Elastic properties and latitudinal zoning of the lithosphere of the Middle Ural region with increased seismicity according to multiwave deep seismic sounding data

Research subject. Lithosphere of the Middle Urals region with increased seismicity.

Aim. Study of the elastic properties and latitudinal zoning of the earth’s crust and upper mantle in the region based on multiwave deep seismic sounding data and the comparison of the identified features of the deep structure with the location of tectonic earthquakes.

Materials and Methods. The gradient velocity sections of the earth’s crust and upper mantle were constructed along the Vizhay– Nizhnyaya Tura–Orsk meridional profile and the fragments of two geotraverses: Kostomuksha–Nizhniy Tagil–Semipalatinsk (Rubin-1) and Nizhniy Tagil–Urengoy (Rubin-2) based on the results of reinterpretation of the Ural deep seismic sounding (DSS) materials using two-dimensional seismic tomography. Sections in isolines of equal velocities were constructed from the first arrivals of compression and shear seismic waves independently of each other and recalculated on the basis of known functional and correlation dependencies into sections reflecting the distribution of a number of elastic parameters in the earth’s crust: density, Poisson’s ratio, Vp/Vs values, bulk and shear modulus.

Results. New data about the deep structure of the Middle Urals region is obtained. Based on materials from the meridional Vizhay–Nizhnyaya Tura–Orsk profile it has been established that the Tagil trough as compared to the Magnitogorsk trough, is characterized by a less thick and higher-speed crust, both in the Vp speed of propagation of compression waves and in the Vs speed of shear waves. At the same time, the Vp/Vs velocity ratio in it is reduced as compared to the Magnitogorsk trough, which indicates greater tectonic disturbance. In the middle part of the earth’s crust a weakened, softened layer is identified, characterized by a reduced Vp velocity of compression waves and reduced Vp/Vs values and Poisson’s ratio. The hypocenters of local earthquakes are mainly confined to this layer. Blocks in the earth’s crust with contrasting density and elastic properties, including latitudinal zones of increased fracturing, have been identified and physically characterized. These zones correspond mainly to latitudinal dislocations, identified earlier according to other criteria, and clarify their location and elastic properties. One of the signs of similar fractured zones cutting across the Ural structures has been identified, namely an increased speed of propagation of shear waves at a reduced speed of compression. The epicenters of Ural earthquakes are confined to the intersection of high-velocity blocks of the earth’s crust of the Tagil-Magnitogorsk trough and Cis-Ural foredeep with identified latitudinal zones of increased fracturing.

Conclusion. The combined interpretation of compression and shear seismic waves made it possible to obtain new data on the deep structure of the Middle Ural region of increased seismicity and to explain the pattern of localization of the epicenters of the Ural earthquakes in this region by its increased tectonic disturbance. The results of the study confirm the geological informational content of multiwave seismicity and can be used in the future to assess the material composition of the earth’s crust, its stress-strain state, development history, as well as for seismic and mineragenic forecasting.

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