The 1999 Mw 6.0 earthquake sequence in the southern Baikal rift, Asia, and its seismotectonic implications

The 1999 February 25 earthquake (Mw= 6.0) is the largest event to have occurred in the Baikal rift system since the 1959 M= 6.8 earthquake. This earthquake sequence took place in the southern basin of Lake Baikal, a region characterized by a sharp transition from strike-slip faulting on the Main Sayan Fault to normal faulting along the border faults of the lake. Thanks to the development of the regional network and a temporal station installed after the main shock, we performed a relocation of the aftershocks to describe the rupture area and to determine fault-plane solutions. We show that the main shock was caused by normal faulting on an intrabasin fault striking northeast and steeply dipping to the northwest. Aftershock hypocentres did not cluster at the same depth but were spread from 5 to 25 km depth. Stress inversion carried out using focal mechanisms of the sequence reveals a radial extensive stress regime with a minimum principal stress axis σ3 (N155°E) slightly oblique to the regional stress field (N139°E), which depicts a stronger intermediate σ2-axis. This behaviour suggests transient stress perturbation induced by the main shock in the neighbouring zones of the rupture. Strain release deduced from GPS data and historical seismicity shows that within the southern tip of the Baikal rift basin, a significant stress release has occurred for at least two centuries, probably through active normal faulting shared among several faults, while the Main Sayan strike-slip fault further west was undergoing interseismic strain accumulation.