HR: 1340h
AN: T43A-1387    [Abstracts]
TI: A crustal earthquake doublet occurred beneath Ilan: left-lateral strike-slip motion of the upper crust at the transition from the NE tip of the Taiwan mountain range to the SW tip of Okinawa Trough
AU: * Liang, W
EM: wtl@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, POB 1-55, Nankang, Taipei, 115 Taiwan
AU: Lee, J
EM: jclee@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, POB 1-55, Nankang, Taipei, 115 Taiwan
AU: Kuo, B
EM: byk@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, POB 1-55, Nankang, Taipei, 115 Taiwan
AU: Liu, Y
EM: lyh@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, POB 1-55, Nankang, Taipei, 115 Taiwan
AB: We determined 7 focal mechanisms for events associated with a shallow crustal earthquake doublet (Mw=5.7) that occurred beneath the Ilan region, Taiwan on March 5, 2005 by inverting broadband waveforms. Except for one event to the NE of the major aftershock activity region that shows normal faulting, the other 6 all show left-lateral strike-slip faulting. The extensional axes of all these focal mechanisms consistently align in a NW-SE direction, whereas the compressional axes are in a NE-SW direction. This stress regime can be considered as a combination of the compression of the Taiwan mountain belt and the opening of Okinawa Trough and/or the retreat of Ryukyu trench. The relocated earthquake sequence shows a near-vertical E-W planar seismogenic structure that dips toward the south to a depth of 15 km with a horizontal dimension of about 15 km. The later event of the earthquake doublet is ~5 km to the west of the leading one and ~68 seconds behind. These observations confirm that an active seismogenic structure is developing beneath the southern part of the Ilan Plain. The deformation is accommodated by left-lateral strike-slip faulting with minor component of normal fault. This is different from previous interpretations of pure normal faulting and simple extension. An earlier Mw 6.1 event located at the eastern end of the major linear structure on May 5, 2002 also shows the same focal mechanism. Together with the June 5, 1994 Nanao earthquake (Mw=6.5) sequence, which is about 20 km to the south of the recent events, they all illustrated similar focal mechanisms and a nearly E-W trend of aftershock activity. This E-W orientation is consistent with the trend of major geological structures in this region, and in addition, is coincident with the fast direction (E-W) of crustal anisotropy. We thus infer that the severe lateral bending of geological structures beneath the northeastern end of Taiwan mountain range in the vicinity of the Ilan region, a transition area from oblique convergence of Philippine and Eurasian plates to back-arc opening of the Okinawa Trough, may yield the left-lateral strike-slip motion of upper crust.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8107 Continental neotectonics (8002)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005