关键词: 青藏高原, 南北向裂谷, 地震定位, 震源机制, 蛇绿岩混杂带

Abstract: We collect the seismic waveforms of the MS5.6 earthquake occurred in the southern part of Nima, central Tibetan Plateau on July 24, 2009 from simultaneous observation stations: Tibet Seismic Network, Western Tibet (or Y2), and TITAN. The Hypo2000 method is applied to relocate the seismic location accurately. Compared with the result relocated by the Tibet Seismic Network (31.30 °N, 86.10 °E), our result (31.08 °N, 86.05 °E) is more reliable with well-distribution of stations. It is similar to the GCMT result (31.05°N, 86.10°E) which is inverted by the moment tensor localization method. Based on the relocated result of the event, we apply the Cut-and-Paste (CAP) inversion method to invert focal mechanism and focal depth. The result shows that the earthquake has a focal depth of 18.5km from the seal level, and a clear characteristic to be a nearly east-west strike-slip fault. According to surface geology and deep structure in the region from surface to deep. The earthquake.was triggerd on the top of the brittle-ductile transition zone with a low seismic velocity between the middle and upper crust beneath south boundary faults of the Seng-ge Kambab-Lhaguo Tso-Yongzhu-Jiali ophiolite mélange zone (SYMZ), 30km away from the Tangra-Yumco Rift to the west. The occurrence of the earthquake indicates that SYMZ formed in the Late Jurassic has reactivated as an east-west trending strike-slip during the quick uplift of the plateau. Thus, it would be of a great significance to understand the geodynamic mechanisms of east-west trend extension within the Tibetan Plateau.

Key words: Tibetan Plateau, north-south trending rift, seismic location, focal mechanism, ophiolite mélange zone