Denudation and seismicity of the Earth’s crust

Abstract

The distribution N ( h ) of the number of earthquake sources ( N ) through the section of the continental crust ( h ) is similar even in regions with different tectonic styles [1]. Most sources concentrate on a seismoactive layer at a depth of 10‐15 km. The number of sources diminishes abruptly upsection toward the Earth’s surface and more slowly (almost exponentially) downward along the section. A similar distribution of sources of seismic pulses, though on a different scale, is recorded in seismicity excited by underground mining. The maximum of the pulse distribution commonly passes ahead the stope over a distance of 1.5‐2.0 m. The number of pulses abruptly falls toward the stope and diminishes more slowly from a maximum at the source toward the intact rock massif. The seismic pulses reflect the process of rock disintegration, which is maximum at the surface of the mine working (crush zone [2]). The next (less destroyed) zone shows a more ordered fracturing. The fractures here are nearly parallel to the contour of the mine working (oriented fracturing zone). In the third (elastic) zone, the newly formed structural features excited by mining are either insignificant or absent. Thus, the maximum of mine seismicity occupies a transitional position between the crush and oriented fracturing zones. The character of mine seismicity changes with variation of the structural setting. Many relatively weak pulses are typical of the short-range zone of a mine working, and the most important events happen at the weakly active periphery in the long-range zone of the working [3]. Geological‐geophysical data and the fine structure of seismic fields indicate that the structural state of the Earth’s crust corresponds broadly to the scheme of mine seismicity considered above. Near the day surface, the rocks are crushed intensely, and the degree of disintegration gradually decreases downward along the section. In particular, it has been established that the seismoactive layers coincide with low-velocity zones (waveguides). According to [1], the waveguides are fluid-saturated crush zones. From this standpoint, the maximum at a depth of 10‐15 km corresponds to a layer with the maximum disintegration at a given moment. The base of this layer is characterized by the appearance of sporadic but more extended fractures of early generations. The roof is marked by the development of more numerous earthquakes related to the fractures of later generations and mainly represented by relatively weak seismic events (figure). The figure compares two distributions of the N ( h ) type based on the data on seismicity of Eurasia. The location of 16 000 earthquake hypocenters was taken into account. The data array of type 1 distribution comprises the sources of strong earthquakes ( M ≥ 5 ), while the data array of type 2 distribution characterizes the sources of weak earthquakes. One can see that the distribution of strong earthquakes is shifted statistically downward relative to the distribution of weak earthquakes.