DEPTH DISTRIBUTION OF EARTHQUAKES IN THE BAIKAL RIFT SYSTEM AND ITS IMPLICATIONS FOR THE RHEOLOGY OF THE LITHOSPHERE

Abstract

The correspondence between the predicted brittle–plastic transition within the crust and the maximum depth of earthquakes is examined in the case of the Baikal rift, Siberia. Although little accurate information on depths is available through large-and moderate-size earthquakes, there are frequent indications of foci at 20 km depth and more. We have relocated 632 events recorded at nearby stations that occurred between 1971 and 1997, with depth and epicentral uncertainties less than 5 km, over the eastern and southern parts of the Baikal rift. We have compared these results with other depth distributions obtained in previous studies from background seismicity in the NE rift (1365 events in the Kalar-Chara zone and 704 events in the Muya region). The relative abundance of earthquakes is generally low at depths between 0 and 10 km (7–15 per cent) and high between 15 and 25 km (y50 per cent). Earthquake activity is still significant between 25 and 30 km (9–15 per cent) and persists between 30 and 40 km (7–13 per cent). Very few earthquakes are below the Moho. We use empirical constitutive laws to obtain the yield-stress limits of several layers made of dominant lithologies and to examine whether the observed distribution of earthquakes at depth (519 events controlled by a close station and located within the extensional domain of the Baikal rift system) can match the pre-dicted crustal strength proportion with depth and the deeper brittle–ductile transition in the crust. A good fit is obtained by using a quartz rheology at 0–10 km depth and a diabase rheology at 10–45 km depth with a moderate temperature field which corres-ponds to a y100 Myr thermal lithosphere. No dioritic composition of the crust is found necessary. In any case, earthquakes occur at deep crustal levels, where the crust is supposed to be ductile, in a way very similar to what is found in the East African Rift System. From these results we conclude that the seismogenic thickness is y35–40 km in the Baikal rift system and that the depth distribution of earthquakes is at first order proportional to the strength profile found in a rheologically layered crust dominated by a mafic composition in the y10–45 km depth range. An upper mantle core with high strength, however, generally prevents it from reaching stress failure at greater depth.