisp; (1) average velocity of a region, and (2)"single-pair phase velocity. Both measurements are done by matching the real part of the cross-correlation spectrum with the appropriate Bessel function. Advantages of these two types of measurements are that no prior knowledge about surface-wave dispersion in the region is needed, and that phase velocity can be measured up to that period for which the inter-station distance corresponds to one wavelength. GSpecDisp can measure the phase velocity of Rayleigh and Love waves from all possible components of the noise correlation tensor. First, we briefly present the theory behind the methods that are used, and then describe different modules of the package. Finally, we validate the developed algorithms by applying them to synthetic and real data, and by comparison with other methods. The source code of GSpecDisp can be downloaded from: https://github.com/Hamzeh-Sadeghi/GSpecDisp.

  • 39.
    Sgattoni, Giulia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. University of Bologna; University of Iceland.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Einarsson, Pall
    University of Iceland.
    Lucchi, Federico
    University of Bologna.
    Li, Ka Lok
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Sadeghisorkhani, Hamzeh
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    The 2011 unrest at Katla volcano: Characterization and interpretation of the tremor sources2017In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 338, p. 63-78Article in journal (Refereed)
    Abstract [en]

    A 23-hour tremor burst was recorded on July 8-9th 2011 at the Katla subglacial volcano, one of the most active and hazardous volcanoes in Iceland. This was associated with deepening of cauldrons on the ice cap and a glacial flood that caused damage to infrastructure. Increased earthquake activity within the caldera started a few days before and lasted for months afterwards and new seismic activity started on the southern flank. No visible eruption broke the ice and the question arose as to whether this episode relates to a minor subglacial eruption with the tremor being generated by volcanic processes, or by the flood. The tremor signal consisted of bursts with varying amplitude and duration. We have identified and described three different tremor phases, based on amplitude and frequency features. A tremor phase associated with the flood was recorded only at stations closest to the river that flooded, correlating in time with rising water level observed at gauging stations. Using back-projection of double cross-correlations, two other phases have been located near the active ice cauldrons and are interpreted to be caused by volcanic or hydrothermal processes. The greatly increased seismicity and evidence of rapid melting of the glacier may be explained by a minor sub-glacial eruption. A less plausible interpretation is that the tremor was generated by hydrothermal boiling and/or explosions with no magma involved. This may have been induced by pressure drop triggered by the release of water when the glacial flood started. All interpretations require an increase of heat released by the volcano.

  • 40.
    Sgattoni, Giulia
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Univ Bologna, Dept Biol Geol & Environm Sci, Bologna, Italy; Univ Iceland, Inst Sci, Inst Earth Sci, Reykjavik, Iceland.
    Jeddi, Zeinab
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Einarsson, Pall
    Univ Iceland, Inst Sci, Inst Earth Sci, Reykjavik, Iceland.
    Tryggavson, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lucchi, Federico
    Univ Bologna, Dept Biol Geol & Environm Sci, Bologna, Italy.
    Long-period seismic events with strikingly regular temporal patterns on Katla volcano’s south flank (Iceland)2016In: Journal of Volcanology and Geothermal Research, ISSN 0377-0273, E-ISSN 1872-6097, Vol. 324, p. 28-40Article in journal (Refereed)
    Abstract [en]

    Katla is a threatening volcano in Iceland, partly covered by the Myrdalsjokull ice cap. The volcano has a large caldera with several active geothermal areas. A peculiar cluster of long-period seismic events started on Katla's south flank in July 2011, during an unrest episode in the caldera that culminated in a glacier outburst. The seismic events were tightly clustered at shallow depth in the Gvendarfell area, 4 km south of the caldera, under a small glacier stream at the southern margin of Myrdalsjokull. No seismic events were known to have occurred in this area before. The most striking feature of this seismic cluster is its temporal pattern, characterized by regular intervals between repeating seismic events, modulated by a seasonal variation. Remarkable is also the stability of both the time and waveform features over a long time period, around 3.5 years. We have not found any comparable examples in the literature. Both volcanic and glacial processes can produce similar waveforms and therefore have to be considered as potential seismic sources. Discerning between these two causes is critical for monitoring glacier-clad volcanoes and has been controversial at Katla. For this new seismic cluster on the south flank, we regard volcano-related processes as more likely than glacial ones for the following reasons: 1) the seismic activity started during an unrest episode involving sudden melting of the glacier and a jokulhlaup; 2) the glacier stream is small and stagnant; 3) the seismicity remains regular and stable for years; 4) there is no apparent correlation with short-term weather changes, such as rainstorms. We suggest that a small, shallow hydrothermal system was activated on Katla's south flank in 2011, either by a minor magmatic injection or by changes of permeability in a local crack system.

  • 41. Stratford, Wanda
    et al.
    Thybo, Hans
    Faleide, Inge
    Olesen, Odleiv
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    New Moho Map for onshore southern Norway2009In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 178, no 3, p. 1755-1765Article in journal (Refereed)
    Abstract [en]

    A recent seismic refraction study across southern Norway has revealed that the up to 2469 m high Southern Scandes Mountains are not isostatically compensated by a thick crust. Rather, the Moho depths are close to average for continental crust with elevations of similar to 1 km. Evidence from new seismic data indicate that beneath the highest topography Moho depths are around 38-40 km. These measurements are similar to 2 km deeper than early estimates interpolated from coarsely spaced refraction profiles, but up to 3 km shallower than Receiver Function estimates for the area. Moho depth variation beneath the mountains roughly correlates with changes in surface topography indicating that topography is, at least to the first order, controlled by crustal thickness. However, the highest mountains do not overlie the thickest crust and additional support for topography, for example from flexural strength in the lithosphere, low densities in the upper-mantle or mantle dynamics, is likely. The relationship between topography and Moho depth breaks down for the Oslo Graben and the Fennoscandian Shield to the east and north. High density lower crustal rocks below Oslo Graben and increasing crust and lithospheric thicknesses below the Fennoscandian Shield may produce a negative correlation between topography and Moho depth.

  • 42.
    Tryggvason, Ari
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Malehmir, Alireza
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Rodrigues-Tablante, Johiris
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences. Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geophysics.
    Weihed, Par
    Luleå University, Ore Geology.
    Reflection Seismic Investigation in the Western Part of the Paleoproterozoic VHMS-Bearing Skellefte District, Northern Sweden: Reflection Seismic Investigation in the Western Part of the Paleoproterozoic VHMS-Bearing Skellefte District, Northern Sweden2006In: Economic Geology, Vol. 101, p. 1039-1054Article in journal (Refereed)
    Abstract [en]

    The Skellefte district forms part of the Svecofennian ca. 1.90 to 1.80 Ga, supracrustal sequence and associated

    intrusive rocks in the northern part of Sweden. The western part of the Skellefte district, which is the most

    important metallogenic province in northern Sweden today, hosts major volcanic-hosted massive sulfide

    (VHMS) deposits (e.g., the 23 million metric tons (Mt) Kristineberg Cu-Zn-Pb-Ag-Au deposit). In order to obtain

    a better understanding of the VHMS ore potential at depth, new seismic reflection data were acquired

    along two parallel and 25-km-long profiles in the Kristineberg area in 2003. The data were collected with the

    purpose of obtaining high-resolution images of the top 10 km of the crust and are presented here for the first

    time. Although the structural setting is very complex, the stacked sections reveal numerous reflections that can

    be correlated with surface geology. Visible on both profiles is a pronounced north-dipping band of reflections

    marking a boundary between relatively transparent crust above and significantly more reflective crust beneath

    it. We interpret this reflective crust to represent a structural basement to the ore-bearing Skellefte Group, possibly

    constituting Bothnian basin metasedimentary rocks bordering the Skellefte district to the south. This new

    interpretation is important for the understanding of the tectonic evolution of the Skellefte district and for

    defining exploration strategies in the area. The seismic results suggest that the Kristineberg and Rävliden deposits

    occur on the northern limb of a kilometer-scale local second-order syncline within the hinge zone of a

    major antiform. Results from a profile located approximately 8 km to the west of the Kristineberg mine indicate

    that the Revsund granitoid has a thickness of about 3 to 3.5 km. Ultramafic rocks are also imaged clearly.

    Diffraction patterns and bright-spot reflectivity is interpreted as originating from either mafic to ultramafic intrusions

    or a mineralization zone at 3- to 5-km depth. These results help to identify new prospective areas and

    mineral potential, both downplunge from known ores and on the same stratigraphic horizon on the southern

    limb of the ore-bearing syncline. The seismic reflection profiling has been effective in imaging the major structures

    around the Kristineberg orebody, demonstrating that this technique can be used for delineating complex

    structures significant for mineral exploration.

  • 43.
    Tryggvason, Ari
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Melchiorre, Caterina
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Johansson, Kerstin
    A fast and efficient algorithm to map prerequisites of landslides in sensitive clays based on detailed soil and topographical information2015In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 75, p. 88-95Article in journal (Refereed)
    Abstract [en]

    We present an algorithm developed for GIS-applications in order to produce maps of landside susceptibility in postglacial and glacial sediments in Sweden. The algorithm operates on detailed topographic and Quaternary deposit data. We compare our algorithm to two similar computational schemes based on a global visibility operator and a shadow-casting algorithm. We find that our algorithm produces more reliable results in the vicinity of stable material than the global visibility algorithm. We also conclude that our algorithm is more computationally efficient than the other two methods, which is important when we may want to assess the effects of uncertainty in the data by evaluating many different models. Our method also provides the possibility to take other data into account We show how different soil types with different geotechnical properties may be modelled. Our algorithm may also take depth information, i.e. the thicknesses of the deposits into account. We thus propose that our method may be used to provide more refined maps than the overview maps in areas where more detailed geotechnical/geological data have been acquired. The efficiency of our algorithm suggests that it may replace any global visibility operators used in other applications or processing schemes of gridded map data. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

  • 44.
    Tryggvason, Ari
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Schmelzbach, C
    University of Potsdam.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Traveltime tomographic inversion with simultaneous static corrections: Well worth the effort2009In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 74, no 6, p. WCB25-WCB33Article in journal (Refereed)
    Abstract [en]

    We have developed a first-arrival traveltime inversion scheme that jointly solves for seismic velocities and source and receiver static-time terms. The static-time terms are included to compensate for varying time delays introduced by the near-surface low-velocity layer that is too thin to be resolved by tomography. Results on a real data set consisting of picked first-arrival times from a seismic-reflection 2D/3D experiment in a crystalline environment show that the tomography static-time terms are very similar in values and distribution to refraction-static corrections computed using standard refraction-statics software. When applied to 3D seismic-reflection data, tomography static-time terms produce similar or more coherent seismic-reflection images compared to the images using corrections from standard refraction-static software. Furthermore, the method provides a much more detailed model of the near-surface bedrock velocity than standard software when the static-time terms are included in the inversion. Low-velocity zones in this model correlate with other geologic and geophysical data, suggesting that our method results in a reliable model. In addition to generally being required in seismic-reflection imaging, static corrections are also necessary in traveltime tomography to obtain high-fidelity velocity images of the subsurface.

  • 45.
    Wagner, Frederic
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Double-difference relocation and local-earthquake tomography of the Vp, Vs, and Vp/Vs structure in Hengill, SW IcelandManuscript (preprint) (Other academic)
  • 46.
    Wagner, Frederic
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland G.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Automatic seismic event detection using migration and stacking: a performance and parameter study in Hengill, southwest Iceland2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 3, p. 1866-1877Article in journal (Refereed)
    Abstract [en]

    We investigate the performance of a seismic event detection algorithm using migration and stacking of seismic traces. The focus lies on determining optimal data dependent detection parameters for a data set from a temporary network in the volcanically active Hengill area, southwest Iceland. We test variations of the short-term average to long-term average and Kurtosis functions, calculated from filtered seismic traces, as input data. With optimal detection parameters, our algorithm identified 94 per cent (219 events) of the events detected by the South Iceland Lowlands (SIL) system, that is, the automatic system routinely used on Iceland, as well as a further 209 events, previously missed. The assessed number of incorrect (false) detections was 25 per cent for our algorithm, which was considerably better than that from SIL (40 per cent). Empirical tests show that well-functioning processing parameters can be effectively selected based on analysis of small, representative subsections of data. Our migration approach is more computationally expensive than some alternatives, but not prohibitively so, and it appears well suited to analysis of large swarms of low magnitude events with interevent times on the order of seconds. It is, therefore, an attractive, practical tool for monitoring of natural or anthropogenic seismicity related to, for example, volcanoes, drilling or fluid injection.

  • 47.
    Wagner, Frederic
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Processing automatic seismic event detections: an iterative sorting algorithm improving earthquake hypocentres using interevent cross-correlation2019In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 219, no 2, p. 1268-1280Article in journal (Refereed)
    Abstract [en]

    We present an iterative classification scheme using interevent cross-correlation to update an existing earthquake catalogue with similar events from a list of automatic seismic event detections. The algorithm automatically produces catalogue quality events, with improved hypocentres and reliable P- and S-arrival time information. Detected events are classified into four event categories with the purpose of using the top category, with the highest assessed event quality and highest true-to-false ratio, directly for local earthquake tomography without additional manual analysis. The remaining categories have varying proportions of lower quality events, quality being defined primarily by the number of observed phase onsets, and can be viewed as different priority groups for manual inspection to reduce the time spent by a seismic analyst. A list of 3348 event detections from the geothermally active volcanic region around Hengill, southwest Iceland, produced by our migration and stack detector (Wagner et al. 2017), was processed using a reference catalogue of 1108 manually picked events from the same area. P- and S-phase onset times were automatically determined for the detected events using correlation time lags with respect to manually picked phase arrivals from different multiple reference events at the same station. A significant improvement of the initial hypocentre estimates was achieved after relocating the detected events using the computed phase onset times. The differential time data set resulting from the correlation was successfully used for a double-difference relocation of the final updated catalogue. The routine can potentially be implemented in real-time seismic monitoring environments in combination with a variety of seismic event/phase detectors.

  • 48.
    Yao, Z.S.
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Roberts, R.G
    Tryggvason, A
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Comment on2001In: Geophysical Journal International, ISSN 0956-540X, Vol. 145, p. 307-314Article in journal (Refereed)
    Abstract [en]

    When solving tomographic inversion problems, the resolution matrix is for the equation system can provide very useful information on how well the model parameters can be resolved. Recently, Nolet et NI. (1999)-referred to as NMV hereafter-proposed a onest

  • 49.
    Yordkayhun, Sawasdee
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Norden, Ben
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bergman, Björn
    3D seismic traveltime tomography imaging of the shallow subsurface at the CO2SINK project site, Ketzin, Germany2009In: Geophysics, ISSN 0016-8033, E-ISSN 1942-2156, Vol. 74, no 1, p. G1-G15Article in journal (Refereed)
    Abstract [en]

    A 3D reflection seismic survey was performed in 2005 at the Ketzin carbon dioxide (CO2) pilot geological-storage site (the CO2SINK project) near Berlin, Germany, to image the geological structure of the site to depths of about 1 km. Because of the acquisition geometry, frequency limitations of the source, and artefacts of the data processing, detailed structures shallower than about 150 m were unclear. To obtain structural images of the shallow subsurface, we applied 3D traveltime tomography to data near the top of the Ketzin anticline, where faulting is present. Understanding the shallow subsurface structure is important for long-term monitoring aspects of the project after CO2 has been injected into a saline aquifer at about 650-m depth. We used a 3D traveltime tomography algorithm based on a combination of solving for 3D velocity structure and static corrections in the inversion process to account for artefacts in the velocity structure because of smearing effects from the unconsolidated cover. The resulting velocity model shows low velocities of 800 1200 m/s in the uppermost shallow subsurface of the study area. The velocity reaches about 1800 m/s at a depth of 60-80 m. This coincides approximately with the boundary between Quaternary units, which contain the near-surface freshwater reservoir and the Tertiary clay aquitard. Correlation of tomographic images with a similarity attribute slice at 150 ms (about 150-m depth) indicates that at least one east-west striking fault zone observed in the reflection data might extend into the Tertiary unit. The more detailed images of the shallow subsurface from this study provided valuable information on this potentially risky area.

  • 50.
    Zhang, Fengjiao
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Juhlin, Christopher
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Cosma, Calin
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Pratt, R. Gerhard
    Cross-well seismic waveform tomography for monitoring CO2 injection: a case study from the Ketzin Site, Germany2012In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 189, no 1, p. 629-646Article in journal (Refereed)
    Abstract [en]

    Geological storage of CO2 is one means of mitigating the effects of continued burning of fossil fuels for power generation. An important component in the storage concept is the monitoring of the CO2 distribution at depth. Seismic methods can play a significant role in this monitoring, in particular cross-well methods are of interest due to their high resolution. For these purposes, a series of cross-well seismic surveys were acquired within the framework of the CO2SINK project at Ketzin, Germany, at various stages of an injection test. We study here the potential of applying cross-well seismic waveform tomography to monitor the CO2 injection process. First, we test the method on synthetic data having a similar geometry to that of the real data. After successful application on the synthetic data, we test the method on the real data acquired at the Ketzin Site. Traveltime tomography images of the real data show no observable differences between the surveys. However, seismic waveform tomography difference images show significant differences. A number of these differences are artefacts that can probably be attributed to inconsistent receiver coupling between the different surveys. However, near the injection horizon, below the caprock, a velocity decrease is present that is consistent with that expected from the injection process.

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