Dynamics of Changes in the Stress–Strain State of the Earth’s Crust of the Black Sea Basin in Seismogenesis Models

Abstract

Changes in geodynamic conditions and, accordingly, the stress–strain state of the Earth’s crust affect the nature of seismic activity in the region. The earthquake focal mechanisms are the direct criteria that make it possible to assess the spatiotemporal changes in the crust and to establish general patterns of the formation of regional seismogenesis. The aim of the artilce is to study the deformation modes and kinematic conditions of regional seismogenesis and to investigate the influence of transformations and inversions of stress fields on the spatial and structural-kinematic characteristics of seismogenic ruptures. Stereographic models of seismogenesis based on the earthquake focal mechanisms, developed and tested by the author in studying seismically active regions, are constructed. The results of tectonophysical analysis of kinematic settings and deformation modes of seismogenesis within the Black Sea Basin are presented. The types of deformation modes of rupture formation in the hearth and kinematic conditions of seismogenesis were used as criteria for the stress–strain state of the crust. Collectively, they reflect the nature of the stressed state of the medium (compression or extension) and the direction of compressive or tensile forces. The influence on the processes of seismogenesis of primary and secondary, transformed of the stress fields. Stereographic quasi-models of seismogenesis are constructed, which approximate the values of the parameters of focal mechanisms formed in identical kinematic settings. The main features of local transformations of stress fields are established. The article provides the specific relations between the parameters of the quasimodels of primary field seismogenesis and similar values of the parameters of earthquake focal mechanisms formed in an identical kinematic setting, but under conditions of transformed stress systems. In all cases, transformations occur by rotation of the stress tensor around the direction of action of one of the principal axes with a change in the position of the other two. Changes in the time of kinematic conditions in earthquake sources of the South Coast and Tuapse–Anapa seismogenic zones of the Crimean–Black Sea region are analyzed. Short-term inversion changes in the kinematic conditions of seismogenesis of two types are investigated. In the first case, compression is replaced by stretching in the same direction (and vice versa). In the second case, one of the principal stress axes, which determines the type of kinematic situation (e.g., compression), changes its direction to perpendicular to its original position. The spatial and kinematic parameters of seismic faults, as well as the general patterns of their formation under the influence of inversion stress fields, have been established.