HR: 14:40h
AN: T53D-05    [Abstracts]
TI: Active Tectonics in Western Sichuan, China from Small Earthquake Relocation
AU: * Zhu, A
EM: alzhu@163.com
AF: Institute of Geology, China Earthquake Administration, P.O. Box 9803, Beijing, 100029 China
AU: Xu, X
EM: xiweixu@vip.sina.com
AF: Institute of Geology, China Earthquake Administration, P.O. Box 9803, Beijing, 100029 China
AU: Zhou, Y
EM: zhouysh@163bj.com
AF: Institute of Geology, China Earthquake Administration, P.O. Box 9803, Beijing, 100029 China
AU: Gan, W
EM: wjgan@gps.gov.cn
AF: Institute of Geology, China Earthquake Administration, P.O. Box 9803, Beijing, 100029 China
AB: To investigate detailed tectonic structures and activity within the western Sichuan region we performed double-difference relocations for 10,118 small earthquakes from a catalog of over 13,000 that occurred between 1992 and 2002. Most relocated events occur within the upper crust at depths of 0-15, and fewer at 20-50 km depth. There appears to be an aseismic layer between 15 and 20 km depth in the western Sichuan plateau associating with thrust faulting, while no such layer is indicated elsewhere. The rheological structure for the western Sichuan plateau suggests a ductile granitic layer within the upper crust at depths of 14 to 19 km. We propose that the existence of this aseismic layer may be associated with eastward extrusion of material from the Tibetan plateau and clockwise rotation of the Sichuan-Yunnan block. We analyzed the relocated events in detail along two dominating faults in the study area, the Xianshuihe-Anninghe-Zemuhe fault zone and the Longmenshan fault zone. Strong variations in the seismicity pattern and b values along the 700 km long, left-lateral Xianshuihe-Anninghe-Zemuhe fault zone, are observed in both map view and cross-section, which are consistent in geometry with the surface faulting segmentation. A series of aseismic segments with dimensions of ~30-50 km appears on this fault. We also observe several segments in simple structure having abnormally low b-value from Luhuo to Daofu and from Shimian to Mianning. These may be asperities. The relocated events are aligned along a ~50 km long NWW striking plane that dips 80° to the northeast in the transitional area between the Anninghe fault zone and the Zemuhe fault zone near Xichang, where no corresponding surface fault was found. Focal mechanism solutions show right-lateral movement. A cross-section of hypocenters along the south segment of the Xianshuihe fault zone shows an oblique plane plunging to southeastward with the maximum hypocentral depth of 50 km, corresponding to the surface expression of Gonggashan mountain. This supports a slip rate of the Xianshuihe fault zone that is reduced from ~14-16 mm/a on the northwest segment to 9 mm/a on the southeast segment; the excess slip is accommodated as uplift of Gonggashan mountain. An aseismic layer appears beneath the Longmenshan thrust fault zone. Seismicity reveals tear faults, perpendicular to the strike of the main fault zone within the thrust system and in the foreland basin, showing strike slip features with vertical dips. Back-thrust faults dipping to southeast are also indicated beneath the aseismic layer. Focal mechanisms indicate a combination of thrusting and right-lateral slip, contrary to previous suggestions that the Longmenshan thrust fault zone has a left-lateral slip component.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8123 Dynamics: seismotectonics
SC: Tectonophysics [T]
MN: Fall Meeting 2005