HR: 0830h
AN: S11C-0310    [PDF]
TI: Significant Seismic Anisotropy in the Shallow Unconsolidated Layer beneath the Southwestern Taiwan
AU: * Liang, W
EM: wtl@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O.Box 1-55, Nankang, Taipei, 115 Taiwan
AU: Kuo, B
EM: byk@earth.sinica.edu.tw
AF: Institute of Earth Sciences, Academia Sinica, P.O.Box 1-55, Nankang, Taipei, 115 Taiwan
AB: Taiwan is located on the convergent boundary zones between the Eurasian and the Philippine sea plates. Due to the collision between Taiwan and Luzon arc, the central Taiwan is undergoing remarkable crustal deformation and consequently results in disastrous earthquakes like the 1999 Chi-Chi, Taiwan Earthquake (Mw 7.6). In this study, we have analyzed the seismic shear wave splitting at a borehole station in the Southwestern Taiwan, where is overlain a thick and soft sedimentary layer, to study the subsurface structural deformation. The fast direction and delay time were estimated by applying the waveform cross-correlation technique. Both the direct and free surface reflected S waves show significant splittings. We found their mean polarization of the leading S waves is in ESE-WNW direction, which is compatible with the surface velocity field derived from geodetic GPS measurements. The delay time obtained from the reflected S wave is around 0.08 s at a two-way travel time of 1.01 s, which demonstrates that the anisotropic degree is as large as 8% or 23% in s/km for the top 173 m unconsolidated layer. This value (8%) is similar to that derived from the direct S wave. In addition, it doesn't change much before and after the 1999 Chi-Chi, Taiwan Earthquake. This observation suggests that the tectonic force induced, prevailing crack-like structures might also exist in the very shallow unconsolidated and water-saturated layer and seems to be less sensitive to the stress redistribution after a nearby large earthquake. However, the anisotropy caused by the alignment of mineral grains cannot be completely excluded.
DE: 7205 Continental crust (1242)
DE: 8107 Continental neotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting