Implications for the magnitude of background stress field from focal mechanisms and GPS measurements
Abstract
We investigate the focal mechanisms after the Mw 9.15 2004 Aceh-Andaman earthquake and find a dramatic change of faulting type near the West Andaman Fault and the Seuliman Fault. Before the 2004 earthquake, the type of focal mechanism is mainly strike-slip, while it becomes normal faulting after the earthquake. We study the stress regime from the inversion of focal mechanisms and find a rotation of principal stress axes after the 2004 earthquake. This suggests the deviatoric stress is low, possibly around 3 MPa, which is on the order of the coseismic stress change. We apply a similar analysis for the Mw 7.6 1999 Chi-Chi, Taiwan, earthquake, an inland thrust event that caused a 100-km-long surface rupture. Limited to the lack of earthquakes near the rupture area before and after the mainshock, we primarily focus on analyzing the surface displacements measured by GPS in the interseismic, coseismic and postseismic periods. The direction of interseismic velocity is approximately N300°E, consistent with the direction of regional plate motion. However, the averaged directions of coseismic and postseismic displacements are N315°E and N280°E, respectively. We observe a 50°-100° counter-clockwise rotation of postseismic relative to coseismic displacements on the northern rupture, where the largest coseismic slip occurred. This suggests the magnitude of stress on the shallow crust is similar to the coseismic stress change in this region. Furthermore, an average of 100° clockwise rotation of postseismic relative to coseismic displacement near the surface rupture on the footwall might be related to the activation of secondary faults to the west of the Chi-Chi surface rupture. Further analysis is required to demonstrate the heterogonous stress filed and its implication to the seismic hazard.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.G33A0047H
- Keywords:
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- 1242 Seismic cycle related deformations (6924;
- 7209;
- 7223;
- 7230);
- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 8164 Stresses: crust and lithosphere