Three-dimensional P-wave velocity structure around the sources of Ludian and Qiaojia earthquakes
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摘要: 本文利用2010年1月至2020年6月巧家地震和鲁甸地震震源区周围发生地震事件的走时观测资料，应用双差层析成像方法获得了2014年MS6.5鲁甸地震和2020年5月18日巧家MS5.0地震周边区域中上地壳的P波速度结构。成像结果显示：整个研究区域的速度结构存在很强的非均匀性：在鲁甸地震震源区附近，浅部存在速度高达6.4 km/s的显著高速异常，10 km深度以下存在速度低至约5.8 km/s的明显低速异常；鲁甸地震的初始破裂点位于高低速异常的过渡带；在2010年巧家MS4.8和2020年巧家MS5.0地震的震源区，尽管浅部没有显著的高速异常，但在主震的初始破裂点下方均存在明显的局部低速异常；巧家北部中上地壳存在明显低速异常。Abstract: Using travel times of earthquakes from January 2010 to June 2020 in the region of Qiaojia and Ludian, we obtained three-dimensional P-wave seismic velocity structures around the source areas of Qiaojia and Ludian earthquakes by double-difference tomography. The result show that the velocity structure in this area is strongly heterogeneous, which affects the distribution of active faults. In the source region of the Ludian earthquake, there is a significant P wave high velocity anomaly with a maximum velocity up to 6.4 km/s, and an obvious P wave low velocity anomaly with a minimum velocity of about 5.8 km/s below the depth of 10 km. The occurrence of Ludian earthquake may be related to the difference of mechanical properties between deep and shallow rocks and the presence of fluid in mid-upper crustal rocks. In the source areas of the 2010 Qiaojia MS4.8 earthquake and the 2020 Qiaojia MS5.0 earthquake, although there are no significant high-velocity anomalies in the shallow crust, there are obvious local low-velocity anomalies beneath the initial rupture point of the main shock. In addition, there are obvious low velocity anomalies in the middle and upper crust of northern Qiaojia.
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图 1 研究区周边的活动断裂与2010年以来MS＞4.0地震分布（房立华等，2014）
F1：则木河断裂；F2：小江断裂；F3：大凉山断裂；F4：莲峰断裂；F5：龙树断裂；F6：包谷垴—小河断裂；F7：昭通—鲁甸断裂；F8：会泽—彝良断裂
Figure 1. Distribution of active faults and the epicenters of MS＞4.0 earthquakes in the studied region since 2010
F1：Zemuhe fault；F2：Xiaojiang fault；F3：Daliangshan fault；F4：Lianfeng fault；F5：Longshu fault；F6：Baogunao-Xiaohe fault；F7：Zhaotong-Ludian fault；F8：Huize-Yiliang fault
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图 3 利用L曲线法选择最佳的平滑因子和阻尼因子
（a） 由L曲线法确定光滑因子为200；（b）光滑因子为200时利用L曲线确定阻尼因子为100
Figure 3. Trade-off curves of smoothing weight parameter and damping parameter
（a） Best parameter of smoothing was 200；（b） Best parameter of damping was 100
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图 4 深度z为2，5，8，12，17 km时的P波速度结构分辨率
Figure 4. Checkerboard resolution tests of P wave velocity structure at the depth z of 2，5，8，12 and 17 km
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图 5 深度为2 （a），5 （b），8 （c），12 （d）和17 km （e）时P波水平层析成像结果及余震分布
F3：大凉山断裂；F4：莲峰断裂；F5：龙树断裂；F6：包谷垴—小河断裂；F7：昭通—鲁甸断裂
Figure 5. Horizontal sections of P-wave velocity structure and aftershocks distribution at the depth of 2 （a），5 （b），8 （c），12 （d） and 17 km （e）
F3：Daliangshan fault；F4：Lianfeng fault；F5：Longshu fault；F6：Baogunao-Xiaohe fault；F7：Zhaotong-Ludian fault
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图 6 沿AA′和BB′剖面的P波速度结构和地震分布
图中黑点为2010年1月至2020年6月之间经重新定位的MS＞0.5地震
Figure 6. P-wave velocity structure along vertical cross-sections AA′ and BB′ and distribution of earthquakes
The black dots denote epicenters of earthquakes （MS＞0.5） from January of 2010 to June of 2020
表 1 研究区P波初始速度模型
Table 1 Initial P wave velocity structure
上界面深度/km vP/（km·s−1） vP/vS 0 5.20 1.73 2 5.70 5 5.88 8 6.00 12 6.15 17 6.27 25 5.60 30 6.70 
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