Variations of apparent resistivity before the 2022 M_S6.9 Menyuan earthquake in Qinghai Province revealed by the virtual fault dislocation model

An earthquake with M_S6.9 struck the Menyuan County, Qinghai Province, northwestern China on 8 January 2022. There were four apparent resistivity observation stations with anomalies in an area within 400 km from the epicenter. Using the observed data with high quality and stable annual variation as well as the soil temperature and soil water provided by ERA5 assimilation datasets, a polynomial fitting was performed so as to subtract the normal annual dynamics and to detect the anomalies of the apparent resistivity prior to the earthquake. The results indicate that the above-threshold changes and anisotropic characteristics appeared before the earthquake in the EW channel at the Jinyintan station, NS channel at the Wuwei station, and the EW and N45°W channels at the Shandan station. A virtual fault dislocation model was used to examine the relationship between apparent resistivity changes and seismogenic process of the Menyuan earthquake. In the areas with compression along NNE direction, the decrease changes in the EW channel, which is approximately orthogonal to the direction of the principal compressive strain, has been observed at the Jinyintan station since ten months before the earthquake, the negative anomaly in the NS channel in the early stages of the earthquake preparation （13 months before the main shock） appeared at the Wuwei station, while it turned to an increase change three months before the earthquake. At the Shandan station, which is located in the relative extensional area, no anomalies were detected in the NS channel parallel to the principle tensile strain. However, an increase change was observed in the EW channel one year prior to the earthquake, and an increase change was also recorded in the N45°W channel of Shandan station half-year before the earthquake. Furthermore, at the Jinyintan station, Shandan station, and Wuwei station with epicentral distance of 92 km, 113 km, and 139 km, the corresponding maximum variation of anomalies are −3.0σ, 2.2σ and −2.1σ respectively. In addition, Variations of apparent resistivity before the 2022 M_S6.9 Menyuan earthquake were consistent with the results from rock experiment and theoretical models. Moreover, the spatio-temporal characteristics of the variation of apparent resistivity before the M_S6.9 Menyuan earthquake are likely consistent with the stress accumulation in the source region, as well as with the characteristics of stress accumulation by a high degree in the epicenter and gradual attenuation towards the periphery. Therefore, it can be deduced that the spatio-temporal variation of apparent resistivity before the M_S6.9 Menyuan earthquake may be related to regional medium deformation and stress change.