New interpretation of the reduction phenomenon in the electrical resistivity of rock masses before local earthquakes

Abstract

Based on long-term observational data from the area of Peter the Great Ridge (Pamir), a decrease in the electrical resistivity of the rock massif before local earthquakes has been observed. To explain the observed phenomenon, a hypothesis has been proposed: the structure of the pore space of a rock in the focal zone of an earthquake changes during the compression/tension associated with earthquake preparation. However, the models proposed have only qualitatively interpreted the experimental data. This article attempts to quantify this hypothesis based on 3D modeling of the observed electromagnetic effects. It is shown that variations in the apparent resistivity are due to changes in the electrical resistivity in a narrow fault zone located at the focus of the dipole-dipole sounding system. The changes occur at depths of <2–2.5 km and are due to low-resistivity fluid saturation of the highly porous fracture crushing zone. Small, smooth changes in the resistivity of the containing medium on both sides of the fault are associated with regional stresses in blocks of rocks and are not associated with the local earthquakes. An analysis of electromagnetic monitoring data at the Bishkek test site (Tien Shan) showed that the observed variations in apparent resistivity during pulse soundings are related to stresses and deformations of rocks in a wide region and cannot be used as a tool for predicting earthquakes. It is shown that, for studies of variations in the resistivity of rock massifs, measuring the response of the medium by grounded electrodes is ineffective due to the “static shift effect” generated by 3D inhomogeneities of the upper layer of the medium. To eliminate this effect, it was proposed that pulse technologies with magnetic field measurements made by ungrounded antennas be used.