Tomography of the three dimensional P-wave velocity structure in the source region of the MS7.0 Lushan, Sichuan, earthquake and its surrouding areas
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摘要: 利用四川数字地震台网和流动地震台站在芦山MS7.0地震震后(2013年4月20日—6月23日)记录到的2026次区域地震事件的28188条P波到时资料，采用地震层析成像方法反演得到了芦山地震震源区及其周边区域中上地壳P波三维速度结构. 结果表明，浅部地壳的P波速度异常分布特征与地表地质构造、 地形和岩性密切相关，即成都断陷盆地表现出与第四纪沉积有关的低速异常区；犍为、 乐山一带的川中微升区和川青块体龙门山以西的邻近地带均表现为与构造抬升有关的高速异常；宝兴、 康定附近分布的基性火山岩及火山碎屑岩均呈局部高速异常分布. 芦山地震震源位于高低速异常分界线附近且偏向高速体一侧，其下方存在明显的低速异常分布，可能与流体的存在有关. 流体的作用导致中上地壳内部发震层的弱化，使孕震断层易于破裂，可能对芦山地震起到了触发作用. 芦山地震与汶川地震两次地震的余震密集区相距50 km，这50 km地震空区震源体的深度范围附近目前正处于高速异常区内，加之龙门山断裂带西南段又具有比较典型的断错地貌发育，使得该段地震空区(大邑—邛崃活动断裂破裂空段)现在所处的深浅部构造环境变得复杂，其潜在的地震危险性仍值得进一步关注.Abstract: Using 28188 P-wave travel time data from 2026 regional seismic events recorded by digital seismic network and transportable seismic stations of Sichuan Province after Lushan MS7.0 earthquake, we conducted seismic tomography to obtain the middle-upper crustal 3D P-wave velocity structure in the souce region of Lushan earthquake and its surrounding areas. The findings indicate that the spatial distribution of velocity anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and lithology. Chengdu basin shows low-velocity anomalies associated with the Quaternary sedimentary area; the slight rising area of Qianwei-Leshan in central Sichuan basin and the adjacent district to the west of the Longmenshan fault of Sichuan-Qinghai block are characterized by high-velocity anomalies related to tectonic uplift; the area near Baoxing and Kangding with basic volcanic rocks and volcanic clastic rocks presents obvious high-velocity anomaly. The epicenter of the Lushan earthquake is located near the boundary of the high velocity and low velocity, and obvious low velocity anomaly exists below this region. This low velocity anomaly body may be associated with the presence of fluid. The fluid effect leads to the weakening of seismogenic layer in the interior of middle-upper crust and may trigger the Lushan earthquake. The aftershock areas of Lushan earthquake and Wenchuan earthquake are 50 km apart. At the depth range around the earthquake source, this 50 km seismicity gap exhibits high-velocity anomaly. Combined with the typical dislocation geomorphic structure at the southwest section of the Longmenshan fault, the deep and shallow structures are complex beneath this seismic gap, namely, the Dayi-Qionglai active fault segment that is not ruptured previously, has potential seismic hazards that deserve further study.
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图 1 芦山地震震源区及周边区域地质构造图
① 茂汶—汶川断裂；② 北川—映秀断裂；③ 彭县—灌县断裂；④ 龙门山山前断裂；⑤ 玉科断裂；⑥ 抚边河断裂； ⑦ 岷江断裂；⑧ 蒲江—新津断裂；⑨ 龙泉山断裂；⑩ 鲜水河断裂；⑪ 大渡河断裂；⑫ 合海子断裂；⑬ 玉农希 断裂；⑭ 大凉山断裂；⑮ 金坪断裂；⑯ 保新厂—凤仪断裂；⑰ 荥经—马边—盐津断裂；⑱ 长山镇断裂
Figure 1. Geologic map around the source area of the Lushan MS7.0 main shock and its vicinity The gray asterisk denotes the Wenchuan MS8.0 earthquake,green asterisk denotes the Lushan MS7.0 earthquake.
① Maowen-Wenchuan fault; ② Beichuan-Yingxiu fault; ③ Pengxian-Guanxian fault; ④ Longmenshan front range fault; ⑤ Yuke fault; ⑥ Fubianhe fault; ⑦ Minjiang fault; ⑧ Pujiang-Xinjin fault; ⑨ Longquanshan fault; ⑩ Xianshuihe fault; ⑪ Daduhe fault; ⑫ Hehaizi fault; ⑬ Yunongxi fault; ⑭ Daliangshan fault; ⑮ Jinping fault; ⑯ Baoxinchang-Fengyi fault; ⑰ Xingjing-Mabian-Yanjin fault; ⑱ Changshanzhen fault
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图 2 本文所用地震台站分布(a)和地震震中分布(b). 图(b)左上角为芦山地震震源区地震事件分布局部放大图
Figure 2. Distribution of seismic stations (a) and earthquake epicenters (b) used in this study The asterisk denotes the Lushan MS7.0 main shock. In Fig.(a) denote portable seismic stations, squares denote fixed stations; In Fig.(b),AA′ and BB′ denote two profiles. Upper left corner gives the local amplification of earthquakes distribution in Lushan earthquake source region
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图 3 检测板分辨率测试结果
The depth of each layer is shown on the upper left of each panel,the scale for velocity perturbation (in percentages) is shown at the bottom
Figure 3. Results of checkerboard resolution test
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图 4 研究区域内不同深度层的射线分布
The depth of the layer is shown at the upper left corner of each subfigure, and the hit counts scale is shown at the bottom
Figure 4. Distribution of the counts of the P-wave rays passing through each grid node
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图 5 芦山地震震源区及其周边区域的P 波速度异常分布. 图中黑线为区内主要断裂
Figure 5. P wave velocity perturbation images around the source region of Lushan earthquake at six depth slices (1,8,12,16,20 and 24 km) Black lines denote main faults in the study region,and blue asterisk denotes the Lushan MS7.0 main shock
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图 6 穿过芦山主震震源区的两个相互垂直的P波速度剖面示意图
(a) 垂直于龙门山断裂带的A-A′剖面；(b) 平行于龙门山断裂带的B-B′剖面. 白色星号代表芦山主震投影，黑色星号代表汶川主震投影
Figure 6. P wave tomography along two cross-sections orthogonally passing through the source area of the Lushan main shock
(a) The cross-section A-A′ perpendicular to the Longmenshan fault zone; (b) The cross-section B-B′ along the Longmenshan fault zone. The white asterisk denotes the Lushan MS7.0 main shock,while the gray asterisk denotes the Wenchuan MS8.0 main shock
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