[S-SS09] 地殻構造

[SSS09-P02] Crustal Structure delineation beneath Western Tibet using Local Earthquake Tomography

*Arghya Kusum Dey¹、Niptika Jana¹ (1.Indian Institute of Technology Indian School of Mines Dhanbad)

キーワード：Local Earthquake Tomography, Western Tibet, Karakoram Fault, Vp anomaly

The evolution of the Tibetan Plateau owes its origin to the convergence of the Indian and Eurasian plates since the early Eocene. Our study is focused to understand the tectonic architecture of western margin of Tibet (latitude range 27°to 40°and longitude range 75°to 90°) by obtaining the velocity structure using the Local Earthquake Tomography using data collected by the Y2 seismic network installed across Western Tibet. The extracted raw data was sorted, merged and eventually manually picked for the arrival time of P and S wave. A total of 768 earthquake events recorded from July 2007 to July 2010 was used, out of which only 123 events recorded with a high signal to noise ratio on at least 4 stations. During initial investigation, Velest algorithm was used in order to find out the initial hypocenter locations. The inversion is performed with an initial crustal Vp/Vs ratio of 1.75, with the crustal Vp varying between 5.7-6.4 km/sec, and mantle Vp varying between 8.0-8.2 km/sec. Initial hypocenter location suggest that the events lie in the shallow depth range between 30 km to 40 km, mostly clustering in the southern part of the study area (31⁰-32⁰ N, 78⁰-82⁰ E). Following the hypocentral locations and station corrections obtained from 1D inversion, the 3D inversion is performed using the tomoDD algorithm. Wave speeds of P and S wave and Vp/Vs ratio have been by a 21 x 21 x 18 grid stretched along N-S, E-W and Z-direction respectively. The resolution of the tomography models is deduced by the Derivative weighted sum, suggesting that that the model is well resolved in the mid crustal depth ranges (30 km) as compared to the lower crustal depths (60 km). Our study shows that the relocated events majorly lie along the southern part of the Karakoram fault (KKF) that transects across the study area, as is deduced from a slow anomaly observed from the Vp velocity. The Tibetan crust is comparatively faster north of the KKF. The results closely follow the cluster of relocated events, which is in corroboration with earlier attenuation studies, that highlights the distinction of the Indian and Eurasian crust across the KKF.