ИСТИНА

The territory of the North Caucasus, being part of the extended Crimea-Caucasus-Kopetdag zone of the Iran-Caucasus-Anatolian seismic region, is characterized by the highest seismic hazard in the European part of Russia. As a complementary to seismic hazard several active volcanic centers are known in Northern Caucasus. The largest volcanic center featuring the Elbrus volcano is characterized by high population density, developed infrastructure and economic activity as well as tourism. Results of recent studies designate the Elbrus volcano as is still in active status. The Elbrus volcanic center is also the area of potential seismic hazard. Both types of hazardous activity have been manifested repeatedly during the Holocene and in the Late Pleistocene. At the same time, extended intervals of recurrence of strong earthquakes and catastrophic eruptions may be the cause of Elbrus's actual quiet state and can be associated with the special rheological conditions of the earth's crust. One may suggest, that the accumulation of significant stresses in the earth's crust does not occur because of the extensive crustal magmatic pocket. At the same time, the lack of strong actual seismicity may indicate the emergence of the potential for an explosive volcanic eruption of mount Elbrus. Seismic hazard is naturally estimated by the map of the "General Seismic Zoning of Northern Eurasia" (OSR-97) and is characterized mainly by 8 and 9-levels of seismic intensity (on average soils in terms of the MSK-64 scale). The study of the attenuation of seismic waves with distance provides fundamental information on the inelasticity and scattering properties of the earth's environment, as well as the actual attenuation properties in a particular region. Information on the intensity of attenuation of seismic waves is also necessary for the estimation of focal spectra, seismic energy, seismic moments and other focal parameters of earthquakes, for modeling possible strong ground motions, etc. The attenuation of the amplitudes with distance includes two effects-the total energy losses of the wave for absorption (inelastic losses) and the divergence. By divergence we mean the decrease of the amplitudes with the hypocentral distance which occurs not only because of the geometric spreading (increase in the surface of the wavefront) but also by conversion on the earth's surface, by reflection loss during the passage of the refracting boundaries, and by the influence of the focus orientation. In this article, such fundamental problems as the relative role of the actual path (ray) and ground-and-geological conditions near the site (site effect) have not been studied. The quantitative estimate of the absorption has been carried out. The following methods were used for calculations: 1. Method of simultaneous determination of the absorption coefficient and divergence by means of amplitude curves. The divergence of the direct waves is described by an exponential function with a variable exponent n (r); however, insufficiently detailed experimental data on the amplitude variation with distance are forced to describe the divergence with the help of a constant n. 2. Observations of changes in the shape of the spectrum of earthquake records with distance were used to estimate the absorption regardless of the discrepancy. The initial data is the catalog of earthquakes in the North-Western Caucasus, obtained in the results of instrumental observations of the network of seismic stations of FRC GS RAS. For work, earthquake records recorded from 2013 to 2018 were selected. The signal-to-noise ratio was more than three. All the recorded earthquakes are upper crusts and their depths do not exceed 40 km. Hypocentral distances up to 100 km, local magnitudes ML = 1.6-5.5 As a result of the study, a summary table of the absorption coefficients determined by different methods was obtained. The attenuation of body waves and the quality factor Q is estimated. The values obtained were compared with those calculated earlier by other authors for other seismically active regions of Russia and the World. The results of the study of the attenuation of seismic waves in the environment make it possible to clarify the spatial configuration of local fluid-magmatic structures, based on the information that high absorption of secondary waves is observed in volcanic regions, which may be due to volumes partially filled with melts. This work was carried out with the financial support of the grant of the President of the Russian Federation for the support of scientific schools No. SS 5545.2018.5.