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Seismicity and crustal structure in Iceland
Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
2018 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Uppsala: Acta Universitatis Upsaliensis, 2018. , s. 56
Serie
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1734
Emneord [en]
Earthquake relocation, local earthquake tomography
HSV kategori
Identifikatorer
URN: urn:nbn:se:uu:diva-363337ISBN: 978-91-513-0477-9 (tryckt)OAI: oai:DiVA.org:uu-363337DiVA, id: diva2:1256421
Disputas
2018-12-06, Hambergsalen, Geocentrum, Villavägen 16, Uppsala, 10:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2018-11-16 Laget: 2018-10-16 Sist oppdatert: 2018-11-30
Delarbeid
1. Relocating earthquakes with empirical traveltimes
Åpne denne publikasjonen i ny fane eller vindu >>Relocating earthquakes with empirical traveltimes
2018 (engelsk)Inngår i: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 214, nr 3, s. 2098-2114Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

A strategy is proposed to incorporate effects of 3-D velocity variations on earthquake locationsusing empirical traveltimes (ETTs). Traveltime residuals are interpolated from those predictedusing a 1-D velocity reference, mapped on to the hypocentres of corresponding earthquakesfor each station in a network. First, station corrections are computed by averaging the residualsover a fixed scale. Then, summary-ray variograms are used to estimate uncertainty and that,in turn, is used to tune a local fit to neighbouring residuals to refine the corrections. Resulting3-D traveltime estimates are then used as a description of the forward problem in a nonlineargrid-search relocation. Data are weighted according to the estimated uncertainty. Data fromthe Icelandic Southern Iceland Lowlands (SIL) national seismic network are used to test thestrategy. ETTs are estimated forP- andS-waves at 65 stations in the SIL network, basedon four million arrival time readings of 300.000 events registered between 1990 and 2012.ETTs are strongly correlated for the two wave types. The spatial variations of the predictedcorrections are consistently comparable or somewhat less forS-waves thanP-waves. Thisfeature suggests variations of theVP/VSratio in the Icelandic crust. Error estimates are alsostrongly correlated for the two wave types and between nearby stations. Relocations aretested by comparison with explosions and small populations of well-located events withindenser subnetworks. Relocations result in modestly enhanced clustering of explosions andearthquakes and significantly improved depth estimates. Estimates of the random relocationerror are statistically better behaved than those of the SIL system. They are in general reduced,as is expected since 3-D heterogeneity is now partly taken into account.

sted, utgiver, år, opplag, sider
Oxford University Press, 2018
Emneord
Body waves, earthquake monitoring
HSV kategori
Identifikatorer
urn:nbn:se:uu:diva-363251 (URN)10.1093/gji/ggy246 (DOI)000439648000037 ()
Forskningsfinansiär
EU, FP7, Seventh Framework Programme, 308377
Tilgjengelig fra: 2018-10-15 Laget: 2018-10-15 Sist oppdatert: 2018-12-05bibliografisk kontrollert
2. Seismicity of the Hengill area, SW Iceland: Details revealed by catalog relocation and collapsing
Åpne denne publikasjonen i ny fane eller vindu >>Seismicity of the Hengill area, SW Iceland: Details revealed by catalog relocation and collapsing
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
HSV kategori
Identifikatorer
urn:nbn:se:uu:diva-327037 (URN)
Tilgjengelig fra: 2017-07-31 Laget: 2017-07-31 Sist oppdatert: 2018-10-16
3. Local earthquake tomography and earthquake relocation in the Tjörnes Fracture Zone (North Iceland)
Åpne denne publikasjonen i ny fane eller vindu >>Local earthquake tomography and earthquake relocation in the Tjörnes Fracture Zone (North Iceland)
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
HSV kategori
Identifikatorer
urn:nbn:se:uu:diva-363252 (URN)
Tilgjengelig fra: 2018-10-15 Laget: 2018-10-15 Sist oppdatert: 2018-10-16

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