九寨沟MS7.0地震前地磁异常

Geomagnetic anomalies prior to the JiuzhaigouMS7.0 earthquake in 2017

摘要:

分析距离九寨沟M_S7.0地震震中分别为65 km、254 km和238 km的松潘地磁台、成都地磁台和天水地磁台2017年连续观测结果，发现九寨沟地震前，松潘地磁台出现异常变化，主要表现为2017年5月开始，松潘地磁台长时间垂直分量累积变化量达40 nT。短时间内也存在地磁场突变，最高可达10 nT，而成都地磁台和天水地磁台地磁场无明显异常。通过对观测数据的详细检查发现松潘地磁台很多观测日的最后一个观测值与后一观测日的第一个观测值相差超过1 nT，因此认为九寨沟地震前观测数据的异常是由观测系统异常导致。对三个台站地磁场垂直分量日变化的统计分析发现垂直分量在不同时段相关性保持一致，说明地震前短时间内不存在地磁垂直分量的明显异常变化，三个台站地磁场日变化的差异是由地下介质性质不同导致。岩石圈磁场在震中西侧区域出现正负异常交替现象，这可能与九寨沟地震的孕育有关。

Abstract:

The continuous measurement data of Songpan (SPA), Chengdu (CDP), and Tianshui (TSY) magnetic stations in 2017, which are 64, 254, and 238 km away from the epicenter of Jiuzhaigou M_S7.0 earthquake, were analyzed in this paper. Geomagnetic anomalies recorded by SPA before the M_S7.0 earthquake mainly showed that the cumulative change of long-term vertical component had reached 40 nT since May 2017. There were also short-term step changes, up to 10 nT. No obvious anomalies showed in the magnetic field of CDP and TSY. According to the detailed inspection of raw data, it is found that the difference between the last observation value of many observation days at SPA and the first observation value of next day is more than 1 nT. Therefore, it is considered that the anomaly in the observation data before Jiuzhaigou earthquake is caused by the anomaly of observation system. The statistical results of diurnal variation in vertical component of the three stations revealed consistent with each other in different time, indicating that there is no obvious abnormal change of geomagnetic vertical component in a short time before the earthquake, and the difference of magnetic field at the three stations is resulted from the different properties of underground medium. An alternation of positive and negative anomalies in the lithospheric magnetic field was shown in the west area of epicenter, which may be related to the seismogenic process of Jiuzhaigou earthquake