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  • 1.
    Adamaki, Angeliki K.
    et al.
    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.
    Precursory Activity Before Larger Events in Greece Revealed by Aggregated Seismicity Data2017In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 174, no 3, p. 1331-1343Article in journal (Refereed)
    Abstract [en]

    We investigate the seismicity rate behaviour in and around Greece during 2009, seeking significant changes in rate preceding larger events. For individual larger events it is difficult to clearly distinguish precursory rate changes from other, possibly unrelated, variations in seismicity. However, when we aggregate seismicity data occurring within a radius of 10 km and in a 50-day window prior to earthquakes with, e. g. magnitude C3.5, the resulting aggregated time series show a clearly increasing trend starting 2-3 weeks prior to the "mainshock'' time. We apply statistical tests to investigate if the observed behaviour may be simply consistent with random (poissonian) variations, or, as some earlier studies suggest, with clustering in the sense that high activity rates at some time may imply increased rates later, and thus (randomly) greater probability of larger coming events than for periods of lower seismicity. In this case, rate increases have little useful predictive power. Using data from the entire catalogue, the aggregated rate changes before larger events are clearly and strongly statistically significant and cannot be explained by such clustering. To test this we choose events at random from the catalogue as potential "mainshocks''. The events preceding the randomly chosen earthquakes show less pronounced rate increases compared to the observed rate changes prior to larger events. Similar behaviour is observed in data sub-sets. However, statistical confidence decreases for geographical subsets containing few "mainshocks'' as it does when data are weighted such that "mainshocks'' with many preceding events are strongly downweighted relative to those with fewer. The analyses suggest that genuine changes in aggregated rate do occur prior to larger events and that this behaviour is not due to a small number of mainshocks with many preceding events dominating the analysis. It does not automatically follow that it will be possible to routinely observe precursory changes prior to individual larger events, but there is a possibility that this may be feasible, e. g. with better data from more sensitive networks.

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  • 2.
    Adamaki, Angeliki
    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.
    EVIDENCE OF PRECURSORY PATTERNS IN AGGREGATED TIME SERIES2016In: Bulletin of the Geological Society of Greece, vol. L, 2016, Proceedings of the 14th Intern. Congress, Thessaloniki, May 2016, 2016, Vol. 50Conference paper (Refereed)
    Abstract [en]

    We investigate temporal changes in seismic activity observed in the West Corinth Gulfand North-West Peloponnese during 2008 to 2010. Two major earthquake sequencestook place in the area at that time (in 2008 and 2010). Our aim is to analyse Greekseismicity to attempt to confirm the existence or non-existence of seismic precursorsprior to the strongest earthquakes. Perhaps because the area is geologically andtectonically complex, we found that it was not possible to fit the data well using aconsistent Epidemic Type Aftershock Sequence (ETAS) model. Nor could weunambiguously identify foreshocks to individual mainshocks. Therefore we soughtpatterns in aggregated foreshock catalogues. We set a magnitude threshold (M3.5)above which all the earthquakes detected in the study area are considered as“mainshocks”, and we combined all data preceding these into a single foreshockcatalogue. This reveals an increase in seismicity rate not robustly observable forindividual cases. The observed effect is significantly greater than that consistent withstochastic models, including ETAS, thus indicating genuine foreshock activity withpotential useful precursory power, if sufficient data is available, i.e. if the magnitudeof completeness is sufficiently low.

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  • 3.
    Amini, Samar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Raeesi, Mohammad
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Fault slip and rupture properties of the December 2017 Hojedk triplet in Eastern IranManuscript (preprint) (Other academic)
  • 4.
    Amini, Samar
    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.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Directivity analysis of the 2017 December Kerman earthquakes in Eastern Iran2020In: Journal of Seismology, ISSN 1383-4649, E-ISSN 1573-157X, Vol. 24, p. 531-547Article in journal (Refereed)
    Abstract [en]

    Using an empirical Green’s function (EGF) approach and data from local to regional distances we analyzed rupture propagation directivity in the three mainshocks (ML 6.0–6.1) and in six of the largest aftershocks (ML 5.0 – 5.5) of the 2017 Kerman, Iran, seismic sequence. The EGF procedure was based on data from smaller events (ML 4.0 – 4.8). Deconvolution was applied separately to P and S phases. Using the P-wave data, we calculated relative source-time functions and examined azimuthal variations in rupture duration. In the S-wave analysis, we investigated along strike rupture directivity of the mainshocks and the largest aftershocks by evaluating azimuthal variation of the amplitude spectra. Two of the mainshocks and four of the aftershocks clearly showed rupture propagation from the south-east toward the north-west. The third mainshock and one of the aftershocks suggested almost bilateral rupture propagation, and one aftershock showed rupture directivity to the southeast. It seems that the rupture propagation direction in the area is generally to the north-west and the events which have different propagation directions are located within the NW and SE ends of the faulting area. We suggest that the general rupture propagation direction in the area is steered by regional tectonic stress field regarding the faulting orientations which have been affected by stress redistribution around a restraining bend.

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  • 5.
    Amini, Samar
    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.
    Raeesi, Mohammad
    SeisAnalysis AS, Bergen, Norway.
    Shomali, Zaher Hossein
    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.
    Zarifi, Zoya
    Univ Western Ontario, Dept Earth Sci, London, ON, Canada.
    Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet2018In: Journal of Seismology, ISSN 1383-4649, E-ISSN 1573-157X, Vol. 22, no 4, p. 815-831Article in journal (Refereed)
    Abstract [en]

    An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.

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  • 6.
    Amini, Samar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Shomali, Z. Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Koyi, Hemin
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Solid Earth Geology.
    Roberts, Roland G.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Tomographic upper-mantle velocity structure beneath the Iranian Plateau2012In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 554-557, p. 42-49Article in journal (Refereed)
    Abstract [en]

    The Iranian plateau is one of the most structurally complex and tectonically inhomogeneous regions in the world. In this study, we analyze Pn arrival-times from regional seismicity in order to resolve lateral velocity variations within the uppermost-mantle under the Iranian Plateau. More than 48,000 Pn first arrival times selected from the EHB catalog were used with epicentral distances of 200 to 1600 km. We used regularized isotropic and anisotropic damped least-squares inversion to image lateral velocity variations in the upper mantle. Our velocity model, with high lateral resolution, shows positive anomalies in the Zagros mountain belt with a distinct transition approximately along the Main Zagros Thrust to the lower mantle velocity zone of Central Iran. Anomalously low velocities are observed predominantly beneath NW Iran and eastern Turkey, suggesting a zone of relatively weak mantle. Low velocity region under the Damavand volcano reveals the hot upper mantle beneath the central Alborz mountains.

  • 7.
    DeFelipe, Irene
    et al.
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Alcalde, Juan
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Ivandic, Monika
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Marti, David
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain.;Lith SCCL, Ave Farners 16, Sta Coloma De Farners 17430, Spain..
    Ruiz, Mario
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Marzan, Ignacio
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Diaz, Jordi
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Ayarza, Puy
    Univ Salamanca, Dept Geol, Plaza Merced S-N, Salamanca 37008, Spain..
    Palomeras, Imma
    Univ Salamanca, Dept Geol, Plaza Merced S-N, Salamanca 37008, Spain..
    Fernandez-Turiel, Jose-Luis
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Molina, Cecilia
    CSIC, C Serrano 117, Madrid 28006, Spain..
    Bernal, Isabel
    CSIC, C Serrano 117, Madrid 28006, Spain..
    Brown, Larry
    Cornell Univ, Dept Earth & Atmospher Sci, 112 Hollister Dr, Ithaca, NY 14853 USA..
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Carbonell, Ramon
    CSIC, GEO3BCN, Geosci Barcelona, C Lluis Sole i Sabaris S-N, Barcelona 08028, Spain..
    Reassessing the lithosphere: SeisDARE, an open-access seismic data repository2021In: Earth System Science Data, ISSN 1866-3508, E-ISSN 1866-3516, Vol. 13, no 3, p. 1053-1071Article in journal (Refereed)
    Abstract [en]

    Seismic reflection data (normal incidence and wide angle) are unique assets for solid Earth sciences as they provide critical information about the physical properties and structure of the lithosphere as well as about the shallow subsurface for exploration purposes. The resolution of these seismic data is highly appreciated; however they are logistically complex and expensive to acquire, and their geographical coverage is limited. Therefore, it is essential to make the most of the data that have already been acquired. The collation and dissemination of seismic open-access data are then key to promote accurate and innovative research and to enhance new interpretations of legacy data. This work presents the Seismic DAta REpository (SeisDARE), which is, to our knowledge, one of the first comprehensive open-access online databases that stores seismic data registered with a permanent identifier (DOI). The datasets included here are openly accessible online and guarantee the FAIR (findable, accessible, interoperable, reusable) principles of data management, granting the inclusion of each dataset in a statistics referencing database so its impact can be measured. SeisDARE includes seismic data acquired in the last 4 decades in the Iberian Peninsula and Morocco. These areas have attracted the attention of international researchers in the fields of geology and geophysics due to the exceptional outcrops of the Variscan and Alpine orogens and wide foreland basins, the crustal structure of the offshore margins that resulted from a complex plate kinematic evolution, and the vast quantities of natural resources contained within. This database has been built thanks to a network of national and international institutions, promoting a multidisciplinary research and is open for international data exchange and collaborations. As part of this international collaboration, and as a model for inclusion of other global seismic datasets, SeisDARE also hosts seismic data acquired in Hardeman County, Texas (USA), within the COCORP project (Consortium for Continental Reflection Profiling). SeisDARE aims to make easily accessible old and recently acquired seismic data and to establish a framework for future seismic data management plans. SeisDARE is freely available at https://digital.csic.es/handle/10261/101879 (a detailed list of the datasets can be found in Table 1), bringing endless research and teaching opportunities to the scientific, industrial, and educational communities.

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  • 8.
    Eken, Tuna
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Plomerova, Jaroslova
    GFU, Prague.
    Vecsey, Ludek
    GFU, Prague.
    Babuska, Vladislav
    GFU, Prague.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Shomali, Hossein
    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.
    Effects of seismic anisotropy on P-velocity tomography of the Baltic Shield2012In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 188, no 2, p. 600-612Article in journal (Refereed)
    Abstract [en]

    We investigate possible effects of neglecting seismic anisotropy on standard isotropic P-velocity tomographic images of the upper mantle beneath the Baltic shield. Isotropic inversions of teleseismic P- and S-wave traveltimes exhibit alternating high- and low-velocity heterogeneities down to depths of over 400 km. Differences in tomographic inversions of SV- and SH-wave traveltimes are distinct down to depths of about 200 km and are associated with anisotropy of the lithospheric mantle. Anisotropic structures of the upper mantle affect both the P and S traveltimes, shear-wave splitting as well as the P polarization directions. Joint inversion for isotropic and anisotropic velocity perturbations is not feasible due to the limited 3-D ray coverage of available data. Therefore, we correct the input traveltimes for anisotropic contributions derived from independent analyses and then perform standard isotropic inversions. These corrections are derived either directly from directional deviations of P-wave propagation or are calculated in anisotropic models retrieved by joint inversions of body-wave anisotropic parameters (P-residual spheres and shear-wave splitting). These anisotropic models are also used to fit backazimuth variations of P-wave polarization directions. General features of tomographic images calculated from the original and the anisotropy-corrected data are similar. Amplitudes of the velocity perturbations decrease below similar to 200 km depth, that is in the sub-lithospheric mantle. In general, large-scale anisotropy related to the fabrics of the continental mantle lithosphere can contaminate tomographic images in some parts of models and should not be ignored.

  • 9.
    Eken, Tuna
    et al.
    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.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Hieronymus, Christoph F.
    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.
    S and P velocity heterogeneities within the upper mantle below the Baltic Shield2008In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 462, no 1-4, p. 109-124Article in journal (Refereed)
    Abstract [en]

    Upper mantle structure beneath the Baltic (Fennoscandian) Shield is investigated using non-linear tomographic inversion of relative arrival-time residuals. 52 selected teleseismic earthquakes recorded by 45 broadband stations of the Swedish National Seismological Network (SNSN) provide 1532 good quality S-wave relative arrival times. SV and SH arrival-time residuals were initially analyzed independently, providing two separate models. These reveal several consistent major features, many of which are also consistent with P-wave results. Lateral velocity variations of ± 3–4% are observed to depths of at least 470 km. The correlation between the SH and SV models is investigated and shows a pattern of minor but significant differences down to around 150–200 km depth, below which the models are essentially similar. Direct cell by cell comparison of the model velocities reveals a similar pattern, with velocity differences between the models of up to 4%. Numerical tests show that differences in the two S-wave models can only be partially attributed to noise and limited resolution, and some features are attributed to the effect of large scale anisotropy. One of the significant and sharp discrepancies between the S models coincides with a presumed boundary between Archean and Proterozic domains, suggesting different anisotropic characteristics in the two regions.

  • 10.
    Fälth, Billy
    et al.
    Clay Technology AB, Lund, Sweden.
    Hökmark, Harald
    ClayTech AB, Lund, Sverige.
    Lund, Björn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Mai, Martin
    King Abdullah University of Science and Technology, Saudi Arabia.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Munier, Raymond
    Svensk kärnbränslehantering AB.
    Simulating earthquake rupture and off-fault fracture response: Application to the safety assessment of the Swedish nuclear waste repository2015In: Bulletin of The Seismological Society of America (BSSA), ISSN 0037-1106, E-ISSN 1943-3573, Vol. 105, no 1, p. 134-151Article in journal (Refereed)
    Abstract [en]

    To assess the long‐term safety of a deep repository of spent nuclear fuel, upper bound estimates of seismically induced secondary fracture shear displacements are needed. For this purpose, we analyze a model including an earthquake fault, which is surrounded by a number of smaller discontinuities representing fractures on which secondary displacements may be induced. Initial stresses are applied and a rupture is initiated at a predefined hypocenter and propagated at a specified rupture speed. During rupture we monitor shear displacements taking place on the nearby fracture planes in response to static as well as dynamic effects. As a numerical tool, we use the 3Dimensional Distinct Element Code (3DEC) because it has the capability to handle numerous discontinuities with different orientations and at different locations simultaneously. In tests performed to benchmark the capability of our method to generate and propagate seismic waves, 3DEC generates results in good agreement with results from both Stokes solution and the Compsyn code package. In a preliminary application of our method to the nuclear waste repository site at Forsmark, southern Sweden, we assume end‐glacial stress conditions and rupture on a shallow, gently dipping, highly prestressed fault with low residual strength. The rupture generates nearly complete stress drop and an Mw 5.6 event on the 12  km2 rupture area. Of the 1584 secondary fractures (150 m radius), with a wide range of orientations and locations relative to the fault, a majority move less than 5 mm. The maximum shear displacement is some tens of millimeters at 200 m fault‐fracture distance.

  • 11. Gregersen, S.
    et al.
    Voss, P.
    Shomali, Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Grad, M.
    Roberts, Roland G.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Physical differences in the deep lithosphere of northern and central Europe2006In: Geological Society of London, Memoirs, Vol. 32, p. 313-322Article in journal (Refereed)
    Abstract [en]

    A number of large-scale integrated studies, including the TOR and POLONAISE'97 projects, with an emphasis on seismic methods, have been used to elucidate the southwestern boundary (suture zone) between the East European Craton and the Phanerozoic terranes of Western Europe. Results indicate that a thick slab of mantle lithosphere below the craton thins southwestwards beneath the Trans-European Suture Zone and is not seen south of the Variscan front. The thinning is not gradual, but is interrupted by at least two abrupt deep boundaries, the most significant of which corresponds to the surface position of the Tornquist Zone, a major fault. The present geometry of the lithosphere is the result of modification of the margin of the Neoproterozoic continent Baltica by Phanerozoic processes, including the development of the Tornquist Zone and the stretching of the lithosphere in a broad central block SW of this zone. Seismic results and their interpretations from the TOR tomographic project are presented and compared with results from the POLONAISE'97 controlled source project to the SE. Both investigations have shown high-angle, non-symmetrical features extending deep into the mantle.

  • 12.
    Gregersen, S.
    et al.
    GEUS,Denmark.
    Voss, P.
    GEUS,Denmark.
    Shomali, Z. Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Grad, M.
    University of Warsaw,Poland.
    Roberts, Roland G.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Physical differences in the deep lithosphere of Northern and Central Europe2006In: European Lithosphere Dynamics / [ed] Gee, D.G. and Stephenson, R.a., Geological Society of London , 2006, 32, p. 313-322Chapter in book (Other academic)
    Abstract [en]

    A number of large-scale integrated studies, including the TOR and POLONAISE’97 projects, with an emphasis on seismicmethods, have been used to elucidate the southwestern boundary (suture zone) between the East European Craton and the Phanerozoicterranes of Western Europe. Results indicate that a thick slab of mantle lithosphere below the craton thins southwestwards beneath theTrans-European Suture Zone and is not seen south of the Variscan front. The thinning is not gradual, but is interrupted by at least twoabrupt deep boundaries, the most significant of which corresponds to the surface position of the Tornquist Zone, a major fault. Thepresent geometry of the lithosphere is the result of modification of the margin of the Neoproterozoic continent Baltica by Phanerozoicprocesses, including the development of the Tornquist Zone and the stretching of the lithosphere in a broad central block SW of thiszone. Seismic results and their interpretations from the TOR tomographic project are presented and compared with results from thePOLONAISE’97 controlled source project to the SE. Both investigations have shown high-angle, non-symmetrical features extendingdeep into the mantle.

  • 13.
    Hagos, Lijam
    et al.
    Uppsala University.
    Roberts, Roland
    Uppsala University.
    Slunga, Ragnar
    Uppsala University.
    Bödvarsson, Reynir
    Uppsala University.
    Lund, Björn
    Uppsala University.
    A preliminary study regarding measures of earthquake risk in Sweden2004Report (Other scientific)
  • 14.
    Joshi, Niranjan
    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.
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Probabilistic Seismic Hazard Assessment of SwedenIn: Natural hazards and earth system sciences, ISSN 1561-8633, E-ISSN 1684-9981Article in journal (Refereed)
    Abstract [en]

    Assessing seismic hazard in stable continental regions (SCR) such as Sweden poses unique challenges compared to active seismic regions. With diffuse seismicity, low seismicity rate, few large magnitude earthquakes and little strong motion data, estimating recurrence parameters and determining appropriate attenuation relationships is challenging. This study presents a probabilistic seismic hazard assessment of Sweden based on a recent earthquake catalogue which includes a large number of events with magnitudes ranging from 5.9 to -1.4, enabling recurrence parameters to be calculated for more source areas than in previous studies, and with less uncertainty. Recent ground motion models developed specifically for stable continental regions, including Fennoscandia, are used in logic trees accounting for their uncertainty and the hazard is calculated using the OpenQuake engine.The results are presented in the form of mean peak ground acceleration (PGA) maps at 475 and 2500 year return periods and hazard curves for four seismically active areas in Sweden. We find the highest hazard in the northernmost part of the country, in the post-glacial fault province. This is in contrast to previous studies, which have not considered the high seismic activity on the post-glacial faults. We also find relatively high hazard along the northeast coast and in southwestern Sweden, whereas the southeast and the mountain region to the northwest have low hazard.For a 475 year return period we estimate the highest PGAs to be 0.04-0.05g, in the far north, and for a 2500 year return period it is 0.1-0.15g in the same area. Significant uncertainties remain to be addressed in regards to the SCR seismicity in Sweden, including the homogenization of small local magnitudes with large moment magnitudes, the occurrence of large events in areas with little prior seismicity and the uncertainties surrounding the potential for large earthquakes on the post-glacial faults in northern Fennoscandia.

  • 15.
    Joshi, Niranjan
    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.
    Tryggvason, Ari
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Incompleteness of natural disaster data and its implications on the interpretation of trendsManuscript (preprint) (Other academic)
    Abstract [en]

    We use data from a leading global database, primarily the reported numbers of disasters of different types and associated fatalities, to elucidate long-term trends in natural disaster impact. There are very strong upward trends in the number of reported disasters. However, we show that these trends are strongly biased by progressively improving reporting. Applying established methods based on analysing evolving differences in the patterns of large (many fatality) to small (fewer fatality) events can be used to compensate for this bias. For all disaster types investigated the long-term (multi-decade) true number of disasters appears to be unexpectedly stable over time, albeit with a significant temporary increase 1980-90. However, the corresponding numbers for the total number of fatalities from the weather-related disasters have declined strongly, due to very effective mitigation for  larger disasters. Data for the recent decades shows a systematic decrease in the number of larger (more fatality) disasters compared to smaller. In contrast, for the geophysical disasters, predominantly earthquakes, the relative numbers of larger versus smaller disasters is fairly constant, as are total fatalities.

  • 16.
    Joshi, Niranjan
    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.
    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.
    Earthquake disaster fatality data: Temporally stable power-law behavior and effects of underreportingIn: Seismological Research Letters, ISSN 0895-0695, E-ISSN 1938-2057Article in journal (Refereed)
    Abstract [en]

    We investigate changes in the global reported fatalities from earthquake disasters in theleading global disaster database EM-DAT. Drawing parallels with the Gutenberg-Richterfrequency-magnitude analysis, in terms of disaster frequency versus the number of casual-ties, we see a significant overlap of the curves and improving levels of completeness oversix 20-year periods. This implies a decrease in underreporting with time. We find that theapparent strong upwards trend in the number of (reported) earthquake disasters in EM-DATis caused by a gradually improved reporting primarily of events killing fewer than 10 peo-ple. An implication of our findings is that the true (reported and unreported) number ofearthquake disasters, according to the EM-DAT definition, has been surprisingly constantover, at least, the last 100 years. We also show that the average annual number of peoplekilled in earthquake disasters is relatively unaffected by spurious trends in reporting, and hasremained remarkably constant, at least since the 1960s, despite population increase.

  • 17.
    Jónsdóttir, Kristín
    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.
    Böðvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Glacial long period seismic events at Katla volcano, Iceland2009In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 36, no 11, article id L11402Article in journal (Refereed)
    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.

  • 18.
    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.

  • 19. Kind, R.
    et al.
    Sodoudi, F.
    Yuan, X.
    Shomali, Zaher Hossein
    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.
    Gee, David G.
    Eken, T.
    Bianchi, M.
    Tilmann, F.
    Balling, N.
    Jacobsen, B. H.
    Kumar, P.
    Geissler, W. H.
    Scandinavia: A former Tibet?2013In: Geochemistry Geophysics Geosystems, E-ISSN 1525-2027, Vol. 14, no 10, p. 4479-4487Article in journal (Refereed)
    Abstract [en]

    The Himalaya and the Tibetan Plateau are uplifted by the ongoing northward underthrusting of the Indian continental lithosphere below Tibet resulting in lithospheric stacking. The layered structure of the Tibetan upper mantle is imaged by seismic methods, most detailed with the receiver function method. Tibet is considered as a place where the development of a future craton is currently under way. Here we study the upper mantle from Germany to northern Sweden with seismic S receiver functions and compare the structure below Scandinavia with that below Tibet. Below Proterozoic Scandinavia, we found two low-velocity zones on top of each other, separated by a high-velocity zone. The top of the upper low-velocity zone at about 100 km depth extends from Germany to Archaean northern Sweden. It agrees with the lithosphere-asthenosphere boundary (LAB) below Germany and Denmark. Below Sweden it is known as the 8 degrees discontinuity, or as a mid-lithospheric discontinuity (MLD), similar to observations in North America. Seismic tomography places the LAB near 200 km in Scandinavia, which is close to the top of our deeper low-velocity zone. We also observed the bottom of the asthenosphere (the Lehmann discontinuity) deepening from 180 km in Germany to 260 km below Sweden. Remnants of old subduction in the upper about 100 km below Scandinavia and Finland are known from controlled source seismic experiments and local earthquake studies. Recent tomographic studies indicate delamination of the lithosphere below southern Scandinavia and northern Germany. We are suggesting that the large-scale layered structure in the Scandinavian upper mantle may be caused by processes similar to the ongoing lithospheric stacking in Tibet.

  • 20. Leptokaropoulos, Konstantinos
    et al.
    Adamaki, Angeliki
    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.
    Gkarlaouni, Charikleia
    Paradisopoulou, Parthena
    Impact of Magnitude Uncertainties on Seismic Catalogue Properties2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 2, p. 940-951Article in journal (Refereed)
    Abstract [en]

    Catalogue-based studies are of central importance in seismological research, to investigate the temporal, spatial and size distribution of earthquakes in specified study areas. Methods for estimating the fundamental catalogue parameters like the Gutenberg–Richter (G-R) b-value and the completeness magnitude (Mc) are well established and routinely applied. However, the magnitudes reported in seismicity catalogues contain measurement uncertainties which may significantly distort the estimation of the derived parameters. In this study, we use numerical simulations of synthetic data sets to assess the reliability of different methods for determining b-value and Mc, assuming the G-R law validity. After contaminating the synthetic catalogues with Gaussian noise (with selected standard deviations), the analysis is performed for numerous data sets of different sample size (N). The noise introduced to the data generally leads to a systematic overestimation of magnitudes close to and above Mc. This fact causes an increase of the average number of events above Mc, which in turn leads to an apparent decrease of the b-value. This may result to a significant overestimation of seismicity rate even well above the actual completeness level. The b-value can in general be reliably estimated even for relatively small data sets (N < 1000) when only magnitudes higher than the actual completeness level are used. Nevertheless, a correction of the total number of events belonging in each magnitude class (i.e. 0.1 unit) should be considered, to deal with the magnitude uncertainty effect. Because magnitude uncertainties (here with the form of Gaussian noise) are inevitable in all instrumental catalogues, this finding is fundamental for seismicity rate and seismic hazard assessment analyses. Also important is that for some data analyses significant bias cannot necessarily be avoided by choosing a high Mc value for analysis. In such cases, there may be a risk of severe miscalculation of seismicity rate regardless the selected magnitude threshold, unless possible bias is properly assessed.

  • 21.
    Li, Ka Lok
    et al.
    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.
    Sgattoni, Giulia
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics. Univ Iceland, Inst Earth Sci, Reykjavik, Iceland.;Univ Bologna, Dept Geol, Bologna, Italy..
    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.
    Locating tremor using stacked products of correlations2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 7, p. 3156-3164Article in journal (Refereed)
    Abstract [en]

    We introduce a back-projection method to locate tremor sources using products of cross-correlation envelopes of time series between seismic stations. For a given subset of n stations, we calculate the (n - 1)th-order product of cross-correlation envelopes and we stack the back-projected products over combinations of station subsets. We show that compared to existing correlation methods and for realistic signal and noise characteristics, this way of combining information can significantly reduce the effects of correlated (spurious or irrelevant signals) and uncorrelated noise. Each back-projected product constitutes an individual localized estimate of the source locations, as opposed to a hyperbola for the existing correlation techniques, assuming a uniform velocity in two dimensions. We demonstrate the method with synthetic examples and a real-data example from tremor at Katla Volcano, Iceland, in July 2011. Despite very complex near-surface structure, including strong topography and thick ice cover, the method appears to produce robust estimates of tremor location.

  • 22.
    Li, Ka Lok
    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. Univ Iceland, Inst Earth Sci, Reykjavik, Iceland; Univ Bologna, Dept Geol, Bologna, Italy.
    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.
    Gudmundsson, Ólafur
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    A double-correlation tremor-location method2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 208, no 2, p. 1231-1236Article in journal (Refereed)
    Abstract [en]

    A double-correlation method is introduced to locate tremor sources based on stacks of complex, doubly-correlated tremor records of multiple triplets of seismographs back projected to hypothetical source locations in a geographic grid. Peaks in the resulting stack of moduli are inferred source locations. The stack of the moduli is a robust measure of energy radiated from a point source or point sources even when the velocity information is imprecise. Application to real data shows how double correlation focuses the source mapping compared to the common single correlation approach. Synthetic tests demonstrate the robustness of the method and its resolution limitations which are controlled by the station geometry, the finite frequency of the signal, the quality of the used velocity information and noise level. Both random noise and signal or noise correlated at time shifts that are inconsistent with the assumed velocity structure can be effectively suppressed. Assuming a surface wave velocity, we can constrain the source location even if the surface wave component does not dominate. The method can also in principle be used with body waves in 3-D, although this requires more data and seismographs placed near the source for depth resolution.

  • 23.
    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.

  • 24.
    Lindman, Mattias
    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. Geofysik.
    Jonsdottir, Kristin
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Roberts, Roland
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Lund, Björn
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Bödvarsson, Reynir
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Earthquakes Descaled: On Waiting Time Distributions and Scaling Laws2005In: Physical Review Letters, Vol. 94, no 10, p. 108501-Article in journal (Refereed)
    Abstract [en]

    Recently, several authors have used waiting time distributions for large earthquake data sets to draw conclusions regarding the physics of earthquake processes. We show, theoretically and by simulation, that a characteristic kink in observed waiting time distributions does not have the physical significance of separating correlated and uncorrelated earthquakes. It also follows from our discussion that the Omori law is not trivially related to a proposed scaling law and that caution must be taken before the spatial scaling exponent of the law is interpreted as a fractal dimension of seismicity.

  • 25.
    Lindman, Mattias
    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. Geofysik.
    Jonsdottir, Kristin
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Roberts, Roland
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Lund, Björn
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Bödvarsson, Reynir
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Lindman et al. reply2006In: Physical Review Letters, Vol. 96, no 10, p. 109802-Article in journal (Refereed)
  • 26.
    Lindman, Mattias
    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. Geofysik.
    Lund, Björn
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Roberts, Roland
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Jonsdottir, Kristin
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Geophysics. Geofysik.
    Physics of the Omori law: Inferences from interevent time distributions and pore pressure diffusion modeling2006In: Tectonophysics, Vol. 424, no 3-4, p. 209-222Article in journal (Refereed)
    Abstract [en]

    Empirical laws and statistics of earthquakes are valuable as a basis for a better understanding of the earthquake cycle. In this paper we focus on the postseismic phase and the physics of aftershock sequences. Using interevent time distributions for a catalogue of Icelandic seismicity, we infer that the parameter C2 in the Omori law, often considered to represent incomplete detection of aftershocks, is at least in part related to the physics of the earthquake process. We investigate the role of postseismic pore pressure diffusion after two Icelandic earthquakes on the rate of aftershocks and what we can infer about the physical meaning of C2 from the diffusion process. Using the Mohr–Coulomb failure criterion we obtain a rate of triggered points in our diffusion model that agrees with the modified Omori law, with a value of C2 that is consistent with data. Our pore pressure diffusion model suggests that C2 is related to the process of reducing high pore pressure gradients existing across a fault zone at short times after a main shock.

  • 27.
    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
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Geofysik.
    Ragnar, Slunga
    Earthquake activity in Sweden. Study in connection with a proposed nuclear waste repository in Forsmark or Oskarshamn2006Report (Other scientific)
  • 28.
    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)
  • 29.
    Roberts, Roland
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Carbonell, Ramon
    CSIC.
    Adamaki, Angeliki
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ivandic, Monika
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Deep Seismic Sounding Data2017Report (Other (popular science, discussion, etc.))
    Abstract [en]

    This document is an interim report within a work package of the SERA project. The document lists a considerable number of previous Deep Seismic Sounding (DSS) projects, where data is available in some form. DSS projects are large scale, logistically complex, and there can be some problems in obtaining formal permission to use the very large seismic sources which may be necessary to be able to successfully record seismic signals penetrating to the relevant depths and distances. This implies that in some cases it would be very difficult to repeat the projects, or conduct a similar project along the same recording profile. This means that even older data can be potentially very valuable, and not all such data is available in modern, digital form (e.g. time series), and some metadata descriptions may be complicated or incomplete.In the text below, we discuss what is meant by DSS data; some of the complications related to this type of data, metadata, different forms of data, and some common types of derived (processed data) which exist. We also present information on a considerable number of DSS projects related to the European area, and list some (but far from all) relevant publications. Note that the term “database” is used in AERA. For DSS data, much data is available in well-structured and maintained digital databases, some is available in digital form only as images of seismic sections, and some data exists only in analogue form e.g. as plotted seismic sections. We consider all such types of data to be relevant, and include them in the “database” concept, as discussed below.

    Download full text (pdf)
    fulltext
  • 30.
    Roberts, Roland
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Carbonell, Ramon
    Adamaki, Angeliki
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ivandic, Monika
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Position Document on Future DSS Data Accessibility2018Report (Other (popular science, discussion, etc.))
    Download full text (pdf)
    fulltext
  • 31.
    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.

  • 32.
    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.

  • 33.
    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.

  • 34. Vinnik, Lev
    et al.
    Oreshin, Sergey
    Makeyeva, Larissa
    Peregoudov, Dmitriy
    Kozlovskaya, Elena
    Pedersen, Helle
    Plomerova, Jaroslava
    Achauer, Ulrich
    Kissling, Eduard
    Sanina, Irina
    Jamsen, Teppo
    Silvennoinen, Hanna
    Pequegnat, Catherine
    Hurskainen, Riitta
    Guiguet, Robert
    Hausmann, Helmut
    Jedlicka, Petr
    Aleshin, Igor
    Bourova, Ekaterina
    Bödvarsson, Reynir
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Bruckl, Evald
    Eken, Tuna
    Heikkinen, Pekka
    Houseman, Gregory
    Johnsen, Helge
    Kremenetskaya, Elena
    Komminaho, Kari
    Munzarova, Helena
    Roberts, Roland
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Ruzek, Bohuslav
    Shomali, Zaher Hossein
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
    Schweitzer, Johannes
    Shaumyan, Artem
    Vecsey, Ludek
    Volosov, Sergei
    Anisotropic lithosphere under the Fennoscandian shield from P receiver functions and SKS waveforms of the POLENET/LAPNET array2014In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. 628, p. 45-54Article in journal (Refereed)
    Abstract [en]

    Seismic azimuthal anisotropy is the key evidence of the past and present strains in the upper mantle. The standard analysis of shear-wave splitting with the SKS techniques is useful in mapping lateral variations but it is insensitive to depth of anisotropy and to variations of anisotropy with depth. To retrieve the depth localized anisotropy under the Fennoscandian shield, we inverted P-wave receiver functions of the POLENET/LAPNET array in northern Finland jointly with SKS recordings. Shear-wave anisotropy of similar to 2.5% with the fast direction of 40 degrees-60 degrees in a depth range from the Moho to similar to 110 km is a robust result of the inversion. The obtained direction is nearly normal to the azimuth of the maximum horizontal compressional stress in the lithosphere, but a recent origin of this anisotropy is in doubt. This anisotropy may be frozen since the Precambrian, but it shows no clear relation with the trends of the Precambrian tectonics. The upper anisotropic layer accounts for similar to 40% of shear-wave splitting in SKS, and to explain the rest another anisotropic layer is required. The top of the second layer with a practically similar fast direction is found at a depth of 200-240 km. This direction is close to the current APM direction of the lithosphere with implication that the inferred anisotropy may be related with the current plate motion, and the anisotropic layer belongs to the asthenosphere. The bottom of this layer is uncertain, but it is at least 320 km deep. In a depth range from 160 km to 200-240 km the fast anisotropy direction is 110-150 degrees. Origin of this direction is unclear. 

  • 35.
    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.

  • 36.
    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.

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