HR: 1330h
AN: S22B-0455    [PDF]
TI: Shear-wave Seismic Attenuation in Southern California and Taiwan: A Comparison
AU: * Wang, Y
EM: s0622018@cc.ncu.edu.tw
AF: National Central University, Institute of Geophysics, Taoyuan, 320 Taiwan
AU: * Wang, Y
EM: s0622018@cc.ncu.edu.tw
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Ma, K
EM: fong@earth.gep.ncu.edu.tw
AF: National Central University, Institute of Geophysics, Taoyuan, 320 Taiwan
AU: Ma, K
EM: fong@earth.gep.ncu.edu.tw
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AU: Mooney, W D
EM: mooney@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025 United States
AB: There have been numerous studies of the 3D seismic P- and S-wave velocity structure of both Southern California and Taiwan. These studies have provided a framework for the relocation of seismicity and for investigations of the composition of the crust and uppermost mantle. Here we address another useful seismological parameter, attenuation, or inverse Q. Attenuation studies are important because this parameter is sensitive to such properties as temperature and fluid content, both of which can be important for the interpretation of active tectonic processes. Schlotterbeck and Abers (2001) determined the 3D Q-structure of Southern California. They found relatively low Q within the Los Angeles basin and Transverse Ranges, compared with the Mojave Desert to the east. They also found low Q in the Salton Trough. Thus, there is a large degree of lateral variation in Q values even within the generally low-Q western USA. It is interesting to compare crustal attenuation in Southern California with attenuation determined for the island of Taiwan. These two regions are similar geologically, and both exhibit high levels of seismic activity. Furthermore, the dense strong motion network in Taiwan provides abundant data to determine the 3D Qs structure. An inversion for Qs in Taiwan shows a close correlation with geology. At shallow depths (4-9 km), relatively high Qs is found in NW Taiwan, while low Qs is found in SW Taiwan. The relative low Qs of SW Taiwan appears to be related to thicker sediments, while the higher values found to the NW are associated with crystalline rocks of the foreland belt. It is also possible to distinguish the Qs values between western and eastern Taiwan: high Qs is found at depth (27-38 km) beneath eastern Taiwan, and this zone can be correlated with the actively subducting slab that marks the active margin of eastern Taiwan. In contrast, a zone of low Qs is found beneath the Central Range at depths of 30-40 km. This low Qs zone correlates with a low velocity zone in both P- and S-waves, as determined from seismic tomography studies. One of the most remarkable features is the observation that the causative fault of the 1999 Mw = 7.6 Chi-Chi earthquake (the Chelungpu fault) exhibits clear lateral (cross-fault) variations in Qs, with the footwall showing higher Qs. The 3D Qs image of the crust of Taiwan is of higher resolution that what is presently available for Southern California. Nevertheless, in both regions we can see that there is a clear correlation between Q values and deep geologic structure. In addition, we anticipate that these results will provide important constraints of P-wave and S-wave Q values to be used in the synthesis of strong ground motions in both regions.
DE: 7203 Body wave propagation
DE: 7218 Lithosphere and upper mantle
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8123 Dynamics, seismotectonics
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Seismology [S]
MN: 2003 Fall Meeting