关键词: 发震构造, 感应电流, 流体, 源兆

Abstract:

On April 24, 2021, a spatially correlated anomaly in geomagnetic diurnal variation was observed in the Sichuan-Yunnan region of China. Subsequently, on May 21, 2021, the M_S6.4 Yangbi earthquake occurred. The earthquake prediction research department suggests that this anomaly is related to the seismic event. The spatial correlation method for geomagnetic daily variation is a seismic geomagnetic field analysis technique with the same physical basis as the geomagnetic low point displacement method, geomagnetic loading-unloading response ratio method, and geomagnetic daily ratio method. This study uses spatially correlated geomagnetic anomaly data to assume the existence of subsurface currents, and inverts their possible distribution and shape using the Biot-Savart law and Monte Carlo modeling. An underground current distribution model potentially generating the observed spatial geomagnetic anomalies is established to explore its quantitative relationship with the Yangbi earthquake.

Based on the principle of spatial correlation anomalies, a “current in situ recurrence” phenomenon was detected in the region six months before the earthquake, with anomalies extending over 500km, meeting established anomaly criteria. Observation data from April 24 and 28, 2021, were used for underground current calculations. Stations near the epicenter were prioritized, supplemented by data from Sichuan, Yunnan, and Chongqing to account for the sparse station distribution west of the epicenter.

Using the Biot-Savart law and the Monte Carlo method, the three-dimensional distribution of induced currents-the abnormal source of geomagnetic phase-reversal anomalies observed within one month before the Yangbi earthquake-was inverted and located. Their spatiotemporal distribution patterns and their relationship with regional structures and seismic activity were examined. Based on these current distributions, the structural features of the middle and lower crust in the Yangbi earthquake area were outlined. It is inferred that during the short-term and imminent stage of the earthquake, long-period induced currents repeatedly appeared in the upper, middle, and lower crust within structural zones, suggesting that these sites repeatedly experienced fluid activity in the run-up to the event.

According to current-source localization of the abnormal phase distortion in the daily variation of the geomagnetic vertical component prior to the earthquake, mountain-shaped induced current surfaces repeatedly emerged in the source region and its vicinity, spanning the upper, middle, and lower crustal levels of the structure during the short-term and imminent stage of the Yangbi M_S6.4 earthquake. The current peaks were located in the middle to upper crust, with their bases at the top of the mantle. These peak-shaped induced currents persisted for several hours and fluctuated over time like ocean waves, rising and falling in succession. The same regions reappeared repeatedly, while areas on both sides of the epicenter alternated in occurrence. The peak-shaped induced current surface represents an electrical structural interface at the top of a transient, peak-shaped, high-conductivity channel. This surface, composed of multiple structural layers, shares the spatiotemporal variability of the underlying conductive channel, which rapidly assembles and disintegrates on an hourly scale. Thus, during the short-term and imminent stage of the Yangbi earthquake, the recurrent long-period induced currents in different crustal levels reflect repeated episodes of fluid activity within the structural framework.

The formation mechanism of the peak-shaped induced current surface is proposed to be linked to electrically isolated, fluid-rich conductors within the high-conductivity belt. Driven by the upwelling of deep mantle-derived thermal fluids, the high-resistance blocks flanking the conductive zone undergo outward arching. As mantle fluids penetrate and interact with fluids in the conductive belt, the previously disconnected fluid-bearing conductors become electrically connected, forming a transient peak-shaped high-conductivity channel. The long-period induced currents distributed along its upper surface constitute the peak-shaped induced current surface. Therefore, the occurrence of such peak-shaped induced currents is likely tied to deep tectonic processes involving fluids.

Key words: structure, induced current, fluid source, megawatt