Body-Wave Attenuation across the Baikal Rift Zone, Siberia

The Baikal rift zone (BRZ) is the seismically most active continental rift on Earth. It lies along the junction of the stable Siberian platform and the mobile Sayan-Baikal foldbelt. In spite of intensive geophysical and geodynamic studies during the past decade, some fundamental questions regarding its formation and evolution remain unanswered. One of the key constraints for the various models proposed for the formation and evolution of the BRZ is the existence, geometry, and property of the assumably upwarped asthenosphere beneath the rift. Recently, several seismic tomographic studies resulted in different conclusions, and thus there has been a renewed interest in the deep structure beneath the rift. Spatial variation of t$^*$, which is defined as travel time over $Q$, across the rift serves as an independent set of observations to provide more constraints on the models and tomographic images. The upwarped asthenosphere is expected to be associated with a region of high t$^*$ relative to the areas unaffected by the rifting process. We have measured P- and S-wave t$^*$ along a 1200 km profile traversing the Siberian platform, the BRZ, and the Sayan-Baikal-Mongolian foldbelt. Both the spectral ratio and the common spectrum approaches were used, and the two approaches generated similar results. An area of increased t$^*$ values is observed, which corresponds to a highly attenuative region at the depth of 50-150 km. Comparison of P- and S-wave results suggests a few percent of partial melting. Those results provide independent support for some of the recent tomographic models which suggest a broad, upwarped low velocity asthenosphere beneath the BRZ .