Velocity structures and aftershock distribution in the source region of the 2017 Jiuzhaigou MS7.0 earthquake
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摘要: 采用九寨沟MS7.0 （MW6.5）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。Abstract: Using the arrival times from the aftershocks of 2017 Jiuzhaigou MS7.0 （MW6.5） 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 MS7.0 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.
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Keywords:
- Jiuzhaigou earthquake /
- tomography /
- aftershocks relocation /
- seismic velocity
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图 1 九寨沟MS7.0地震及其余震震中、震源区附近台站及主要断裂分布
Figure 1. Location of the Jiuzhaigou MS7.0 earthquake and distribution of aftershocks，stations and faults near the main-shock region
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图 2 一维P波速度（a）、S波速度（b）反演结果和走时残差随迭代次数的变化（c）
Figure 2. P-velocity （a）， S-velocity （b） obtained by one-dimensional inversion and the curve of data residual versus iteration number （c）
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图 4 一维重定位前（a）、后（b）地震走时残差均方根统计图
Figure 4. The traveltime residuals root-mean square histograms of original events （a） and relocated events （b）
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图 3 一维重定位后的震中分布（a）及震源沿纬度（b）和经度（c）方向的纵向投影
Figure 3. Distribution of relocated hypocenters （a）， the vertical projection of aftershocks along latitude （b） and longitude （c）
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图 5 L-曲线（a）和三维反演网格分布（b）
Figure 5. The L-curve （a） and the distribution of inversion grids （b）
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图 6 P波（a）、S波（b）速度在5，10，15和20 km深度处的水平剖面和纵向剖面AA′ 的初始棋盘格模型
Figure 6. The initial checkboard models for P-wave （a），S-wave （b） velocity on the horizontal profile at depth of 5， 10， 15，20 km as well as on the vertical profile AA′
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图 7 P波（a）、S波（b）速度在5，10，15和20 km深度的水平剖面和纵向剖面AA′ 的检测板测试结果
Figure 7. The results of checkboard tests for P-wave （a），S-wave （b） velocity on the horizontal profile at depth of 5， 10， 15， 20 km as well as on the vertical profile AA′
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图 9 AA′ 剖面上的P波 （a）和S波 （b）速度成像结果
Figure 9. The tomography images of vP （a） and vS （b） along vertical profile AA′
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