ASYMMETRIC UPWARP OF THE ASTHENOSPHERE BENEATH THE BAIKAL RIFT-ZONE, SIBERIA

In the summer of 1991 we installed 27 seismic stations about lake Baik al, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal rift z one and adjacent regions. Sixty-six teleseismic events with high signa l-to-noise ratio were recorded. Travel time and Q analysis of teleseis ms characterize an upwarp of the lithosphere-asthenosphere boundary un der Baikal. Theoretical arrival times were calculated by using the Int ernational Association of Seismology and Physics of the Earth's interi or 1991 Earth model, and travel time residuals were found by subtracti ng computed arrival times from observed ones. A three-dimensional down ward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our resul ts suggest that (1) the lithosphere-asthenosphere transition upwarps b eneath the rift zone, (2) the upwarp has an asymmetric shape, (3) the velocity contrast is -4.9% in the asthenosphere, (4) the density contr ast is -0.6%, and (5) the P wave attenuation contrast t is 0.1 s.