虎牙断裂的断层滑动及地震危险性定量研究

On August 8, 2017, a strong earthquake occurred on an unmapped extension of the Huya fault, Sichuan, China, causing severe damages on local communities. This earthquake represents a northward migration of strong earthquakes along a single strike-slip fault of 120-130 km-long, most of which was unzipped by the four Ms6.2-7.2 earthquakes in the past half century. This earthquake definitely plays an important role in active tectonics and stress pattern of eastern Tibet, and draws public attention on its seismogenic fault Huya. Investigations for the slip rate, contemporary tectonic deformation, the stage of the seismic cycle, tectonic stress accumulation and coseismic and postseismic stress loading caused by historical earthquakes along Huya fault, are with great seismological and social significance, specially in the situation of recovery running of the famous Jiuzahigou National Park and a rapid expansion of sight-seeing in the incoming years. This proposal will concentrate on the slip and seismic hazard quantitative evaluation on Huya fault by using geodetic measurements. Firstly, we are planning to develop a unified source rupture model of 2017 Jiuzhaigou earthquake to infer the rupture details of historical characteristic earthquakes occurred on Huya fault, and check the existence of a seismic gap. Secondly, we will preform a geodetic profile to capture the deformation and slip rate. By simulating the geodetic data using the finite element method, we try to study the different tectonic characteristics of the fault segments and state of stress accumulation. Finally, we will evaluate the combined contribution of interseismic stress accumulation, coseismic and postseismic stress loading caused by characteristic earthquakes and surround historical earthquakes, and provide the probability of the different magnitude earthquake occurrence on Huya fault.

2017年九寨沟地震Ms7.0发生于虎牙断裂北段，是半个世纪以来Ms6-7级地震第5次撕裂展布仅120km、滑动速率较慢的虎牙断裂，与1973-1976年间发生的4次Ms6.0以上地震组成该断裂特征地震序列。九寨沟地震的发生势必影响青藏高原东缘应力环境及地震背景，其孕震断层虎牙断裂现今运动状态如何，仅45年即经历5次Ms6-7级强震后是否还存在地震空区，构造应力加载及历史地震同震、震后应力影响将形成何种地震危险性态势，解答这些问题不仅是对科学问题的认识，并且随着九寨沟国家公园重新开放及游客数量攀升而具有紧迫、严峻的社会意义。本项目拟围绕虎牙断裂地震危险性开展以下工作：1)建立九寨沟地震精细破裂模型，为特征地震破裂习性提供参考，核查地震空区；2)基于观测剖面及运动学模型，研究分段活动性及应力累积状态；3)研究震间应力累积、同震、震后应力加载共同形成的应力分布及演化规律，给定地震发生概率。

2017年九寨沟地震Ms7.0发生于虎牙断裂北段，是半个世纪以来Ms6-7级地震第5次撕裂展布仅120km、滑动速率较慢的虎牙断裂。九寨沟地震的发生势必影响青藏高原东缘应力环境及地震背景。本项目围绕: 虎牙断裂现今运动状态；经历5次Ms6-7级强震后是否还存在地震空区; 构造应力加载及历史地震同震、震后应力影响将形成何种地震危险性态势等三方面内容开展，并取得了如下进展：.1）九寨沟地震震源模型。联合静态/高频GNSS、InSAR、远震地震波，对断层几何及震源运动学参数进行约束。滑动沿走向25-30km长度、倾向18-20km展布于倾角为88°的断层面。破裂同时向北西向和南东向双侧传播，释放总地震矩9.0×1018 Nm，破裂传播速度为3.2km/s。滑动分布由两个凹凸体组成，静态应力降分别为3.3Mpa和3.6Mpa。.2）虎牙断裂历史强震及汶川地震对九寨沟地震应力扰动。1960年章腊地震、1973年地震、1976-08-16地震的同震显著加速了九寨沟地震的发生，而1976-08-22地震、1976-08-23地震和汶川地震则对该断层面具有应力加载。汶川震后形变GPS观测约束的九寨沟区域下地壳粘滞系数为1×1019 Pa∙s，并显示汶川地震近10年的震后效应加载了0.036bar，在九寨沟地震孕震的最后阶段起到了关键的推进效应。.3）虎牙断裂的大震空段及其潜在地震危险性。九寨沟地震对岷江断裂北段约30km长具有的同震、震后应力加载效应；对塔藏断裂西北段的同震、震后应力加载进一步增加玛沁-玛曲段的地震危险性。虎牙特征地震的重复周期下限值不少于1000年，近50年的强震频度表明该断裂已处于一个地震周期的晚期阶段。位于虎牙断裂带上此次滑动南部终点至1976年序列地震北端的滑动空区具备孕育Mw6-7的地震危险性。