九寨沟M_S7.0地震的前兆异常时空演化特征及其分析

Temporal and Spatial Evolution of Precursory Anomalies of the Jiuzhaigou M_S7.0 Earthquake and its Analysis

以2017年8月8日九寨沟MS7.0地震震中为中心,对震前不同震中距前兆异常的时空演化过程进行分析。研究表明,震中距100km范围内无前兆异常,前兆异常主要分布在震中距100～500km范围内。震前2年新增前兆异常开始逐渐增多;不同震中距范围内的新增异常测项所占比例在震前1.5年左右达到最大,在震前又降低。在震中距300km范围内的异常测项所占比例相对较高。前兆异常时空演化过程存在3个不同的变化阶段,即α阶段(前兆异常的早期阶段,包括α_1和α_2,震前1000天以上)的异常主要分布在震中东北方向和西南方向,异常具有向外扩展的特征;β阶段(前兆异常的中期阶段,震前500～1000天)的异常表现为大范围出现异常;γ阶段(前兆异常的短期阶段,包括γ_1和γ_2,震前500天内)的异常分布在震中的东北方向,表现为远源区的异常向震中收缩过程(γ_1)和近源区的异常向外扩展的过程(γ_2)。通过与其他震例对比分析发现,前兆异常的3个阶段现象可能是大震前的共性特征,大震前前兆异常的显著增加对发震时间的预测具有一定的指示意义。

In this paper, the epicenter of the Jiuzhaigou Ms7.0 earthquake on August 8,2017 is taken as the center, and the evolution process of the precursory anomalies of different epicentral distances before the earthquake is analyzed. The result shows that there was no precursory anomaly within the range of 100 km from the epicenter, and the precursory anomalies were mainly distributed in the epicentral distance from 100 to 500 km. The number of new anomalies increased gradually 2 years before the earthquake. The ratio of anomaly items in different epicentral distances reached the maximum around 1.5 years before the earthquake and decreased again before the earthquake and the ratio of abnormal items in the epicenter distance 300 km was relatively high. There were three different stages of spatiotemporal evolution of precursory anomalies: during the α stage(early stage of precursory anomaly,includin α1 and α2,more than 1000 days before the main earthquake), the anomalies are mainly distributed in the northeast and southwest of the epicentral area, It is shown that the precursors have the characteristics of outward expansion. During the βstage(Intermediate stage of precursory anomaly,between 500 to 1000 days before the main earthquake), there was a large range of anomalies. During the γ stage(Recent stage of precursory anomaly,including γ1 and γ2,500 days before the main earthquake), the anomalies were distributed in the northeast of the epicentral area. The anomalies converged to epicenter(γ1) in the far source region and expanded outwards(γ2) in the near source region. Compared with other earthquake cases, it is found that the three-stage phenomena of precursory anomalies may be the common characteristics before major earthquakes. The significant increase of precursory anomalies before major earthquakes has some indication significance for time prediction of earthquakes.