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Seismicity of the Hengill area, SW Iceland: Details revealed by catalog relocation and collapsing
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.ORCID iD: 0000-0002-1402-8341
Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Geophysics.
Icelandic Metereological Office - IMO.
(English)Manuscript (preprint) (Other academic)
National Category
Geophysics
Identifiers
URN: urn:nbn:se:uu:diva-327037OAI: oai:DiVA.org:uu-327037DiVA, id: diva2:1129057
Available from: 2017-07-31 Created: 2017-07-31 Last updated: 2018-10-16
In thesis
1. Location and Relocation of Seismic Sources
Open this publication in new window or tab >>Location and Relocation of Seismic Sources
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This dissertation is a comprehensive summary of four papers on the development and application of new strategies for locating tremor and relocating events in earthquake catalogs.

In the first paper, two new strategies for relocating events in a catalog are introduced. The seismicity pattern of an earthquake catalog is often used to delineate seismically active faults. However, the delineation is often hindered by the diffuseness of earthquake locations in the catalog. To reduce the diffuseness and simplify the seismicity pattern, a relocation and a collapsing method are developed and applied. The relocation method uses the catalog event density as an a priori constraint for relocations in a Bayesian inversion. The catalog event density is expressed in terms of the combined probability distribution of all events in the catalog. The collapsing method uses the same catalog density as an attractor for focusing the seismicity in an iterative scheme. These two strategies are applied to an aftershock sequence after a pair of earthquakes which occurred in southwest Iceland, 2008. The seismicity pattern is simplified by application of the methods and the faults of the mainshocks are delineated by the reworked catalog.

In the second paper, the spatial distribution of seismicity of the Hengill region, southwest Iceland is analyzed. The relocation and collapsing methods developed in the first paper and a non-linear relocation strategy using empirical traveltime tables are used to process a catalog collected by the Icelandic Meteorological Office. The reworked catalog reproduces details of the spatial distribution of seismicity that independently emerges from relative relocations of a small subset of the catalog events. The processed catalog is then used to estimate the depth to the brittle-ductile transition. The estimates show that in general the northern part of the area, dominated by volcanic processes, has a shallower depth than the southern part, where tectonic deformation predominates.

In the third and the fourth papers, two back-projection methods using inter-station cross correlations are proposed for locating tremor sources. For the first method, double correlations, defined as the cross correlations of correlations from two station pairs sharing a common reference station, are back projected. For the second method, the products of correlation envelopes from a group of stations sharing a common reference station are back projected. Back projecting these combinations of correlations, instead of single correlations, suppresses random noise and reduces the strong geometrical signature caused by the station configuration. These two methods are tested with volcanic tremor at Katla volcano, Iceland. The inferred source locations agree with surface observations related to volcanic events which occurred during the tremor period.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. p. 73
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1532
Keywords
Earthquake location, Earthquake relocation, Tremor location, Bayesian inversion, Correlation, Earthquake catalog, Iceland, Seismicity, Interferometry, Inverse theory, Volcanic tremor
National Category
Geophysics
Research subject
Geophysics with specialization in Seismology
Identifiers
urn:nbn:se:uu:diva-327038 (URN)978-91-513-0013-9 (ISBN)
Public defence
2017-09-15, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2017-08-23 Created: 2017-08-01 Last updated: 2017-09-08
2. Seismicity and crustal structure in Iceland
Open this publication in new window or tab >>Seismicity and crustal structure in Iceland
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The main goal of this Ph.D. thesis is to improve locations of earthquake hypocenters and to resolve heterogeneous crustal structure and its effects on travel times. The data and case studies are drawn from the Icelandic national SIL network and the temporary NICE project deploy-ment in the Tjörnes Fracture Zone. Iceland presents complex tectonics and active volcanism, consequences of its position astride the Mid-Atlantic Ridge between the European and North American plates and on top of a melting anomaly in the mantle below. Studies focused on characterizing the seismicity and the crustal structure are prerequisites for further seismologi-cal studies in Iceland, e.g., on seismic sources, the evolution of volcanic systems, activity on seismic faults and seismic hazard, among others.

Different methods have been explored. First, we estimated empirically travel-time functions of seismic waves and their uncertainties for 65 stations in the Icelandic permanent network (SIL) using arrival times. The estimated travel-time functions and uncertainties were used to relocate the complete catalog applying a nested-search algorithm to this non-linear problem. The clearest changes in locations compared to the SIL solutions were obtained in the peripheral areas of the network, in particular in the Tjörnes Fracture Zone (North Iceland) and on the Reykjanes Peninsula (South Iceland).

Relocations with empirical travel times were used complementary with constrained earth-quake relocation and the collapsing methods of Li et al. [2016] to study the seismicity in the Hengill area (South Iceland). Patterns in the seismicity in the final locations reproduce lin-eations previously found in relative relocations in the area. The brittle-ductile transition was estimated, obtaining a smaller depth in the northern part of the region, dominated by volcanic processes, compared with the south, controlled by tectonic processes. Furthermore, the Hengill fissure swarm that hosts two large geothermal power plants, was found to have deeper penetrat-ing earthquakes than adjacent volcanic areas, presumably because it is more effectively cooled.

Local earthquake tomography was used to solve simultaneously for earthquake location and velocity structure in the Tjörnes Fracture Zone, using data from the temporary network installed during the North ICeland Experiment, and data from the permanent SIL network. 3-D velocity models for P- and S-waves were obtained for the area and used to relocate the complete SIL catalog. The results demonstrate significant structures associated with the different branches of this complex transform region, e.g. low velocities along the Husavík-Flatey Fault (HFF), penetrating to about 10 km depth. Low Vp/Vs ratios were also mapped at depth along the HFF indicating presence of highly compressible fluids in the middle crust. In general, the seismicity pattern was shifted towards the surface from SIL locations and clarified in its lateral distribution. This highlighted a zone of concentrated deformation in the Tjörnes Microplate, which intersections with the two main strands of the overall zone coincide with changes in their geometry and character.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 56
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1734
Keywords
Earthquake relocation, local earthquake tomography
National Category
Geophysics
Identifiers
urn:nbn:se:uu:diva-363337 (URN)978-91-513-0477-9 (ISBN)
Public defence
2018-12-06, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2018-11-16 Created: 2018-10-16 Last updated: 2018-11-30

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Gudmundsson, Ólafur

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