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.

中文翻译：

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作为大陆地壳变形性质表现的深度地震震源分布

世界上具有不同构造变形条件的七个发震区的一个例子证实了弱地震限于地球大陆地壳中 5 公里和 10 公里深度的两个层位的可靠性，而与区域无关。在大多数情况下，从最大值到 0.7 级的极值宽度不超过 2-3 公里。弱地震数量的这种显着的双峰深度分布是从强度增加和地壳材料不均匀性随深度增加的角度来解释的（Mogi，1962；Scholz，1968；Mori 和 Abercombie，1997）。假设 5 公里深度附近弱地震的集中是由以下事实决定的：由于地壳中材料的不均匀性较高，脆性断裂强度较低，断裂过程在发展为重大地震事件之前停止，因为强度增加的地点阻碍了其发展。在更深的深度（9-15 公里），破裂的开始更有可能发展成更大的事件，因为这些深度的岩石材料由于在脆性 - 韧性转变中的围压和温度的增长而变得更加均质和更耐用区。这导致 10 公里深度附近较大地震的数量相对增加，为地震数量的深度分布提供了第二个极值。这种含义也解释了在地震学中观察到的强地震被限制在底部而不是发震层的顶部。同时，在大于 15-16 公里的深度几乎完全没有相对强烈的地震，这可能表明材料的弹性特性和脆韧过渡带下部和更深处的变形性质发生了急剧变化。在这里，脆性地震变形让位于塑性地震流。