用形变资料反演1976年唐山地震序列的破裂分布

Rupture distribution of the 1976 Tangshan earthquake sequence inverted from geodetic data

1976年唐山发生了7.8级地震,相继又发生了两次大余震——滦县7.1级地震和宁河6.9级地震.地震发生在观测条件比较好的地区,水准测量和三角测量测得了地震的同震位移场.本研究采用原始水准测量数据,而不是采用根据水准数据处理的地面沉降图像,和三角测量数据反演了该地震序列的破裂分布.模型构建中考虑了滦县地震和宁河地震的断层形态和大小.结果表明,唐山地震主震断层有明显的右旋走滑性质,最大走向滑动错距>6 m,位于断层南段,北段的走滑分量明显小于南段.主震总地震矩达2.58×10^(20)N·m,与地震波反演得到的地震矩的量级相当;滦县地震断层总体表现为左旋正断层,释放地震矩达4.95×10^(19)N·m;宁河地震断层总体表现为右旋正断层,释放地震矩达3.94×10^(19)N·m,比地震波反演的地震矩大一个量级.据此可以推测唐山地震的无震滑移主要发生在宁河地震断层的西部上,滑动性质以正断层为主.该结果对于唐山地震序列后的动力学演变过程及余震发生机理有一定参考.

The Tangshan 1976 M7.8 earthquake was followed by two large aftershocks （the Luanxian M7.1 earthquake and the Ninghe M6.9 earthquake）. The earthquake sequence occurred in an area with good geodetic observation, and horizontal and vertical co-seismic displacements are obtained by triangulation net and levelling net. In this study, by using raw levelling data（not the subsidence image processed from raw levelling data） and triangulation observation data, we obtain the rupture distribution of strike slip and dip slip component on seismogenic faults. The geometry and size of the seismic faults of the Luanxian and Ninghe earthquake are considered in our model construction. The result shows that the seismic fault of the Tangshan mainshock manifested left-lateral strike slip with a maximum value greater than 6 m in the southern segment. The strike slip component on the northern segment is much less than that of the southern segment. The total seismic moment of the Tangshan mainshock is 2.58×10^20N·m, which is consistent with that determined by seismic waves. The seismic fault of the Luanxian earthquake shows left-lateral normal fault with total moment of 4.95×10^19N·m.The seismic fault of the Ninghe earthquake shows as a right-lateral normal fault with a total moment of 3.94×10^19N·m, which is an order of magnitude greater than the moment determined by seismic waveform. It is inferred that the aseismic slip of the Tangshan earthquake occurred on the western part of the Ninghe earthquake fault, demonstrated nearly normal fault property, which has important significances for geodynamic process and mechanism of aftershock occurrence after the Tangshan earthquake.