关键词: 积石山M_S6.2地震, 地电阻率, 异常特征, 自适应变化幅度

Abstract:

The 2023 M_S6.2 Jishishan earthquake was the largest seismic event in Gansu Province since the 2013 M_S6.6 Minxian-Zhangxian earthquake. Due to its high intensity and significant casualties, it drew widespread public attention and reignited discussions regarding the predictability of earthquakes. Earthquake prediction remains a challenging scientific endeavor critical to public safety. Among the various geophysical techniques employed for short-to medium-term earthquake monitoring, apparent resistivity observation has shown considerable potential. Anomalous variations in apparent resistivity relative to background values, particularly their spatiotemporal evolution prior to seismic events, have become a focal point in earthquake research.

This study investigates apparent resistivity anomalies preceding the 2023 Jishishan earthquake using data from stations located within a 300km radius of the epicenter. Two analytical methods were applied: the original curve method and the adaptive change magnitude method. These analyses were supplemented by pre-earthquake anomaly verification and retrospective evaluation of historical seismic cases to explore the characteristics of apparent resistivity anomalies associated with the event.

The key findings are as follows: 1)Apparent resistivity anomalies were detected at four stations within 300km of the epicenter, with the earliest anomaly recorded at Wuwei Station, located north of the epicenter. Between 2 and 7 months before the earthquake, anomalies emerged sequentially at stations to the northeast and southeast of the epicenter, with no anomalies observed in the southwest direction. The spatial distribution of anomalies suggests that the observed signals were not generated directly at the seismic source but were instead induced by regional stress redistribution linked to tectonic activity. The anomalous stations are interpreted as stress-sensitive sites. Under the influence of NNW-directed compressive stress from the northeastern margin of the Qinghai-Tibet Plateau, these sites—particularly Wuwei, Wushengyi, Dingxi, and Tongwei—experienced heightened compressive deformation, thereby enhancing the likelihood of resistivity anomalies. 2)Analysis using the original curve method revealed a sharp decline at Tongwei Station during the two months preceding the earthquake, indicating a short-term anomaly. Wushengyi and Dingxi stations exhibited year-scale variations in high/low values, while Wuwei Station showed a reduction in annual variation amplitude. These three stations thus demonstrated medium-term anomalies, with nearly synchronous onset times. Using the adaptive change magnitude method, the anomaly at Tongwei Station began in September 2023 on the N20°W and EW' profiles, with magnitudes of 0.09 Ω·m and 0.12 Ω·m, respectively. These anomalies coincided with threshold exceedances of 0.07% and 0.2%. For the EW' profile, an additional anomaly began in October 2023(0.12 Ω·m, 0.4%threshold exceedance). At Wuwei Station, the NS profile anomaly began in May 2023 with a magnitude of 0.2 Ω·m and a 0.15%threshold exceedance. At Dingxi Station, the EW profile anomaly began in April 2023(0.04 Ω·m, 0.2%threshold exceedance).

In conclusion, the deviations in apparent resistivity prior to the Jishishan M_S6.2 earthquake, together with timely anomaly verification, hold scientific value for advancing earthquake prediction capabilities. This study contributes to the growing body of evidence supporting the role of apparent resistivity anomalies as reliable seismic precursors and provides methodological guidance for anomaly extraction, characterization, and practical application in earthquake forecasting.

Key words: Jishishan M_S6.2 earthquake, apparent resistivity, anomaly characteristics, adaptivity variation amplitude