2019年6月17日长宁MS 6.0地震前地震活动和水位的加卸载响应比异常研究

Evolutions of LURR Anomaly Based on Seismicity and Groundwater Level Data before the June 17,2019 MS6.0 Changning Earthquake

加卸载响应比(LURR)方法是通过固体潮加卸载过程中的某一物理参数的响应差异探查区域应力场演化。本文将这一方法应用于2019年6月17日四川长宁MS6.0地震,根据LURR时序演化探查震源区介质的应力状态变化,提取可能的震前异常信息。首先采用贝尼奥夫应变作为响应量计算LURR,通过固体潮在地震断层面的最优滑动方向上引起的库仑破坏应力变化来判断加载还是卸载,结果显示LURR值自2018年年初开始快速增加并在2018年年中达到峰值,此后异常持续至地震。在此基础上对该地区的大地形变和地下井水位资料进行了LURR分析,发现水富水平摆倾斜仪的东西和南北分量在LURR出现异常的同期发生明显偏转,而地震周边的地下井水位高值变化过程与长宁地震也有较好的对应关系。研究表明在长宁地震发生前,存在地震活动和地下水位的LURR异常,且异常时间与水富倾斜同步,暗示震源区介质存在明显的应力积累过程。

The load/unload response ratio(LURR)is a method to explore the evolution of regional stress field by detecting the difference of responses during the loading and unloading induced by tidal stress.Taking MS6.0 Changning earthquake as a case study,we analyzed the evolution of LURR time series,the change of stress state of source media,and the possible pre-earthquake anomaly.Firstly,the Benioff strain of small earthquakes are adopted as the loading/unloading response,and the change of Coulomb failure stress induced by earth tide,in the tectonically preferred slip direction on the fault surface of main shock,is used for differentiating the loading and unloading periods.The results show that the LURR values have anomalous increased since the beginning of 2018 and reached the peak in mid-2018.Meanwhile,the LURR method is also applied to the groundwater level data in the near field wells,and the pre-earthquake anomalies are observed too.In addition,we find that the changes can also be supported by the variations of the geodetic time series observed in the same neighborhoods during the same periods.Obvious changes in the EW and NS components of SF horizontal pendulum tiltmeter are observed during the period of LURR anomalies,which indicate that significant anomalies could be observed in LURR time series produced using both seismicity and groundwater level change as data input,suggesting the high stress accumulation in source media.