Three-dimensional seismic velocity structure beneath the M6.4 Yangbi，Yunnan earthquake region
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摘要: 利用2015年1月至2021年5月28日期间我国云南省漾濞县及周边地区固定台站和漾濞地震后布设的流动台站所记录到的近震资料，使用双差层析成像方法获得了该地震震区的高分辨率地壳三维速度结构和震源位置。重定位结果显示，漾濞M6.4地震序列主要沿NW−SE向展布，与维西—乔后—巍山断裂走向一致，地震主要集中在4—10 km的深度范围，呈约80°高倾角分布。结合定位结果与三维速度结构显示：漾濞M6.4地震序列的空间分布与速度结构变化具有相关性，主震位于P波、S波高低速异常交界处，这种介质物性变化的交界地带可能有利于中强地震的孕育和发生，余震主要分布在低P波速度、高S波速度和低波速比的脆性区域；沿漾濞地震序列的分布走向，主震两侧呈现完全不同的速度结构，其西北部具有明显的高P波速度、低S波速度特征，该地区高密度、强韧性的地层可能是阻挡漾濞地震的NW向破裂而呈单向破裂特征的原因。Abstract: This paper collected the seismic travel-time data both from temporary stations employed after Yangbi M6.4 earthquake and the seismic networks in Yangbi region and its vicinity from January of 2015 to 28 May, 2021 and performed the high-resolution inversion for three-dimensional velocity structure and accurate hypocentral locations by using double-difference seismic tomography method. The relocation results show that the sequence spread in the NW−SE direction along the Weixi-Qiaohou-Weishan fault. The focal depth in the area is generally in the range of 2−5 km with high dip angle of 80°. The three-dimensional velocity structure shows that the spatial distribution characteristics of the Yangbi M6.4 earthquake sequence are closely related to the velocity structure. The epicenter of the Yangbi M6.4 main shock was located near the P- and S-wave high-to-low-velocity anomaly transitional zones, which are favorable for occurrence of moderate-strong earthquakes, and the aftershocks are mainly distributed in the brittle region with low vP, high vS and low vP/vS. In addition, along the strike of Yangbi earthquake sequence, there are totally different velocity structures on both sides of main earthquake, and significant higher vP and lower vS anomalies are observed in the northwest of the Yangbi M6.4 main shock compared with the southeast part, which may obstruct the northwestward slipping of the seismogenic fault of Yangbi M6.4 earthquake, leading to the striking unilateral source rupture.
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图 1 漾濞地区构造背景、台站和2010年以来M4.0以上地震分布图
Figure 1. Tectonic settings，seismic stations and M≥4.0 earthquakes since 2010 in Yangbi region
图 2 射线路径、台站及地震分布
Figure 2. Geographical distribution of seismic stations （triangles），earthquake hypocenters （circles） and ray paths （blue lines） considered in this study
图 4 本文选取的最佳阻尼因子（a）和平滑因子（b）的折中曲线图
Figure 4. Trade-off curves for determining optimal damping （a） and smoothing （b） parameters
图 5 深度为5 km （a），10 km （b）和15 km （c）时P波（左）和S波（右）检测板的测试结果（0.1°×0.1°）
Figure 5. P （left） and S （right） wave checkboard resolution tests at the depth of 5 km （a），10 km （b） and 15 km （c） with the grid 0.1°×0.1°
图 6 漾濞M6.4地震序列重定位前（a）、后（b）的空间分布变化
Figure 6. Spatial distribution variation of the Yangbi M6.4 earthquake sequence before （a） and after （b） relocation
图 7 漾濞M6.4地震序列重定位前（a）、后（b）的震源深度变化
Figure 7. Focal depth variation of the Yangbi M6.4 earthquake sequence before （a） and after （b） relocation
图 8 重定位前（红）、后（蓝）走时残差直方图
Figure 8. Histograms of travel time residual before （red） and after （blue） relocation
图 9 不同深度z的vP （左）和vS （右）分布
黑色圆圈代表地震， 图（b）中AA′和BB′分别为平行、垂直于漾濞地震序列走向的剖面
Figure 9. vP （left） and vS （right） distribution at the depth z of 5，10 and 15 km
The circles represent earthquakes，and in Fig. （b） AA′ and BB′ are the two profiles parallel to and perpendicular with the strike of the Yangbi sequence，respectively
图 11 vP，vS和vP/vS沿纵向剖面的变化及漾濞地震序列震中分布图
Figure 11. vP，vS and vP/vS images along different vertical profiles and epicenter distribution of Yangbi earthquake sequence
表 1 本文所用的P波初始速度模型
Table 1. Initial P wave velocity model used in this study
深度/km vP/（km·s−1） 深度/km vP/（km·s−1） 0 4.4 20 6.0 5 5.6 25 6.1 10 5.8 30 6.3 15 5.9 50 8.0 
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