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| Titel |
Transient deformation indicating rheological variation in the South Iceland Seismic Zone. |
| VerfasserIn |
Thóra Árnadóttir, Sigrún Hreinsdóttir, Karolina Michalczewska, Halldór Geirsson, Benedikt Ófeigsson |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250082589
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| Zusammenfassung |
| In June 2000 and May 2008, two sets of magnitude 6.5 and 6.0 main shocks struck the South
Iceland Seismic Zone (SISZ) – a transform zone that accommodates E-W left lateral shear at
depth by faulting on numerous N-S oriented faults in the brittle crust. Both earthquake
episodes consisted of a pair of main shocks of similar size rupturing closely spaced faults,
where static and dynamic stress changes generated by the first event triggered the second
main shock further west. The June 2000 earthquakes occurred in the central part of
the SISZ, and the May 2008 events in the western part, close to the Hengill triple
junction.
Since June 2000 we have performed annual GPS measurements in a geodetic network
in South Iceland and a number of continuous GPS stations have been installed.
The aim is to monitor crustal deformation caused by plate motion, earthquakes,
post-seismic transients, and magma movement. GPS and InSAR observations revealed
rapidly decaying deformation transients in the SISZ following the 2000 main shocks
caused by poro-elastic rebound (~2 months) and a longer term signal lasting for
about 4 years, explained by visco-elastic relaxation of the lower crust and upper
mantle with viscosities of 1019 Pa s and 3Ã1018 Pa s, respectively. The geodetic
studies also suggest that the seismic moment released in these earthquakes was at
most half of the accumulated stress since the last major earthquake sequence in
1896–1912. Thus, we may expect magnitude 6–7 events in South Iceland in the near
future.
Here, we report pronounced GPS station velocity changes, in particular west of the 2008
epicentral area – still significant in 2012. This result is rather surprising as the 2008 main
shocks were smaller (magnitude 6.0) than the 2000 events (magnitude 6.5). Rheological
models obtained from studies of the post-seismic deformation following the June 2000 main
shocks do not apply directly to the May 2008 events. The rapid transient (~ week) we
observe in 2008 is inconsistent with motion due to poro-elastic rebound, and the
longer-term transient requires thinner crust and lower viscosities than obtained
from studies of the June 2000 events. In addition to plate motion and post-seismic
transients, the deformation field is complicated by subsidence and contraction in the
Hengill area as a result of fluid withdrawal for geothermal energy production at
two power plants. The overall pattern of deformation since May 2008, however,
suggests that the crustal structure in the western part of the SISZ is different from the
central part. We also note an increase in strain rates in the epicentral area of the May
2008 main shocks from 2004 to 2007, indicating a link between the strain rate
anomaly and subsequent earthquakes. Frequent measurements in dense geodetic
networks therefore aid in identify crustal rheology and possible locations of future main
shocks. |
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