Abstract:

Earthquake prediction based on changes in the resistivity of rock and soil media is a systematic endeavor that integrates field observation, scientific research, and forecasting practice. Apparent resistivity observation, which is widely used in earthquake monitoring and prediction in China, employs the direct-current method to continuously measure temporal changes in resistivity within a fixed subsurface investigation volume. Analytical calculations and numerical analyses of apparent resistivity observations conducted at the surface and underground have been developed by computing the potential distribution of the DC steady-current field. During the establishment of observation stations, electrical sounding should be carried out to invert the electrical structure of subsurface formations, and analytical calculations should be performed to determine the theoretical range of observational values, thereby verifying the correctness of the actual observation array configuration. Both analytical computation and numerical analysis can also be used to investigate the influence of resistivity variations in different parts of the medium on the observations. These methods provide reasonable explanations for three types of annual variation and for the characteristics of diurnal variation in apparent resistivity observations. In anomaly verification, they also enable quantitative evaluation of the interference amplitudes caused by leakage currents and electrically anomalous bodies. In the vertical direction, the one-dimensional sensitivity coefficient distribution reveals the extent to which resistivity changes at different depths affect the observations. The more detailed three-dimensional sensitivity coefficient distribution further reveals the differences in the effects of resistivity changes in different regions of the monitoring area on the observations. Therefore, the sensitivity coefficient distribution characterizes the local features of apparent resistivity observations and allows the influence of resistivity changes in different regions on the observations to be rapidly determined qualitatively. Petrophysical experiments and theoretical resistivity models have revealed the relationship between anisotropic resistivity decreases in water-bearing rock-soil media and the directional propagation of microcracks during compressive stress loading. In shallow subsurface layers, microcrack systems are predominantly aligned with the direction of the maximum horizontal principal compressive stress. During stress loading, the apparent resistivity measured in the direction perpendicular to the maximum principal compressive stress shows the largest variation amplitude, whereas that measured in the parallel direction shows the smallest variation, and that in an oblique direction exhibits an intermediate response. Through analysis of geoelectrical resistivity anisotropy, the orientation of the regional horizontal stress field can be inferred. The relative deformation accumulation around the epicenter before earthquakes has been analyzed using a virtual fault dislocation model. By using regional deformation as an intermediate bridge, a preliminary link has been established between anomalous changes at observation stations and the late-stage seismogenic process of distant earthquakes, and the expected patterns of apparent resistivity anomalies related to seismogenic processes have thus been obtained.

When observation stations are located within compressional enhancement zones during the seismogenic process, apparent resistivity exhibits anomalous decreases, whereas stations located in dilatational domains show anomalous increases. Transitional zones between compression and dilatation typically display weak or negligible anomalous variations. Based on earthquake case studies, persistent apparent resistivity anomalies lasting from several months to approximately two years serve as magnitude indicators for earthquake prediction, providing important constraints on potential epicentral locations and earthquake magnitudes. By contrast, short-impending anomalies characterized by accelerated variations or high-frequency perturbations act as temporal indicators and provide predictive information on earthquake timing. In operational earthquake forecasting, the spatial location and magnitude should first be preliminarily estimated using magnitude indicators, followed by short-term time prediction through continuous monitoring of temporal indicators within the pre-identified risk zone. On this basis, medium- and short-term earthquake predictions have been carried out from the perspective of interpreting field anomalies in terms of their seismic source processes. In recent years, the 2022 M_S6.8 Luding, 2023 M_S6.2 Jishishan, and 2024 M_S7.1 Wushi earthquakes were successfully predicted using the apparent resistivity method. However, the relevant research is scattered across domestic and international literature spanning more than 60 years. This paper therefore reviews and summarizes the main theoretical foundations of earthquake prediction based on apparent resistivity observations, so as to facilitate their understanding and application in geoelectrical earthquake prediction practice.

Key words: apparent resistivity, earthquake, anomaly, earthquake prediction, basic theory, regional deformation

摘要：

基于岩土介质电阻率变化开展地震预测, 是野外观测、科学研究和预测实践相互结合与促进的系统性工作。中国应用于地震监测预报的地电阻率观测, 采用直流电法连续测量地下固定探测体积内介质电阻率随时间的变化。通过计算直流稳恒电流场的电位分布, 实现了地电阻率地表和井下观测的解析计算和数值分析。在垂直方向, 一维影响系数分布揭示了不同深度地层电阻率变化对观测的影响程度; 更为细致的三维影响系数分布, 则揭示了测区不同区域地层电阻率变化对观测影响的差异性, 明确了地电阻率观测的局域性特点。岩石物理实验和电阻率理论模型揭示出压应力加载过程中, 含水岩土介质电阻率具有各向异性特征的下降变化与微裂隙定向扩展之间的关系。通过断层虚位错模式分析地震前震中周围相对的变形积累特征, 以区域变形为中间桥梁, 电磁学科建立了观测站的异常变化与远距离地震晚期孕育过程之间的联系过程, 得到了与孕震过程有关的地电阻率异常的预期形态, 在此基础上开展“以源观场”的地震中短期预测, 对近年来发生的2022年泸定 M_S6.8、2023年积石山 M_S6.2 和2024年乌什 M_S7.1 等地震做出了较好的预测。以上研究内容散见于时间跨度超过60年的国内外文献之中。文中对基于地电阻率观测开展地震预测的主要基础理论进行回顾总结, 以便地电分析人员能较为便捷地了解并应用于地震预测实践。

关键词: 地电阻率, 地震, 异常变化, 地震预测, 理论基础, 区域变形