HR: 15:10h
AN: S23D-07    [Abstracts]
TI: Spatio-temporal variation of seismogenic stress field in the source region of the 2004 Sumatra-Andaman earthquake
AU: * Oishi, M
EM: oishi_m@seis.nagoya-u.ac.jp
AF: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-City, Aichi, 464-8602, Japan
AU: Sagiya, T
EM: sagiya@seis.nagoya-u.ac.jp
AF: Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya-City, Aichi, 464-8602, Japan
AU: Sato, T
EM: tamao@cc.hirosaki-u.ac.jp
AF: Faculty of Science and Technology, Hirosaki University, 3 bunkyo cho, Hirosaki City, Aomori, 036-8561, Japan
AB: The 2004 Sumatra-Andaman earthquake occurred in an oblique subduction zone. The plate convergence direction significantly changes along the trench in the source region. The aftershocks activity and its variety of source mechanisms indicate the stress field in the source region is very complex. We conducted seismological analyses to investigate spatio-temporal variation of the stress filed in detail. We calculate angular deviation of earthquake slip directions from the plate convergence directions based on the Global CMT solutions (1976-2007, 209events). We also compare the coseismic slip direction (Tsai et al. 2005, Chlieh et al. 2007) with the plate convergence direction. Both results indicate that the coseismic slip direction is normal to the trench in the southern portion of the source region. This result implies that the fore-arc sliver motion accommodates the trench parallel component of the oblique plate convergence. On the other hand, in the northern portion around Andaman Islands, the fault slip direction deviates from the plate convergence direction by about 40 degrees, which is not perpendicular to the trench. The result suggests that stress partitioning due to the fore-arc sliver motion is not complete in the northern part. The large width of the fore-arc sliver and an effect of collision with the Eurasian plate at the northern edge of the sliver may be responsible for the incomplete stress partitioning. In order to discuss spatio-temporal stress variation with CMT solutions, we also conduct relocation of hypocenters in the source region by using the modified Joint hypocenter determination (MJHD) method. Initial hypocenter locations are assumed based on the ISC catalogue and Engdahl(2007), and 196300 P wave arrivals of 1807 events are reanalyzed. As a result, the average travel time residual reduced from 8.2 sec to 0.4 sec, and configuration of the subducted slab becomes clear. Combining the relocation result with the Global CMT solutions, various characteristics are identified. For example, afertshocks with high angle reverse faulting mechanism occurred just beneath the trench axis in the southern part. On the contrary, normal fault earthquakes occurred under the trench in the northern part, indicating a significant stress variation along the trench. In the southern part, normal fault events occurred near the plate boundary only after the mainshock, implying significant temporal stress change due to the mainshock.
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: 2007 Fall Meeting