Activation of rifting processes in the Northern Cis-Baikal region: A case study of the Kichera earthquake sequence of 1999

The paper addresses the spatiotemporal development of the Kichera sequence of earthquakes of 1999 (more than 6000 events over the year) within the Kichera depression, terminating on land in the Northern Baikal basin; the series was the most intense of all earthquake sequences recorded in the Northern Cis-Baikal region (NCBR) since 1960. The spatial coordinates of earthquakes showed that the source rupture, originating in the area of the Kichera-Upper Angara interbasin mountainous isthmus, propagated in the SW direction toward Lake Baikal. Stresses in the sources of the two strongest shocks (Mw = 6.0 and 5.6) of the sequence were released along fault planes striking NE (normal type) and E-W (normal-strike-slip type). Focal mechanisms of aftershocks revealed the presence of differently oriented faults motions on which were controlled by a large rifting fault striking NE. The Kichera earthquakes are shown to have occurred under seismotectonic conditions dominated by NW-SE extension and to have been accompanied by active normal faulting promoting longitudinal growth of the Upper Angara depression and deepening of the Kichera depression. The seismotectonic strain rates calculated for the NCBR before and after 1999 were of the order of (0.1–1.0) × 10^?10 yr^?1, whereas their values were two to three orders larger during 1999. Thus, the Kichera earthquakes confirmed the high seismic potential of the NCBR and showed that this rift segment developed through growth of depressions and destruction of interbasin mountainous isthmuses.