Seismicity of the northern section of the Tanlu fault zone in the Bohai Bay and its implications

The northern section of the Tanlu fault zone, extending through Liaoning Province, is considered active at present, at which the 1975 M-s 7. 3 Haicheng earthquake occurred. In this study, we re-pick up 780 events around this fault and then from which relocate 546 small ones using a double-difference relocation method (HypoDD) based on the seismic data recorded by Bohai Bay seismic network and Liaoning Provincial Seismic Network from May 2005 to April 2006. The horizontal and vertical location uncertainties are 1. 5 and 1. 9 km, respectively. The location results show that the seismicity in the main fault of this section is not active. While the banded dense distributions of quakes are present along both the Haichenghe fault and the Taizihe fault east of the main fault in the Bohai aforementioed. General Cut and Paste method (gCAP) is used to calculate the focal mechanism solutions of these events, from which four solutions are reliable. In combination with previous relevant studies, the focal mechanisms of the small earthquakes in the study region are dominated by strike-slip and normal faulting, which is consistent with the regional tectonic stress field. The focal depths on the Haichenghe fault and the Taizihe fault extend downward to the mid-lower crust. Combining with heat flow data and petrophysical experiments, this work speculates that the middle and lower crust beneath study region is likely composed of dry mafic rocks.