芦山Ms 7.0地震前的跨断层形变亚失稳前兆特征

Precursory Characteristics of Meta-Instability of Cross-Fault Deformation Before the Lushan Ms 7.0 Earthquake

2013年芦山Ms 7.0级地震发生前数月内,发震断裂的宝兴跨断层短水准场地逆断加速,相邻的鲜水河断裂侏倭、虚墟、沟普、龙灯坝、老乾宁跨断层综合场地观测到准同步压性转折等异常现象。基于震区周边半径300 km内跨断层形变时空演变分析,结合GPS、卫星遥感、井水位、地震活动等辅助资料进行机理探讨。结果显示:震前半年多,断层形变从有利于应力积累的“继承性”转为“逆继承性”;震前3个月,观测到与震区相邻的鲜水河断裂反向异常增多增强,灰色关联度张压分量时序变化与亚失稳实验所得应力-时间过程相似,发震的龙门山断裂反向变化场地也明显增多,显示出断层活动协同化及加速特征,很可能是该次地震前的亚失稳前兆表现。

Objectives:The Lushan Ms 7.0 earthquake on April 20,2013,occurred in the southern segment of the Longmenshan fault,which is the boundary zone of collision and compression between the Bayan Har block and the South China block.Within a few months before the earthquake,the Baoxing mobile cross-fault short-leveling site in the seismogenic fault displayed thrust accelerating,25 km from the epicenter;the quasi-synchronous compressive turning was observed in the adjacent Xianshuihe fault,Zhuwo,Xuxu,Goupu,Longdengba,Laoqianning mobile cross-fault comprehensive sites for baseline and leveling,and the Xuxu and Longdengba sites also showed right-lateral turning,147-295 km from the epicenter.We attempt to explore the mechanism of these abnormal phenomena and its relationship with the Lushan earthquake.Methods:Based on analysis of the spatio temporal evolution of mobile cross-fault deformation within the radius of 300 km around the epicenter where above sites are located,combined with continuous GPS,satellite remote sensing,well water level,seismic activities and other auxiliary data,the precursory characteristics of meta-instability of cross-fault deformation before the Lushan Ms 7.0 earthquake is discussed.Results:Over six months before the earthquake,the“inherited”fault activity which was conducive to stress accumulation slowed down obviously at first,deviating from linear variation,later changed to reverse inheritance state.Three months before the earthquake it was observed that,the“reverse”anomalies(contrary to the tectonic setting)of the Xianshuihe fault adjacent to the earthquake area increased and strengthened,the compressive turning obviously accelerated,the temporal variation of tension-compression component of grey correlation index was similar to the stress-time process obtained from the meta-instability experiment;the“reverse”change sites of the Longmenshan fault also increased significantly(at the same time,there existed Baoxing anomaly that may be related to pre-slip of the seismogenic fault).Moreover,the auxiliary data containing continuous GPS,satellite remote sensing,well water level and seismic activities also had supporting evidences.Conclusions:All of above showed characteristics of fault activities synergism and acceleration.It was very likely to be the precursor performance of meta-instability before the Lushan earthquake.