郯庐断裂渤海湾北段地震活动及其指示意义

郯庐断裂渤海湾北段纵贯辽宁省,现今仍然活动,1975年海城MS7.3地震就发生在该断裂系统上.利用2005年5月至2006年4月布设的渤海湾流动地震台阵观测数据,结合辽宁省固定地震台网数据,重新拾取了780个区域小地震,并使用双差地震定位方法获得了546个地震事件的定位结果.通过自助法得到双差定位的平均水平误差和深度误差分别为1.5和1.9km.结果显示在观测周期内,郯庐断裂渤海湾北段主断裂地震不活跃,但是主断裂东侧的海城河断裂和太子河断裂小震沿断裂呈条带状密集分布.利用gCAP(general Cut and Paste)方法计算了区域地震震源机制解,选取出4个可信的震源机制解;综合前人已有结果,认为研究区的小地震震源机制主要以走滑与正断为主,与区域构造应力场吻合良好.海城河和太子河断裂的地震活动深达中下地壳,结合热流数据和岩石物理实验,推测研究区中下地壳由干燥的镁铁质岩石组成.

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_S7.3 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.