The characteristics of 3D P-wave velocity structure of Middle-upper crust and reservoir water infiltration-diffusion in Xinfengjiang Reservoir of Guangdong

In this paper, we determined the Xingfengjiang (XFJ for short) Reservoir area earthquake sequence location from June 2007 to July 2014 and 3D P-wave velocity structure by a simultaneous inversion method. On that basis, we study the 3D P-wave velocity structure feature and the distribution characteristics of earthquake sources. The results show that the P-wave velocity presents obvious lateral and vertical inhomogeneity in the middle-upper crust. The P-wave velocity of XFJ reservoir is higher than that in the margin in shallow crust. In 5 similar to 10 km depth upper crust, there exists a NW-trending high velocity zone (HVZ) and 2 low velocity discontinuity zones (LVDZ) from Baitian at the downstream of dam to Xichang at the end of the reservoir. The 2 LVDZs are respectively located in the M6.1 hypocentral area and at the intersection of Renzishi fault and Nanshan-Aotou fault. At the 2 LVDZs, we speculated that the reservoir water infiltrates into 13 similar to 14 km depth crust along the Renzishi fault and Shijiao-Xingang-Baitian fault. In 10 similar to 14 km depth crust, with the NE-trending Daping-Yanqian fault as a boundary, it is a HVZ with maximum velocity 6.2 km.s(-1) in the NW side, in the SE side it is a significant low velocity zone (LVZ) from Huilong at the center of the XFJ reservoir to Baitian at the downstream of dam, it is the area of reservoir water infiltration-diffusion also, and it is the most concentrated area of mid-strong earthquakes too. Most of earthquakes occurred at the HVZ, the gradient belt between different velocity blocks, or the side of infiltration channel. The M6.1 hypocentral area is located on the gradient belt above 2 HVZs whose volume is about 5x5x5 km(3) and 7x8x5 km(3) respectively.