基于密集台阵的长宁6.0级地震区浅部精细速度结构研究

2019年6月17日长宁6.0级地震，是迄今为止四川盆地东南缘震级最大、破坏性最强的地震。目前对其成因及发震构造的认识尚存争议，但震区浅部构造对此次地震的控制作用却不容忽视。原因如下：长宁6.0级地震序列的震源深度浅（均值约3.0 km），表明浅部构造发生活动；震区多处有深约3km的采盐井，存在注水活动致使先存断层再活化的可能。已有的震区速度结构模型尚不足以为上述问题的解决提供精细结构约束。为此，本项目拟在长宁6.0级主震区开展三分量、短周期、超密集线性台阵观测并搜集震区已有短周期密集台阵数据。基于噪声自相关照明技术，利用高频体波信息，揭示研究区5km以浅的精细反射结构；基于面波噪声成像技术，反演5km以浅的S波速度结构。联合反射结构和S波速度，构建长宁6.0级地震区5km以浅精细速度结构模型，为解决长宁主余震序列的发震构造争议，降低盐矿、页岩气开采活动诱发地震的风险提供地震学依据。

The Changning Ms 6.0 earthquake on June 17, 2019 is thus far the largest and most destructive earthquake in the southeast margin of the Sichuan Basin. Although the seismogenic mechanism and structure of this event is still controversial, it is cannot be ignored that the shallow structure plays main role on occurrence of this earthquake. The reasons are as follows: the main shock and its aftershocks is shallow since their average hypocenter depths are about 3 km, indicating the tectonic activity in the shallow structure. Meanwhile, there are many salt mining wells with a depth of about 3 km, and water injection probably causes the reactivation of pre-existing faults. Therefore, the fine structure in the shallow part of the Changning seismic zone plays great significance for the scientific determination of the seismogenic structure of the Changning Ms6.0 earthquake sequence. Previous seismic velocity structure models are not fine enough to provide velocity structure constraints for solving the scientific issues discussed above.Consequently,this research plans to deploy a seismic dense array consisting of 250 three-component, short-period seismographers across the Changning Ms6.0 main shock region. Using the daylight seismic imaging from ambient noise autocorrelations, the high-frequency body wave signals are retrieved to reveal the fine reflection structure shallower than 5km. Meanwhile, we conduct high-frequency ambient-noise tomography to invert shear-velocity structure shallower than 5km. Finally, we combine the body-wave reflectivity response and S-wave velocity to construct the high-resolution structure model in the top 5km beneath the Changning 6.0 earthquake region. Our velocity model can provide seismological foundation to resolve the seismogenic tectonic disputes, to understand how the shallow structures control seismic activity and to reduce risk of induced earthquake by both of water injection in salt mining and shale gas hydraulic fracturing in the region of Changning Ms6.0 earthquake.