FAULTING WITHIN THE WESTERN BAIKAL RIFT AS CHARACTERIZED BY EARTHQUAKE STUDIES

Earthquake relocations, focal mechanism determinations, waveform modeling, and stress analyses using focal mechanism information are used to examine the faulting processes within the western Baikal rift (95-degrees to 108.5-degrees). Results of the study show that high angle (dip greater-than-or-equal-to 70-degrees) faulting is most likely to occur in the more youthful parts of the rift and is most likely to be associated with M greater-than-or-equal-to 4.5 events. Like the eastern rift, nearly all earthquakes occurred on faults dipping > 30-degrees, suggesting that in no part of the rift is significant deformation occurring on low angle structures. Focal mechanism information does not indicate there is a sharp boundary to the western rift as observed in the eastern rift. Less normal faulting (rakes -60-degrees to -120-degrees) occurs in the western rift than the eastern rift. Waveform modeling of the two largest earthquakes (1950 Mondy and 1959 Srednebaikal) in the western rift gives focal depths of 10 to 15 km. The 1959 event appears to have occurred along the eastern boundary fault on Lake Baikal just to the north of Proval Bay, a feature created a M = 7.1 earthquake in 1862. The 1950 event may have occurred along the southern boundary fault of the Tunka depression. Stress orientation information indicates a near horizontal, NW-SE to WNW-ESE striking sigma-3 in the eastern part of the western rift.