Geophysical processes and biosphere: article

Аnnotation: On the example of seven seismogenic regions of the world with different conditions of tectonic deformation, the reliability of the occurrence of weak earthquakes to two horizons in the continental crust at depths of 5 km and 10 km, regardless of the region considered, is confirmed. The width of extremes at the level of 0.7 from the maximum in most cases does not exceed 2-3 km. This pronounced bi-modal distribution with a depth of the numbers of weak earthquakes is interpreted from the standpoint of increasing strength and decreasing heterogeneity of the material of the Earth’s crust with depth [Mogi, 1962; Scholz, 1968; Mori, Abercombie, 1997]. It is assumed that the concentration of weak earthquakes near a depth of 5 km is due to the fact that, due to the relatively high degree of heterogeneity and low brittle fracture strength of the crust material, the fracture process stops earlier before it develops into a major seismic event due to an obstacle to its development in view of the site with increased strength. At greater depths (9-15 km), any onset of a rupture is more likely to continue to grow into a larger event, since the rock material at these depths becomes more homogeneous and more durable due to the growth of constraining pressure and temperature within the zone of «brittle-plastic transition». This leads to a relative increase in the number of larger earthquakes near a depth of 10 km, providing a second extremum in the distribution with a depth of earthquake numbers. Within the framework of such ideas, the confinedness of strong earthquakes observed in seismology is more likely to be at the bottom of the seismogenic layer than at its roof. At the same time, the almost complete absence of relatively strong earthquakes at depths of more than 15-16 km may indicate a sharp change in the elastic properties of the material and the nature of the deformation in the lower part of the «brittle-plastic transition» zone and deeper. Here brittle seismogenic deformation gives way to a plastic aseismic flow.