Abstract:

The M_S6.0 Changning earthquake in Sichuan Province caused heavy casualties and property losses. Many scholars have carried out a lot of research on the tectonic background and seismogenic mechanism of the earthquake. However, whether the triggering mechanism of the earthquake is related to fluid injection remains controversial, and further comprehensive analysis should be conducted based on multidisciplinary observation data. The preparation and occurrence of large earthquakes are often closely related to crustal deformation and mass density changes. The comprehensive study of these two methods is conducive to capturing the real crustal dynamics information. To study the process of material migration in the earth's crust before and after the earthquake and its triggering mechanism, the characteristics of gravity changes before and after the earthquake, the crustal vertical deformation data and their relationship were analyzed by using the gravity and GNSS data in Sichuan.

The characteristics of regional gravity change before and after the earthquake show that the epicenter of the Changning earthquake is located in the gradient zone of positive and negative gravity change anomalies and presents obvious reverse before and after the earthquake. According to the summary of previous earthquake cases, this region may have entered the short- and medium-term stages of earthquake preparation. The wavelet multi-scale decomposition method is used to process the regional gravity change characteristics of two periods before and after the earthquake, and the gravity change characteristics of different depths and scales are obtained. The results show that in the shallow part of the seismic region, the local variation characteristics of the dispersion are obvious, mainly distributed in the boundary of the active fault zone and block. The trend change characteristics are significant in the deep part, and the gravity gradient zone associated with the earthquake is found. In theory, there is an approximate ratio relationship between surface gravity change and crustal vertical deformation, and different ratio coefficients contain different geophysical meanings. The relationship between gravity changes and crustal vertical deformation at four sites around the epicenter is extracted. The results are inconsistent with the approximate law and show different characteristics, including subsurface material migration information at each location.

Based on the above research results, the process of material migration in the crust before and after the Changning earthquake and its triggering mechanism are comprehensively analyzed and discussed. The results show that the long-term driving force of the Changning earthquake is the result of the deep migration of materials from the Qinghai-Tibetan plateau to the southern boundary of the Sichuan Basin. In the shallow surface of the epicenter and the surrounding area, there may be some cavities or airbags and other Spaces, and the loss and filling phenomenon of gas, liquid, or high-density substances may occur before and after the earthquake. Combined with the previous research results on the seismogenic mechanism of the Changning earthquake, it is indicated that the factors such as salt mining, gas production and wastewater reinjection in this area may be one of the reasons for the triggering of the Changning earthquake.

Key words: Gravity change, crustal vertical deformation, ratio relationship, wavelet multi-scale decomposition, Changning earthquake

摘要：

为研究2019年四川长宁地震前后地壳内部物质的运移过程, 文中利用流动重力和GNSS资料对地震前后的重力变化特征、 地壳垂直形变及其关系进行了深入分析。地震震中处于重力变化正、 负异常的梯度带上, 并在地震前后呈现明显反向。小波多尺度分解表明, 浅部的离散性局部变化特征明显, 深部的趋势变化特征显著。震中周边4个站点的重力变化与地壳垂直形变的关系与近似规律不一致。长宁震区长期的动力是青藏高原的物质在深部向四川盆地南边界运移的结果。在震中及周边区域的地表浅层可能存在一些空洞或气囊等空间, 在地震前后出现气体、 液体或高密度物质流失和填充的现象。结合前人对长宁地震发震机制的研究成果分析认为, 该区域的采盐、 采气及废水回注等因素可能是触发四川长宁地震的原因之一。

关键词: 重力变化, 地壳垂直形变, 比值关系, 小波多尺度分解, 长宁地震