Seismogenic extensional faults in the southwestern tip of Okinawa trough: with constraint of coseismic ground slips of 2005 Ilan earthquakes, northeastern Taiwan

HR: 14:55h
AN: T33E-06 [Abstracts]
TI: Seismogenic extensional faults in the southwestern tip of Okinawa trough: with constraint of coseismic ground slips of 2005 Ilan earthquakes, northeastern Taiwan
AU: * Lai, K
EM: d92224003@ntu.edu.tw
AF: Institute of Geosciences, National Taiwan University, No.1, sec 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chen, Y
T33E-06 AF: Institute of Geosciences, National Taiwan University, No.1, sec 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Wu, Y
T33E-06 AF: Institute of Geosciences, National Taiwan University, No.1, sec 4, Roosevelt Road, Taipei, 106 Taiwan
AU: Chang, C
T33E-06 AF: Seismological Observation Center of the Central Weather Bureau, No.64, Gongyuan Road, Taipei, 100 Taiwan
AU: Wu, C
T33E-06 AF: Seismological Observation Center of the Central Weather Bureau, No.64, Gongyuan Road, Taipei, 100 Taiwan
AU: Kuochen, H
T33E-06 AF: Seismological Observation Center of the Central Weather Bureau, No.64, Gongyuan Road, Taipei, 100 Taiwan
AU: Kuo, Y
T33E-06 AF: Institute of Geosciences, National Taiwan University, No.1, sec 4, Roosevelt Road, Taipei, 106 Taiwan
AB: Two similar magnitude (ML=5.9) earthquakes occurred on March 6, 2005 at Ilan, northeastern Taiwan. Associated with the main shocks a large amount of aftershocks were recorded by Central Weather Bureau Seismic Network (CWBSN). This event provided an opportunity to explore the seismogenic structures by coseismic ground slips in Ilan Plain, an alluvial delta with sediments more than one thousand meters in depth. First of all, we relocated the aftershocks and determined the focal mechanisms of larger ones, and then identified the 3D fault geometry by analyzing the spatial seismicity distribution. We secondly conducted baseline corrections to determine the centimeter-scale coseismic displacement from strong-motion records to further inverse the fault model. Our results show the epicenters are assembled in a nearly E-W zone in the southern Ilan Plain. By 3D spatial analysis, two potential śf-shaped fault planes can be distinguished: one is dipping to the north with steeper angle (~80 degree), and another dipping to the south branching out from the former one with gentler angle (~55 degree). Two main shocks actually occurred along the junction of derived two fault-planes at depth of 7 km. On the other hand, the coseismic displacements determined from strong-motion records show that in the north of the seismic cluster all stations coseismically moved northwestward and stations in the south moved southeastward, implying two fault planes mentioned above slipped at the same time. Using ground slips and dislocation model, two fault planes (N70E, 75NW from near surface down to 10 km in depth; N65E, 60SE from 7 km to 10 km in depth) are tested by both inversed and forwarded simulations. Further combining the focal solutions, the derived extensional mechanism and the fault geometry well coincide with the possible origin of the opening Ilan Plain, which is the southwestern tip of the current back-arc spreading, Okinawa Trough.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8107 Continental neotectonics (8002)
DE: 8109 Continental tectonics: extensional (0905)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005