Further Studies Of Seismic Variability At The Shagan River Test Site PDF Download

Large samples of teleseismic P wave data recorded from Shagan River underground explosions have been systematically analyzed in an attempt to develop a better quantitative understanding of the sources of m sub b variability observed for these explosions. Results indicate that large differences in station-corrected m sub b residuals between explosions in close proximity are associated with changes in the near-source proximity are associated with changes in the near-source P wave propagation paths to teleseismic distances. Projection of the m sub b residuals from seismic stations in continental Europe back into the corresponding P wave initiation area near the source reveals the existence of an anomalous volume of material located northwest of the test site at a depth of about 100 km. This anomalous volume apparently defocuses energy out of paths to certain stations and redirects this energy into the paths to other stations. Presumably this anomalous volume corresponds to some sharp lateral variation in physical properties, in particular P wave velocity at this depth. Available geological maps indicate no surface expression of this anomaly. In some cases, the observed patterns of m sub b residual variation can be correlated with semilar patterns of variation in either spectral composite, coda complexity or arrival times of the P wave signals. The sense of these variations in such that enhanced high frequency energy, decreased coda complexity and delayed arrival times are all associated with positive shift in the m sub b residuals.