利用四川数字地震台网和流动地震台站在芦山 MS 7．0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．%Using 28 188 P-wave travel time data from 2 026 regional seismic events recorded by digital seismic network and transportable seismic stations of Sichuan Province after Lushan MS 7.0 earthquake,we conducted seismic tomo-graphy to obtain the middle-upper crustal 3D P-wave velocity structure in the souce region of Lushan earthquake and its surrounding areas.The findings indi-cate 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 vol-canic 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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