Structural Control of Seismic Activity along the Southeastern Edge of the Bureya Massif

Analysis of the relief and geophysical fields showed the seismic zone of the southeastern margin of the Bureya massif to be confined to a ring structure, where signs of shear deformation were revealed: displacement of submeridional structures by NE faults and oblique mantle detachment. Gravity field analysis indicates that strike-slip displacements were initiated by NE- and NW-oriented heterogeneities. Anomalies of the released seismic energy also have NE and NW directions, but the intensity and size of the first direction are much greater. The strongest seismic sources with M ≥ 5 are confined to deep-seated NE intense gravity anomalies, suggesting that activation of NE structures in this seismic zone is the main process. The area where sources with 5 > M ≥ 4 cluster on the surface is confined to the narrow depression formed above the edge of the deep NW-oriented intense anomaly which tends to pinch out laterally. In the near-surface horizons, according to the gravity field analysis data, submeridional anomalies are displaced by NE-oriented structures to form a transtensional fault. Thus, the area of enhanced seismic activity represents a tectonic center with NE- and NW-oriented structures aligned. These structures initiate rotational and shear deformations, which probably reflects the joint influence of the collision between the Indian and Eurasian plates, on the one hand, and the Pacific subduction zone, on the other, on intraplate processes.