2017年九寨沟MS7.0地震震源区速度结构与余震分布

采用九寨沟M_S7.0 （M_W6.5）地震的余震直达P波、S波走时数据，通过体波走时层析成像方法，获得了震源区及其邻区的P波和S波速度结构，并利用成像结果对余震进行了重定位。结果显示:余震主要集中分布于高、低速异常交界处偏低速异常一侧，呈走向NNW，倾向SW，倾角较高的分布特征；余震序列两侧的P波、S波速度结构揭示了发震断层两侧介质性质的差异，即上盘为刚性较强的高地震波速度区，下盘为刚性较弱的低地震波速度区。由余震分布特征和地震波速度结构推断:九寨沟地震发生在上地壳底部，发震断层具有上盘地震波速度高、下盘地震波速度低的特征；主震引起的后续破裂在上地壳内部的剧烈形变区内传播，破裂能量终止于25 km深度附近。 

Velocity structures and aftershock distribution in the source region of the 2017 Jiuzhaigou MS7.0 earthquake

Using the arrival times from the aftershocks of 2017 Jiuzhaigou M_S7.0 （M_W6.5） earthquake, the P- and S-wave velocity structures in the source region and its adjacent area are obtained by means of the body wave tomography method. The results show that the aftershocks, characterized by NNW-striking, SW-trending and a high dip angle, are mainly concentrated on one side of the low-velocity anomalies at the intersection of high- and low-velocity anomalies. The P- and S-wave velocity structures reveal the differences of crustal property on both sides of the seismogenic fault: the hanging-wall is a high-velocity zone with strong rigidity while the footwall is a low-velocity zone with weak rigidity. According to the distribution of aftershocks and the seismic velocity structure, it can be inferred ultimately that the 2017 Jiu-zhaigou M_S7.0 earthquake occurred on the lower interface of the upper crust, following with the rupture propagated in a dramatically deformed area in the upper crust and terminated near the depth of 25 km, and the seismogenic fault has the property of low-velocity hanging-wall and high-velocity footwall.