2014年2月12日Mw6.9于田地震震源破裂过程及对周围断层的应力影响

Rupture process of 12 February 2014,Yutian M_w6.9 earthquake and stress change on nearby faults

2014年2月12日,在新疆于田县发生了里氏7.3级地震.在该地震震中附近,前人研究证明发育了大量规模不同的活动断层（如康西瓦断裂与贡嘎错断裂等）.根据地震触发理论,地震发生后因地壳同震变形会导致其周边不同性质断裂破裂应力发生变化,进而影响其地震的潜在危险性.本文利用地震远场波形记录,反演了该地震滑动模型.之后,根据弹性无限半空间位错理论,计算了该地震在近场范围内活动断裂上的同震应力变化.其目的在于讨论于田地震引起的附近断裂上的库仑应力变化以及这些活动断裂可能潜在的地震危险性.在地震发生后,从国际地震学联合会（IRIS）地震数据中心,下载了震中距离介于30°-90°的地震远场波形记录,为保证台站方位角分布均匀,从中挑选了27个不同方位角的高信噪比地震记录参与理论地震图的生成和波形反演过程.我们采用广义射线理论计算生成远场理论地震波形数据.每个子断层参数的反演则利用基于全局化反演的快速模拟退火反演方法.在有限断层反演过程中,我们采用了强调波形拟合的相关误差函数作为待反演的目标函数,拟合的断层参数使目标函数为最小.之后,根据弹性无限半空间位错理论,以库仑破裂准则为基础,结合反演得到的地震震源机制解和地震位错模型,计算该地震引起的近场断层面上库仑应力的变化.由远场波形计算结果可以看到,于田地震的震源深度为10km,地震断层的倾角约71.9°,破裂面上最大的同震位移达到210cm,以左旋走滑为主并具有正倾滑分量,地震能量主要在前15s内释放.由此得到该地震的地震矩为2.91×1019 N·m,地震震级为Mw6.9.于田地震引发的余震,大致分布在三个区域内：普鲁断裂北部、康西瓦断裂东部和贡嘎错断裂中部.弹性应力计算结果表明,于田地震导致阿尔金断裂西段、普鲁断裂中段、康西瓦断裂东段和贡嘎错断裂中段的静态库仑应力明显增加,其中以康西瓦断裂东段和贡嘎错断裂中段应力增量为最大,分别达到了0.05 MPa和0.04 MPa.大量研究证明,当地震所导致的库仑应力变化大于0.01 MPa时将具有明显的地震触发作用.根据本文结果,2014年于田Mw6.9地震使普鲁断裂、贡嘎错断裂和康西瓦断裂上的库仑应力增量均超过了触发阈值,具有被触发出地震的潜在危险.因此,在以后的地震学研究中,应加强对该三条断裂地震危险性的研究和监测.此外,近6年以来,研究区域发生了3次6级以上的地震.这些地震均沿着贡嘎错断裂,由南西向北东迁移,逐步靠近阿尔金断裂,并且逐渐由正倾滑型地震转变为走滑型地震.阿尔金断裂的走滑速率达到了9mm·a-1,所以,尽管本次地震导致的阿尔金断裂库仑应力增量小于0.01 MPa,阿尔金的地震危险性也应该加强监测.

On February 12 2014,an Ms7.3earthquake hit Yutian,Xinjiang,the epicenter is at the intersection of Karakax fault and Gonggacuo fault.Earthquake triggering theory indicates that Coulomb stress on nearby faults will change because of crustal coseismic slip after earthquake,and it will affect earthquake potential risk.In this paper,we estimate focal mechanism and rupture process with far field seismic wave data,and calculate coseismic stress change on nearby faults around epicenter.The purpose is to discuss Coulomb stress change and seismic potential hazard on these faults caused by Yutian earthquake.After the earthquake,we download seismic far field wave data of which epicenter is between 30°to 90°from IRIS.We select27 high SNR（signal to noise ratio）seismic record to make theoretical seismogram.We use generalized ray theory to get synthetic seismic wave map,and sub fault parameter inverse process is based on simulated annealing algorithms.In the way of changing fault parameter to fit actual and synthetic wave form,optimum solution of every sub fault is found.Based on fault rupture inversion model and infinite elastic half space theory,we calculate Coulomb stress change on nearby faults caused by the earthquake.The inversion results show that source depth of earthquake is 10 km,dip angle is 71.9°,largest coseismic displacement is 210 cm.Seismic moment is 2.91×1019 N·m,the main seismic energy is released in former 10 second.Aftershocks are mainly distributed in three regions：north Pulu fault,east Karakax fault and centre Gonggacuo fault.Stress increased significantly on western segment of Altyn fault,central part of Pulu fault,eastern segment of Karakax fault and central segment of Gonggacuo fault.Among them the largest stress change is 0.05 MPa on Karakax fault and 0.04 MPa on Gonggacuo fault.Former research shows that,Coulomb stress change caused by earthquake larger than 0.01 MPa will dramatically increase seismic risk on faults.In our research,stress change on Pulu fault,Gonggacuo fault and Karakax fault exceed triggering threshold,so all these three faults have seismic risk.In the near future seismic potential hazard on these faults should be closely monitored.In the past 6years,3 moderately strong earthquakes happen in the study area,epicenters migrate from Gonggacuo fault to Altyn fault,from southwest to northeast.Though stress change on Altyn fault is much lower,in consideration of 9mm·a-1 slip rate,seismic risk on Altyn fault should cause enough attention.