On the Difference in Physical Mechanisms of Different Depth Earthquakes and Their Ionospheric Response Patterns

Abstract

According to the paradox of seismicity, earthquakes that follow the mechanism of ordinary brittle failure cannot occur at depths below a few tens of km. Several models have been proposed to explain deeper earthquakes, but these models have not been convincingly supported by data on the depth changes in earthquake source parameters. In this paper we present the examples of seismicity cutoff at the depth level of the Earth’s crust despite the undoubted continuation of similar shear displacements at greater depths. Based on worldwide data, the changes in a number of the average parameters of the earthquakes with depth are demonstrated. The pattern of these changes is consistent with expected differences in the physical mechanisms of the earthquakes with depth and significantly refines the accepted division of earthquakes into shallow, intermediate, and deep. The difference in the physical mechanisms of the earthquakes implies a probable difference in the nature of their precursors. Based on the hourly data of the vertical ionospheric sounding station “Tokyo” for 1957–2020, differences in the character of the seismoionospheric effect are demonstrated for the earthquakes (more than 300 events) occurring at different depths and presumably having different dominant mechanisms of seismogenesis. The average amplitude of the variations in the critical frequency foF2 providing the observed anomalies is estimated to be only 2–3% of the foF2 value. The separation of earthquakes by depth increased the statistical significance of the observed seismoionospheric effect.