Variations of apparent resistivity and its possible seismogenic mechanism before the 18 December 2023 Jishishan Ms6.2 earthquake in Gansu province,China
The destructive earthquake(Ms6.2)occurred in Jishishan County,Gansu Province,northwestern China on December 18,2023,which resulted heavy casualties.The various anomalies of apparent resistivity were recorded a year before the earthquake occurrence at five observation stations in an area within 400 km around the epicenter,which include negative anomalies showing decreased annual changes or high decreasing rates at the Wushengyi,Dingxi,Tongwei(including surface observation and borehole observation)and Wuwei stations;as well as positive ones showing increased annual changes at the Wudu station.For quantitative analysis of the normal annual variations in apparent resistivity,soil temperature and soil water content at different depths from the ERA5 assimilation datasets(ECMWF Reanalysis v5)were used to eliminate environmental effects on the anomalies.The apparent resisitivity data combined with the rock physics experiment and the virtual fault dislocation model indicate possible relationship between the seismogenic mechanism and anomalies of apparent resistivity.The negative anomalies with decline changes of apparent resistivity at the Wushengyi Dingxi,Tongwei and Wuwei stations occurred in an enhancement area of compressive stress related to the seimogeinc process of the Jishishan earthquake.While the positive anomalies at the Wudu station occurred in the areas of relative extension.Moreover,the surface strain calculated by GNSS time series near the area of apparent resistivity anomalies demonstrates that the distribution of apparent resistivity anomalies with decline changes are spatially consistent with the compression zone,but the Wuwei station in the expansion zone of GNSS surface strain,which coincides with the result of the virtual fault dislocation model.Therefore,the relationship between the changes of apparent resistivity at the five stations and the seismogenic mechanism of the Jishishan Ms6.2 earthquake can be described in term of the deformation-apparent resistivity variations.
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