Deep structure of eastern margin of Bayan Har block and its adjacent areas by using teleseismic P-wave tomography

This paper collected the teleseismic P-wave travel data recorded by 203 broadband stations in digital seismic network of Sichuan and its neighboring areas from May 2009 to August 2016. And by using the finite-frequency tomography this paper carried out the inversion for P-wave velocity structure of the crust and upper mantle in the depth of 50−600 km in the eastern margin of Bayan Har block and its surrounding areas. The results show that the velocity structure of the Bayan Har block in the eastern part of the Qinghai-Xizang （Tibetan） Plateau and the surrounding crustal-mantle is characterized by obviously lateral inhomogeneity and zoning. Specifically, the Songpan-Garze trough fold system, the western Qinling and Qilianshan fold systems in the eastern margin of the Tibetan Plateau displayed low velocity, whereas the northwestern margin of the Sichuan basin and the southern edge of the Ordos block in the eastern part of the studied area showed obvious high velocity anomalies. P wave velocity structure characteristics of upper mantle suggested that the uplift of Songpan-Garze block is related to upper mantle upwelling. There are low velocity anomalies in the depth range of 50−250 km in Jiu-zhaigou earthquake area and the surrounding upper mantle in the east margin of Bayan Har block. The three-dimensional P-wave velocity structure also reveals that the source region of Jiuzhaigou M_S7.0 earthquake shows a high-velocity anomaly at the mantle transition zone with depth range of 400−600 km. Therefore, it was deduced that this anomaly may be caused by upwelling of hot asthenosphere material. With the crustal thickening and strong uplift of the Tibetan Plateau, the Sichuan-Qinghai block in the west of Longmenshan fault zone slipped and was pushed to the SE direction, which was influenced by the blocking of high velocity and high strength craton blocks to the eastern Qinghai-Tibet Plateau extrusion, its hard upper crust should form a huge stress accumulation, and Jiuzhaigou is located in the vicinity of the north-east boundary fault intersection at the Songpan-Garze block, where the stress is likely to be concentrated. These factors may result in the occurrence of the Jiuzhaigou M_S7.0 earthquake near the East Kunlun fault zone and the northern section of Minjiang fault.