The eastern Baikal rift is characterized by a succession of young an echelon half-grabens distributed in a 300-km-wide zone of high seismic activity. In this study we present the overall seismotectonic setting of the north Muya region, which is an along-strike transfer zone between two en echelon dip-slip fault systems. Using both regional and local seismological networks, we refine the hypocenters and the single-event focal mechanisms of 704 earthquakes recorded during 3 years, including the main period of activity of a very dense cluster, the Angarakan swarm. Our final hypocenter locations, obtained after determining a region-specific velocity model and applying a master event technique, allow us to observe a widespread and unusual seismic deformation throughout the upper 30 km of the continental crust. Deep earthquakes (15--30 km) define a planar surface related to a large north-dipping basement fault (Upper Muya), and also occur in a small cluster at 23 km depth at the intersection of two major structural trends.

Along more than 150 km of strike length from west to east, maximum focal depths continuously increase from 20 to 30 km, whereas fault dip concomitantly increases from 30¿ to 55¿. On the other hand, most activity of the Angarakan swarm is confined in the ''classical'' depth range of 0--15 km and depicts a narrow and steeply south-dipping seismic band related to a very recent fault scarp at the surface (eastern Kovokta). This fault geometry, which is observed at depth and in the field, and the opposite tilting of regularly 30-km-spaced horsts and grabens, reveal an asymmetrical rift system. The agreement of fault lengths estimated from field studies with rupture lengths estimated from large historical earthquakes strengthens the inference that large earthquakes may occur near the base of the seismogenic layer of the crust, i.e., 30 km. Most of the 39 focal mechanisms determined in the study region show a dominant normal dip-slip displacement on nodal planes striking in the main inherited structural direction WSW-ENE, whereas some limited pure strike-slip faulting mainly occurred in the highly fractured Angarakan zone. From inversion of 37 focal mechanisms, we deduce a tensional stress tensor with a &sgr;3 axis striking N160¿ and a &sgr;2 axis slightly compressional (shape factor R of 0.4).

Comparing these results with stress tensors computed along the eastern Baikal rift and with the focal mechanisms of three strong earthquakes that occurred in the Muya region during this century shows that small-magnitude earthquake data give consistent and accurate constraints on the regional stress regime. Whether this present stress tensor remained stable since the beginning of opening of the Baikal rift and during Quaternary is unknown. Our results confirm that continental lithosphere submitted to rapid extension in an early stage of rifting may retain significantly rigidity. ¿ American Geophysical Union 1993