Abstract:The total length of the seismic profiles in the northeastern regions of Russia and, accordingly, the area of the territories covered by the seismic data interpretations, remains insignificant in comparison with the total area of these regions. At the same time, the geological objects in the northeastern regions attract much attention in view of their prospects, including potential mineral resources. The challenge is to construct the regional models of the crust structure without deep seismic survey data, and to analyze the regional seismicity that depends on the features of the deep crust structure. We develop a density model of the crust structure using the new interpretational gravimetry method. The density modeling results show that the density changes in the crust can be used to estimate the position of a surface separating the lower (quasi-homogeneous) and upper (heterogeneous) parts of the crust, i.e. to assess the density boundary of stratification. This boundary is formed due to a complex of physical and chemical processes that facilitate the transition of the material in the lower part of the crust into the quasi-uniform (homogeneous) state. The study area is the junction zone of the Ayan-Yuryakh anticlinorium and Inyali-Debin synclinorium (62-63°N, 148-152° E). The initial interpretation of the deep seismic survey data on the reference geological-geophysical profile 3-DV was available, so the ambiguity of the density modeling was reduced. In turn, the density modeling results can provide additional information for geological-geophysical interpretation of the DSS results on the sites wherein the seismic profiles go along the fault zones. The relationship between seismic events and the relief of the density boundary of stratification in the crust was studied quantitatively on the basis of the data from the regional catalog of seismic events and the results of the earlier analysis of seismicity in the study area. The analysis shows that 74 % hypocenters are located above the density boundary of stratification. The earthquake hypocenters located at depths ranging from 20 to 35 km are usually confined to the systems of long-living regional crustal faults and occur below the density boundary of stratification. The energy class of such earthquakes does not exceed 9. In the study area, the seismically active zones are mainly confined to the areas of subduction of the density boundary of stratification. Most of the earthquake epicenters (80 %) occur in the zones where the gradient of the relief change of the density boundary does not exceed 1. The number of recorded seismic events practically reduces to zero in the regions where the dip angle of the density boundary exceeds 65°.

Abstract:The total length of the seismic profiles in the northeastern regions of Russia and, accordingly, the area of the territories covered by the seismic data interpretations, remains insignificant in comparison with the total area of these regions. At the same time, the geological objects in the northeastern regions attract much attention in view of their prospects, including potential mineral resources. The challenge is to construct the regional models of the crust structure without deep seismic survey data, and to analyze the regional seismicity that depends on the features of the deep crust structure. We develop a density model of the crust structure using the new interpretational gravimetry method. The density modeling results show that the density changes in the crust can be used to estimate the position of a surface separating the lower (quasi-homogeneous) and upper (heterogeneous) parts of the crust, i.e. to assess the density boundary of stratification. This boundary is formed due to a complex of physical and chemical processes that facilitate the transition of the material in the lower part of the crust into the quasi-uniform (homogeneous) state. The study area is the junction zone of the Ayan-Yuryakh anticlinorium and Inyali-Debin synclinorium (62-63°N, 148-152° E). The initial interpretation of the deep seismic survey data on the reference geological-geophysical profile 3-DV was available, so the ambiguity of the density modeling was reduced. In turn, the density modeling results can provide additional information for geological-geophysical interpretation of the DSS results on the sites wherein the seismic profiles go along the fault zones. The relationship between seismic events and the relief of the density boundary of stratification in the crust was studied quantitatively on the basis of the data from the regional catalog of seismic events and the results of the earlier analysis of seismicity in the study area. The analysis shows that 74 % hypocenters are located above the density boundary of stratification. The earthquake hypocenters located at depths ranging from 20 to 35 km are usually confined to the systems of long-living regional crustal faults and occur below the density boundary of stratification. The energy class of such earthquakes does not exceed 9. In the study area, the seismically active zones are mainly confined to the areas of subduction of the density boundary of stratification. Most of the earthquake epicenters (80 %) occur in the zones where the gradient of the relief change of the density boundary does not exceed 1. The number of recorded seismic events practically reduces to zero in the regions where the dip angle of the density boundary exceeds 65°.

Ore deposits of the Magadan region are now in the focus of comprehensive studies as information on their deep structure is needed for both subsoil prospecting and regional development planning. This article presents the research results for the southeastern flank of the Yana-Kolyma orogenic belt. This area located at the junction with the Okhotsk-Koryak orogenic belt was investigated using the northeastern segment of the regional geophysical profile 3-DV. We analyzed the frequency-energy sections of the crust along the profile, 3D crustal density model of the entire study area, and magnetic, geoelectric and gravimagnetic characteristics of the crust. Complex data interpretation allowed tracing the crustal fault zones, areas wherein the crust material was strongly reworked, and zones of quasi-horizontal stratification. Considering the revealed features of the physical parameters of the crust material, we conclude that the currently accepted boundaries of individual tectonic blocks in the study area need to be adjusted. The northern boundary of the Balygychan uplift should be mapped along the Pautov fault. The Srednekansky branch of the Inyali-Debinsky synclinorium should be considered a transitional block that belongs to the Sugoi synclinorium, and its name should be changed to the Orotukan block.

This study is concerned with quantitative estimation of the relationship between earthquakes and tectonic crustal fragmentation based on a correlation analysis of fault density with seismicity parameters (the number and energy of earthquakes per unit area) for the Sredne Yamskoi seismic junction and adjacent area. The highest level of seismic activity and the highest probability of earthquake occurrence with energy classes K ≥ 12 within areas that have a continental crust with a well pronounced granite layer occur in those areas with the mean fault density. Within areas with a thinner granite layer in the crust, the most likely seismic events are K ≥ 12 earthquakes that occur in areas with lower fault density. We estimated the relationship between the degree of crustal fragmentation and the topography of stratification interfaces in the crust as identified by new interpretative gravimetry. Zones with the lowest degree of fragmentation tend to be areas where the top of the crystalline basement lies deeper.

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