Determination of the NW-trending faults in Xinfengjiang Reservoir dam by using high-precision small earthquake data

Based on the M_L≥1.0 earthquake catalogue of Xinfengjiang Reservoir area in Heyuan of Guangdong Province during the period from 2009 to 2015，this paper delineates three NW-trending small earthquake dense zones in dam-gorge region. According to the principle that the small earthquake clustering occurs in and around active fault plane，this paper calculated the fault plane parameters of the three dense zones including the strike，dip，dip angle，length，depth and geographic position by using both simulated annealing algorithm and Gauss-Newtonian nonlinear inversion algorithm，and finally got the geometric shape of the NW-trending faults. The results show that the three faults are NW--NNW trending in the depth range of 3--12 km，and are of left-lateral strike-slip with a little vertical slip component. Furthermore，the analyses of inversion results combined with previous data indicate that，the Baitian-Shuangtang fault，one of the three NW-trending faults，was proved to be the seismogenic fault of M_S6.1 Xinfengjiang earthquake in 1962，and from Baitian to Shuangtang，the Heyuan fault is interrupted with gap of 5 km，and the gap is filled with part of the Baitian-Shuantang fault. The junction area of endpoints and tangent points of the three faults intersected near the reservoir dam，which leads to stress concentration more easily there，and becomes the most active seismic region in the reservoir.