于田3次强震的断层相互作用及应力触发机制研究

In this project we will focus on two major scientific questions: the stress triggering of the three Yutian mainshocks and their fault motion transferring mechanism. Using interferometry of SAR images acquired before and after an eartquake by Radar satellite, we firstly obtain coseismic deformation of the three Yutian earthquakes. Whenever there is no suitable SAR images for interferometry， we will try to use high-resolution optimal satellite images. With offset tracking technique, we will get coseismic deformation of that specific Yutian earthquake. Based on the coseismic deformation by InSAR or offset tracking, we will analyze the Yutian macro epicenters, the possible surface breaks and their fault motion pattern. We will then apply a joint inversion of seismic data and InSAR measurements or offset tracking data, aiming at obtaining a robust rupture process and slip distribution of the 2008-2014 Yutian earthquakes, based on which we can analyze the kinematic features, the rupture directivity, and the earthquake mechanism of each Yutian earthquake. Using seismic stations distributed close to the Yutian epicenters, such as from Xinjiang or Tibet province, we then invert for the focal mechanisms of the Yutian mainshocks and aftershocks magnitude larger than Ms 4.0. With the solutions of the focal mechanisms, we can obtain the local principal stress axis. Based on the comprehensive analysis of focal mechanism of both the historic events and the 2008-2012-2014 three Yutian events and their rupture directivities, we try to analyze the fault motion transferring mechanism between the 2008 Yutian earthquake and the 2012-2014 Yutian earthquakes. Based on previous seismic structural exploration, local active faults and block model, and the coseismic slip distribution from joint inversion, we then set up a three-dimensional rheology model, with the consideration of the lateral heterogeneity between Tarism basin and Tibetan plateau, to model the stress evolution, including the coulomb stress, normal stress and shear stress, and so on. And finally the stress trigerring effect among the Yutian earthquakes can be studied and the seismic hazards after the 2014 Yutian event be evaluated based on the stress evolution vectors.

本项目拟采用InSAR干涉处理、偏移量追踪等卫星遥感影像处理技术，分别获取2008、2012、2014年3次于田强震的同震形变场、地表破裂带及宏观震中位置等；在此基础上，采用形变数据与地震波数据联合反演，分别获取于田强震群各个地震的发震断层三维构造及其震源时空破裂过程，从而深入研究3次于田强震的震源破裂运动学特征及发震机理等；利用区域地震台网数据反演3次于田强震的余震震源机制解，并综合震源时空破裂过程优势破裂方向、历史地震及其震源机制等，研究走滑断层尾端正断层运动与走滑运动互相转换特征及应力作用机制；利用反演获得的震源时空破裂过程、余震震源机制以及区域块体介质参数，建立三维地质模型，模拟区域中长期动态应力演化过程，重点分析库仑应力、正应力及剪切应力演化特征等，从而研究3次于田强震之间的同震弹性与震后介质粘弹性应力触发机制，认识3次强震之间的转换机制和相互关系，也为未来断层地震危险性趋势分析提供依据。

强烈地震间应力触发机制一直是国际地震学界广泛关注的研究热点。同时，断层间存在相互应力作用，从而导致某一地震发生后，相邻段落或邻近断层的应力状态发生调整或改变，触发或抑制后续强震的发生。新疆于田地区分别于2008年3月21日、2012年8月12日及2014年2月12日发生了Ms7.3、Ms6.3、Ms7.3级强烈地震，称之为于田强震群。此次于田强震群为地震之间的应力触发机制及青藏高原北缘与塔里木盆地交接区域的地震危险性趋势研究提供了独特机会。因而，采用InSAR干涉处理、偏移量追踪等技术，获取了于田强震群的同震形变场、地表破裂带位置等；基于弹性位错模型反演InSAR等数据，获得了于田强震群的发震断层及断层滑动分布，并深入探讨了其孕震机理。结果表明：1）2008年于田地震的地表破裂长度约30km；地表破裂类型以倾斜正断层陡坎和类地堑正断层陡坎为主，有少量水平断错冲沟；InSAR形变场显示，两盘相对运动的趋势大体显示出2008年于田地震以拉张运动为主兼具少量走滑的运动特点；2）发震断层中段和南段上形成较大的滑动分布集中区，最大滑动量达3.2m，而在断层北段则滑动相对较小；反演结果并未出现与走滑断层类似的“浅部滑动缺失”现象，破裂到达地表，且主要集中于0~14km深度；3）模拟获取的青藏高原西北缘中下地壳及上地幔粘滞系数显示，青藏高原西北缘在70km内存在显著的两层软物质，分别位于20-35km及50-70km深度，而塔里木盆地介质在地壳部分完全弹性，厚度达70km；4）2008年于田强震对其周边尤其是震中区域的同震应力触发效应非常显著，量级较大，导致其主震的震中区域余震分布密集；但由于距离较远，2008年于田地震的同震效应对后续2012及2014年于田地震触发效应不明显；5）2008年于田地震对后续2012及2014年的于田地震震后粘弹性效应较为明显，于田地震群之间的震后粘弹性应力触发在10年弛豫时间尺度内达到了库仑应力触发的阀值（0.1MPa），具有显著的触发效应。