2017年四川九寨沟MS7.0地震及余震精定位研究

2017年8月8日21点19分四川省九寨沟县发生M_S7.0级强震，本文应用对相对位置和绝对位置都有较好约束的hypoRelocate精定位方法，对8月15日前M_L大于1.5的854个余震重新定位.重定位使用了6178个Pg走时、3363个Sg走时、158929个地震对的相对走时差以及74个M_L大于3.5的34个地震构造的SH尾波互相关走时.定位结果显示，余震区呈NW向分布，长度约35 km、宽度约12 km.以主震为中心，余震形成西北和东南两个丛集区，其中西北区的余震展布较东南区宽，发震断层近似垂直，与主震的震源机制解吻合.余震深度主要集中在5~15 km，主震深度为16 km.从沿断层走向的垂直剖面可见，主震周围一直延伸到地表的区域，余震相对稀少，出现明显空区，这种现象可能与主震发生时该区域的滑动位移较大，应力得到充分释放有关.另外，我们还发现震后60 h之后余震的深度明显变浅，且西北向的深度明显浅于东南向.通过对比分析流动台站布设前后余震深度的分布特点，我们认为这种时空演化差异主要是由流动台站陆续布设和使用造成的，对余震的主要分布特征影响不大.

A high precision relocation study of the MS7.0 Jiuzhaigou earthquake and the aftershocks occurred in 2017

On August 8^th, an M_S7.0 strong earthquake occurred in Jiuzhaigou, Sichuan province. We utilized the high precision relocation method, hypoRelocate, which has good constraints on both absolute location and relative location, to relocate 854 aftershocks that are larger than M_L1.5 and recorded before August 15th. In relocation, we used 6178 Pg travel times, 3363 Sg travel times, 16365 differential travel times and 74 SH coda cross-correlogram travel times of 34 events that are larger than M_L3.5. The relocation result indicates that the aftershocks are distributed along northwest with a length of 35 km and width of 12 km. The mainshock separates the aftershocks into northwestern crowd and southeastern crowd and the northwestern crowd spreads more widely than the southeastern. The seismogenic fault is nearly vertical that is consistent with the focal mechanism of the mainshock. The mainshock is relocated at the depth of 16 km and most of the aftershocks are between 5 km and 15 km. From the depth profile along the fault, we found that the seismicity near the main shock area is less active and this area extends to the surface. The phenomenon can be explained by that the area is the large slip area of the mainshock and the strain and stress are released adequately. Besides, we also found that the northwestern event crowd is shallower than the southeastern and the events' depths have temporal change that both the two event crowds move shallower after 60 hours after the main shock. After comparison and analysis, we found that the utilization of temporary stations caused the spatial and temporal evolution and that does not affect the main features of the distribution of the aftershocks.