The seismicity and earthquake focal mechanisms of the Baikal Rift Zone

Abstract

Epicentres of earthquakes occurring in the region of the Baikal Rift Zone have a fractal distribution. Temporal variations of the fractal distribution of the epicentral fields of the South Baikal and Tunka depressions show that the spatial structure of the fields is still developing. For practically all regions of the Baikal Rift Zone, the seismic fields typically have clearly defined similarities at different levels of spatial hierarchy and across a wide energy range of earthquakes. The index of seismicity at various scales varies over the zone, with the rift extremities being characterized by the greatest contrast. In this case, even within relatively small areas, there are always places in which the level of seismic activity is less than in the neighbouring areas by a factor of 3 to 12. Analysis of the orientation of the kinematic P and T axes, established from the focal mechanisms of more than 3000 earthquakes which occurred in the Baikal Rift Zone between 1950 and 1991, reveals a clear axial symmetry of the kinematic field. The axis of symmetry, which runs through the northern part of Lake Baikal, divides the area into two parts, mirror images of each other. The transition between the extensive regional kinematic field of rift style and the global compressive kinematic field typical of Western Mongolia and Southern Yakutia is characterized by the existence of aseismic zones, symmetrically located at both extremities of the Baikal Rift Zone.