Deep Velocity Structure and Seismicity of the Trans-Baikal Region (Along the Reference Geological and Geophysical Profile 1-SB)

Abstract

The paper presents analysis of the seismicity and deep structure of the Trans-Baikal region in the section of the reference geophysical profile 1-SB. It was determined that the Earth’s crust and upper mantle has a complex heterogeneous structure. The thickness of the Earth’s crust varies from 40 km in the South-Eastern part of the profile and in the areas of intermountain depressions in the North-Western part, and up to 48 km in the areas of mountain ranges. The values of the boundary velocities along the M boundary also vary greatly, from higher values of 8.4‒8.5 km/s for P-waves and 4.9‒4.95 km/s for S-waves (especially in the South-Eastern part of the profile) to reduced values of 7.8‒8.0 km/s for P-waves and 4.6‒4.7 km/s for S-waves in the section of the Baikal rift zone in the North-Western part of the profile. A strong inhomogeneous structure of the medium in terms of elastic wave velocities, Vp/Vs velocity ratios, and the Poisson’s ratio is determined for the upper and the middle crust at depths of 8‒20 km. The authors determined that zones of increased seismicity are referred to blocks of the Earth’s crust with inhomogeneous velocity structure according to data of differently polarized P- and S-waves. The area of the Baikal rift zone, in the immediate vicinity of the largest Muya earthquake of 1957 with M = 7.6, is characterized by elevated inhomogeneity in the upper part of the Earth’s crust according to the elastic wave velocities and secondary parameters of the medium (Vp/Vs ratio, K* = = Vp/(γ – 1), where γ = Vp/Vs, Poisson’s ratio (σ)). A number of other inhomogeneous deep zones have also been identified in the profile based on anomalies of P- and S-waves velocities and secondary parameters of the medium, which correlate to varying degrees with seismically active sites according to long-term instrumental observations. The established unambiguous connection of large inhomogeneous zones of the upper crust of the Trans-Baikal region with the accumulation of stresses and their discharge in the form of strong earthquakes allows us to make a reasonable medium-term forecast of catastrophic events.