Depth distribution of earthquakes in the Baikal rift system and its implications for the rheology of the lithosphere

Abstract

Summary 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 (∼50 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 predicted 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 corresponds to a ∼100 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 ∼35–40 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 ∼10–45 km depth range. An upper mantle core with high strength, however, generally prevents it from reaching stress failure at greater depth.

Upper mantle P velocity structure beneath the Baikal Rift from modeling regional seismic data

“…[3] Many investigators have suggested that the development of the rift system has not been influenced to any great extent by warm upwelling mantle material beneath the rift because of the paucity of magmatism in the rift system, and the strength of the rifted lithosphere, as manifest in the gravity field [Diament and Kogan, 1990;Ruppel et al, 1993;Petit et al, 1997] and the prevalence of deep crustal earthquakes [Deverchere et al, 1991[Deverchere et al, , 1993[Deverchere et al, , 2001. Some seismic studies of upper mantle structure find little evidence for a significant thermal anomaly beneath the rift system and therefore support this view [e.g., ten Brink and Taylor, 2002], while others report slower-than-average upper mantle velocities beneath portions of the rift system, consistent with the presence of thermally modified lithosphere [e.g., Puzyrev et al, 1978;Logatchev and Zorin, 1992;Gao et al, 1994].…”

Off-rift volcanism in rift zones determined by crustal unloading

“…The stress barrier is maintained only in the elastic layer. Observations of deep seismicity in young rifts suggest a strong midlower crust [17][18][19] that maintains the stresses necessary to create a stress barrier at any crustal depth. If the lower crust and upper mantle are weak, the elastic stresses dissipate and z 2 may coincide with the bottom of the elastic layer.…”

Crust and Mantle of the Baikal Rift Zone From P- And S-Wave Receiver Functions

“…практически до подошвы коры (например (Déverchère et al, 2001)]. Эти землетрясения могут свидетельствовать о пе-реходе к пластическому режиму деформаций на глубинах, превышающих 30 км, и о хрупкости коры на меньших глубинах.…”