Seismic Propagation in the Baikal Rift Zone: A Transition from a Craton to an Orogenic Zone

Paul M. Davis and Shangxing Gao
Department of Earth and Space Sciences
University of California, Los Angeles, CA90095
Contract No. F49620-94-1-0161
Sponsored by AFOSR


Most nuclear tests occur on continents. Continental geology can broadly be separated into stable shields and orogenic zones. Recently it has been recognized that large variations in the seismic properties of the upper mantle occur beneath the continents and these need to be taken into account when using seismograms to discriminate explosions from earthquakes, to measure seismic yield and to locate events. This report documents lateral variation in attenuation, anisotropy, and seismic velocity in a 1200x1500 km area centered on the Baikal rift zone in Siberia, based on two portable array experiments we carried out with colleagues from University of Wisconsin and the Institute of the Earth's Crust, Irkutsk in the summers of 91 and 92 as well as analog regional network data collected in 94 which we have digitized. The attenuation analysis finds that relative t^* has a 0.1 second anomaly at the rift zone and increases to the east into the Sayan-Baikal fold belt. Explosions detonated in this area would have reduced body wave amplitudes at arrays at teleseismic distances such as NORESS with a concomitant underestimate in yield. We have observed S wave splitting of on average 1 s at virtually every station, implying that significant anisotropy is present in the uppermost mantle. Though fast directions are spatially coherent in local regions there is significant variation region to region. Travel time delays shown an unusual pattern of a central peak of about 1 sec surrounded by two troughs of about 0.5 secs. We attribute the combination of low Q, anisotropy and travel time anomalies to small scale convective upwelling beneath the rift zone. Comparison with similar experiments and the global Earth models is taken as evidence that these effects are present worldwide beneath orogenic zones on the continents and should be taken into account as part of any global seismic monitoring system.

(Mantle Anomaly Attenuation Anisotropy Baikal Continental Rift Zone)

S. Gao

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