Abstract:
The process of rock stress-strain state change causes acoustic radiation, which is called rock acoustic emission or geoacoustic emission. The relation between the earthquake preparation process and rock acoustic emission variations, which are called pre-seismic anomalies, has been stated in a number of investigations. The general mechanism of occurrences of these anomalies is associated with the fact that formation of a preparing earthquake source causes changes in the stress-strain state of rocks, surrounding it. One of the kinds of anomalies, occurring at the final stage of earthquake preparation, is the appearance of clearly expressed direction of acoustic activity. The main hypothesis of occurrence of this phenomenon is that a preparing earthquake source impact causes formation of constant direction of principle stress axes at an observation point. In their turn, the direction of these axes determines the primary orientation of acoustic radiation sources. To confirm this hypothesis, axis orientations of the main stresses, determined by the earthquake preparation process, were modeled. The estimates are based on the model constructed within the framework of elasticity linear theory where the Earth crust is considered in the form a homogeneous isotropic elastic half-space and the force impact at a preparing earthquake source is considered in the form of a combination of forces double pairs. Elastic deformation potential energy, accumulated during the earthquake preparation process, is taken into account. In the paper, we used the data from the catalog of earthquake source mechanics «The Global Centroid-Moment-Tensor Catalog» on seismic events occurred near Kamchatka peninsula from 1976 until 2020. As long as the acoustic radiation direction depends on the azimuthal direction to earthquake epicenter, all the considered seismic events were divided into three groups by the method of K-averages according to spatial locations of their epicenters. The modeling results were compared with experimental estimates of the main stress axis directions at Mikizha observation site (52.99$^{\circ}$ N, 158.22$^{\circ}$ E). The estimates were earlier obtained based on the geoacoustic emission directivity anomalies. It was shown that histograms of main stress axis direction distributions agree with the results of estimates for two groups of earthquakes. Modal intervals of distribution histograms fall within the range of experimental estimates from 290$^{\circ}$ to 320$^{\circ}$ and from 20$^{\circ}$ to 50$^{\circ}$ accordingly.
Keywords:geoacoustic emission, pre-seismic anomalies, mathematical modeling, main stress axes.
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