Distribution of Earthquake Foci with Depth as a Manifestation of the Nature of Deformation of the Continental Crust

An example of seven seismogenic regions of the world with different conditions of tectonic deformation confirms the reliability of confinedness of weak earthquakes to two horizons in the Earth’s continental crust at depths of 5 and 10 km, regardless of the region. The width of extrema at the level of 0.7 from the maximum does not exceed 2–3 km in most cases. This markedly pronounced bimodal depth distribution of the numbers of weak earthquakes is interpreted from the standpoint of increasing strength and decreasing inhomogeneity of the material in the Earth’s crust with depth (Mogi, 1962; Scholz, 1968; Mori and Abercombie, 1997). It is assumed that the concentration of weak earthquakes near a depth of 5 km is determined by the fact that, due to a relatively high degree of inhomogeneity and low brittle fracture strength of the material in the crust, the fracture process stops before it develops into a major seismic event because of an obstacle to its development in the form of a site with increased strength. At greater depths (9–15 km), an initiation of rupture more likely develops into a larger event, since the rock material at these depths becomes more homogeneous and more durable due to the growth of confining pressure and temperature within the brittle–ductile transition zone. This leads to a relative increase in the number of larger earthquakes near a depth of 10 km, providing the second extremum in the depth distribution of the number of earthquakes. Such implications also explain the confinedness of strong earthquakes observed in seismology to the bottom rather than to the roof of the seismogenic layer. At the same time, the almost complete absence of relatively strong earthquakes at depths greater than 15–16 km may indicate a sharp change in elastic properties of the material and in the deformation nature in the lower part of the brittle–ductile transition zone and deeper. Here, brittle seismogenic deformation gives way to a plastic aseismic flow.