3D-tectonic analysis of seismicity fields in the south regions of Russian Far East

Abstract

The relevance of the research is caused by the need to detect tectonic reasons of earthquakes in the Far East Russia and to analyze the relation of seismic intensity with rheological state of tectonic media. The aim of the research is to justify the relation of earthquake hypocenter and magnitude distributions with a deep structure and rheological state of tectonic media in the crust and the upper mantle of Northeast Asia. The method used in the study: 3D modeling of earthquake hypocenters and magnitudes distributions and development of layer-bylayer schemes and sections of average magnitude distributions on this basis. Results. In distributions of hypocenters and magnitudes of earthquakes the authors have found out vertical resolution. It is related with rheological heterogeneity of tectonic media in the crust and the upper mantle. The areas characterized by increased magnitudes of earthquakes (M=3,5-6) in the upper layer of the crust up to the depth of 10-15 km are spatially correlated with ancient metamorphic complexes. They are outcropped in the Aldano-Stanovy shield of North Asian craton and the northern flank of North Chinese cratons. Within the Amur plate the maxima of magnitudes are dated for blocks of cratonic type: Arguno-Mamynsky, Dyagdagachinsky and Tszyamusy-Bureninsky. By the depth and shape of the upper seismic layer in these blocks it is possible to define the depth, thickness and behavior of the crystal crust layer roof, as well as the degree of a rework of the crust caused by disjunctive and volcanic processes. In particular, the authors determined the immersion of a granite-metamorphic layer of the crust in the North Chinese craton under Tszyamusy-Bureninsky terrane and crystal complexes of the Aldano-Stanovoy shield under Mongolo-Okhotskaya and Sikhote-Alin fold-thrusted systems. In margin seas the maxima of magnitudes coincide with island arcs (Sakhalin, Kuril) in which basement there are high density oceanic ultrabasites. Zones of lowered viscosity are associated with lowered and low magnitudes of earthquakes (M=2,5-3,2) in the lower crust, subcrustal layer of the upper mantle, and an asthenosphere where they are correlated with low electric resistance and lowered seismic velocity. Three geometrical types of such zones: vertical linear, layered in horizontal, and local izometric are found. The first ones coincide with lithosphere stretching structures and magma-permeable zones of deep faults. The second zones coincide with the boundaries of tectonic layers in sections, and the third ones coincide with the centers of tectonic-magmatic structures of the plume nature. In all cases the low seismicity zones are followed by a complex of geologic-geophysical signs indicating fluid, magmatic or tectonic rework of geological structures corresponding to them. The paper characterizes vertical discontinuance and longitudinal heterogeneity of seismic focal zones in the Sea of Okhotsk and Japanese regions. In the region of Okhotsk Sea the Pacific lithosphere subduction under the plate of the Sea of Okhotsk shows itself in the sections of M (x,y,z)-model, however this subduction is traced only up to the depth of 100 km.

Keywords

• earthquake magnitudes
• rheology
• deep structure of the crust and upper mantle
• Northeast Asia