A New Approach to the Study of the Block Geoelectric Structure of the Lithosphere and Fluid-Saturated Fragments of Faults as Indicators of Zones with Increased Seismicity According to Magnetotelluric Sounding Data for Southern Sakhalin

Abstract

Large steeply dipping geoelectrical structures penetrating the whole lithosphere and extending deeply beneath it are detected on Sakhalin Island by magnetotelluric soundings. The fault system that forms this structure contributes to the flow of telluric currents from the deep horizons of the tectonosphere to the near-surface layers. As a result, the electrical component across the strike of the structure is very sensitive to the presence of conducive galvanically coupled vertical and horizontal elements (objects) in the section through which currents flow. This phenomenon provides the basis for a new approach to a detailed study of the block structure of the lithosphere and its upper part, the Earth’s crust. Based on this effect, which has not been studied earlier in the magnetotelluric works, the block structure of the Earth’s crust on southern Sakhalin is examined. It is found that the interblock joints characterized by a relatively low level of specific electrical resistivity, which indicates their fluid saturation, are fragments of regional or higher-order faults. The location of the foci of crustal earthquakes is closely related to these fragments. It is shown that the urban district of Yuzhno-Sakhalinsk is situated in the area of a triple junction of geoelectrical blocks in the Earth’s crust.