The mechanism of apparent resistivity change before and after the earthquake in Songyuan

Abstract

From 2013 to 2019, there have been frequent moderate-small earthquakes in Songyuan. The conductive structure of rocks had obvious changes before and after the earthquake, and the activity of fluid related to the earthquake was also enhanced significantly. Based on the study of 12 magnetotelluric sounding points which were measured before and after the magnitude 5.1 earthquake in the Songyuan on May 18, 2019, it was found that the apparent resistivity curve had translation before and after the earthquake for some measuring points. It is speculated that the translation may be affected by the following factors: 1) Static effect: The earthquake changed the near-surface electrical structure, which caused non-uniform electrical bodies to appear on the shallow surface. The redistribution of accumulated charges caused the resistivity curve to change. 2) Structural change of underground rocks: The earthquake made the underground stress environment change abruptly, the geometry of the rock pores or fractures in the formation changed, and the movement and filling of the pore fluid caused the resistivity to change; 3) The escape of underground gas: the earthquake enabled the gas stored in the formation to escape. The change of pressure made other pore fluids fill the pore resulting in the variation of resistivity.