四川九寨沟7.0级地震前震情跟踪概述及震后总结

对于强震可否预测与如何科学预测的问题,通过九寨沟地震前的震情跟踪及地球物理场的时空演化总结,科学地认识地震预测工作.四川九寨沟7.0级地震前距离震中400 km范围内,在1~2年内出现形变、地电阻率等地球物理场的异常.异常时间主要从2016年开始,空间上主要沿北东方向分布,在200 km内有流动水准4项异常、GNSS观测1项;200~400 km流动水准1项异常、定点形变2项,GNSS观测1项,地电阻率3项.这些异常变化出现后,地震部门组织了多次现场核实和跟踪;最为典型的核实是2017年6月27日至29日中国地震局台网中心、宁夏地震局、甘肃省地震局组成异常核实小组,对甘宁川交界地区多台地电阻率变化进行了分析研判,2016年7月平凉台井下观测的地电阻率有1年的趋势性破年变异常,NS和EW向地电阻率异常变化率比背景值(背景值年变化为0.6%、0.4%)的明显增大,分别为1.1%、0.6%;天水深井地电阻率从2月份三个测道地电阻率值偏离背景值幅度达到0.4%,宁夏固原EW测项异常下降幅度约为0.4%,计算分析这组异常是由地层电性变化引起;针对上述变化再结合2015年全国7级地震(M 7)大形势危险区判定意见,以及地壳形变多测项异常的变化,预测在未来较短的时间(3个月内)甘宁陕交界地区(平凉、固原、宝鸡)有发生6.0级以上地震的可能,尽管预测意见偏离了九寨沟7.0级地震震中和震级.但通过梳理异常变化的序列,总结分析在监测范围内出现有限的异常时,如何判定地震要素?以期对未来地震预测提供的借鉴.

Overview of tracking of Sichuan Jiuzhaigou M_S 7.0 earthquake in 2017 and its post earthquake precursor anomaly summary

Aiming at whether the strong earthquake can be predicted and how to predict, through the seismological situation tracking before the Jiuzhaigou M_S 7.0 earthquake and the summary of space-time evolution of the geophysical field, we can scientifically understand the work of earthquake prediction. about 1~2 years before the Jiuzhaigou M 7.0 earthquake in Sichuan Province, the anomalies of geophysical fields such as deformation and resistivity(trend anomalies and short-term and impen-ding anomalies) began occurred in areas within 400 km from the epicenter. The anomalies mainly occurred in 2016, and the spatial distribution was mainly along the NE direction. There were four anomalies of deformation flow level and one anomaly of GNSS observation within 200 km from the epicenter, and there were one anomaly of deformation flow level, two fixed-point anomalies, one GNSS observation and three earth resistivity anomalies within 200~400 km from the epicenter. After the occurrence of these anomalous changes, several seismic departments organized on-site verification and tracking;the most typical verification is the joint anomaly verification team composed of the network center of China Seismological Bureau, Ningxia Seismological Bureau and Gansu Seis-mological Bureau from 27 to 29 June 2017, which carried out anomaly verification and tracking, seismic analysis and judgment on the resistivity changes of several stations in the border area of Gansu, Ningxia and Sichuan. It is found that the annual variation of ground resistivity measured in under-ground deep well at Pingliang station lasted for one year from July 2016 and the trend breakthrough the annual anomaly. The annual variation rate of the abnormal amplitude of NS and EW items increased significantly compared with the background values(0.6% and 0.4% of the background changes), which were 1.1% and 0.6% respectively. The deviation of all of three measuring way ground resistivity in deep well at Tianshui station from the background values began from February and reached up to 0.4%, and the abnormal decrease rate of EW items in Guyuan station of Ningxia Hui Autonomous Region was about 0.4%. The analysis shows that the change is caused by the change of electrical properties of underground media. In view of the above changes, combined with the opinions on the determination of the risk areas of the M 7 earthquake(M 7) in China in 2015, as well as the abnormal changes of multiple items of crustal deformation, It is predicted that an earthquake of M≥ 6.0 will occur in the border area between Gansu, Ningxia and Shaanxi(Pingliang, Guyuan and Baoji) in a relatively short period of time in the future(within 3 months). Although the prediction conclusion deviates from the location and magnitude of the Jiuzhaigou M 7.0 earthquake. However, through the discussion on the occurrence of abnormal changes, this paper summarizes and analyzes how to determine the seismic elements when there are limited anomalies in the monitoring range. In order to provide reference for future earthquake prediction.