关键词: 古冶M_S5.1地震, 地电阻率, 地电场, 极低频, 电磁异常

Abstract:

On July 12, 2020, an M_S5.1 occurred in Guye, Hebei Province, and as the largest earthquake in the capital circle in recent years, its unique geographical location has attracted more attention. During an earthquake, the electromagnetic properties of underground media will change, so dense electromagnetic observation stations were arranged in the capital circle. In this study, the data of geoelectric resistivity, geoelectric field, and extremely low frequency(ELF)observation within 400km of the Guye earthquake are analyzed using a combination of time-domain waveform analysis, sliding Fourier analysis with annual variation removed, normalized variation rate method(NVRM), and geo-electric azimuthal method. After eliminating the influencing factors such as operation status, observational environment, and the spatial electromagnetic effect, we analyzed the characteristics of electromagnetic phenomena that may be related to the Guye earthquake preliminarily and found that there was a variation process of “trend decrease—accelerated decrease—postseismic recovery” observed in 6 geoelectric resistivity stations and that the normalized variation rate exceeded the threshold value of ±2.4 in 7 stations within one year before the earthquake. In Luanxian station, the intensity of the geoelectric field in the north-south and north-western directions decreased and then rose back before the earthquake. In addition, the azimuth shifted to the direction of the Guye earthquake in the preseismic period, and then returned to the direction of the Luanxian-Laoting Fault. The ELF stations in Wen'an and Fengning precisely recorded the coseismic change of the 16Hz natural magnetic field, in which the variation of the vertical component is twice larger than that of the horizontal component. Under the condition of large subsurface structure difference beneath the stations, the observed electric values from the two stations are distinctively different; moreover, the coseismic disturbance is submerged by the background noise. The subsurface electric structure was obtained by interpolating and inversing the data collected from the ELF stations in the capital area, which indicates that the Guye earthquake occurs near the boundary of the electric property changes. Meanwhile, it shows high electric resistivity in the northern area, low electric resistivity in the southwestern area, and partially low electric resistivity in Baodi and Wen'an, which is consistent with the location of the abnormally stronger ground motion. Regarding the spatial selectivity of the anomalies, we believe it may be related to the direction of the two main conjugated structures in the capital area, which lie in NEE and NW direction, respectively. And the study also enlightens researchers that the investigation of the mechanism of seismic electromagnetic anomaly should start from the coseismic phenomenon, and then focus on the aspects of seismic signal source and propagation path, because the extremely low-frequency observation band is wide and the coseismic electromagnetic signals can be clearly recorded. There are many effective ways to extract electromagnetic signals related to earthquakes from strong interference background, such as making retrospective analysis of moderately strong earthquakes in time, summarizing the electromagnetic anomaly characteristics of different earthquake events, densifying the electromagnetic observation layout appropriately, so that the abnormal information can be mutually corroborated and a variety of means for fusion and comparative analysis can be developed.

Key words: Guye M_S5.1 earthquake, georesistivity, geoelectrical field, extremely low frequency(ELF), electromagnetic anomaly