Physical Modeling of Current Spreading in a Homogeneous Medium with an Electrically Conductive Vertical Inclusion

Abstract

The phenomenon of electromagnetic triggering of earthquakes was discovered more than 30 years ago as a result of a statistical analysis of the seismicity response of the geophysical sites of the Schmidt Institute of Physics of the Earth, Russian Academy of Sciences (Pamir); and the Joint Institute for High Temperatures, Russian Academy of Sciences (Northern Tien Shan), to artificial electrical impact by direct current pulses supplied to the crust through a grounded dipole. At the same time, according to estimates, the current density at the depth of regional earthquake sources of 5–10 km can be 10^–8–10^–7 A/m². The physical mechanism by which such very weak currents trigger the earthquake’s source is still unclear. All results of laboratory experiments demonstrating the growth of acoustic emission (cracking) under electrical action on rock samples up to their destruction were obtained at a current density level greater than 1 A/m²; however, due to the heterogeneous geoelectric structure of the crust, which includes vertical inclusions in the form of fluid-saturated faults with increased conductivity, with artificial electrical action on the environment, current can be concentrated in real faults. In order to determine the degree of possible current concentration in a model fault in the crust with a conductivity exceeding the conductivity of the host rocks by several orders of magnitude, calculations were carried out for the spread of current from a grounded dipole during field experiments in the Northern Tien Shan on electromagnetic sounding of the crust using the COMSOL Multiphysics© software package. It is shown that in the fault of the crust closest to the dipole, the current concentration increases by an order of magnitude. However, the calculation model does not take into account the possible anisotropy of the conductivity of rocks in the fault. At present, the influence of such anisotropy on the spread of current in the crust can only be established experimentally, and to study this influence, a laboratory setup for the physical modeling of electromagnetic soundings of the crust has been created. This article describes the physical model and the results of the first series of experiments by filling a fault with a material having isotropic properties. The results showed good agreement between the experimental and calculated data, which confirms the applicability of this model for subsequent experiments with fault fill having anisotropic electrical properties. In addition, it was shown that the current concentration in the fault increases with a decrease in the ratio of the width of the fault to its length. In a model narrow fault with a low value of this ratio and high conductivity of the filling material containing mineralized fluid, when the conductivity of this material exceeds the conductivity of the host medium by 50–100 times, the current concentration factor can reach values of 20–25. However, it is noted that even such fairly large values of the current density concentration coefficient do not allow one to eliminate the discrepancy of six and seven orders of magnitude between the current density values in field and laboratory conditions, which makes it difficult to use the results of laboratory experiments to interpret the observed effects of electrical impacts from artificial sources on the seismic regime, which are presented in detail in numerous publications.