Shallow velocity structure at the Shagan River Test Site in Kazakhstan

During 1997 and 1998, twelve chemical explosions were detonated in borehole s at the former Soviet nuclear test site near the Shagan River (STS) in Kaz akhstan. The depths of these explosions ranged from 2.5 to 550 m, while the explosive yield varied from 2 to 25 tons. The purpose of these explosions was for closure of the unused boreholes at STS, and each explosion was reco rded at local distances by a network of seismometers operated by Los Alamos National Laboratory and the Institute of Geophysics for the National Nucle ar Center (NNC). Short-period, fundamental-mode Rayleigh waves (Rg) were ge nerated by these explosions and recorded at the local stations, resultingly the waves exhibited normal dispersion between 0.2 and 3 seconds. Dispersio n curves were generated for each propagation path using the Multiple Filter Analysis and Phase Match Filtering techniques. Tomographic maps of Rg grou p velocity were constructed and show a zone of relatively high velocities f or the southwestern (SW) region of the test site and slow propagation for t he northeastern (NE) region. For 0.5 see Rg, the regions are separated by t he 2.1 km/sec contour, as propagation in the SW is greater than 2.1 km/sec and less in the NE region. At 1.0 see period, the 2.3 km/sec contour separa tes the two regions. Finally, for 1.5 and 2.0 see, the separation between t he two regions is less distinct as velocities in the NE section begin to ap proach the SW except for a low velocity region (< 2.1 km/sec) near the cent er of the test site. Local geologic structure may explain the different reg ions as the SW region is composed predominantly of crystalline intrusive ro cks, while the NE region consists of alluvium, tuff deposits, and Paleozoic sedimentary rocks. Low velocities are also observed along the Shagan River as it passes through the SW region of the test site for shorter period Rg (0.5-1.0 sec). Iterative, least-squares inversions of the Rg group velocity dispersion curves show shear-wave velocities for the southwestern section that are on average 0.4 km/sec higher than the NE region. At depths greater than 1.5 km the statistical difference between the models is no longer sig nificant. The observed group velocities and different velocity structures c orrelate with P-wave complexity and with spatial patterns of magnitude resi duals observed from nuclear explosions at STS, and may help to evaluate the mechanisms behind those observations.