Analysis of Short-Period P-Coda Measurements for Presumed Underground Nuclear Explosions in Eurasia

摘要

Characteristics of P-coda measurements at NORSAR were investigated for presumed underground nuclear explosions in the Soviet Union. For explosions in the Semipalatinsk region, coda magnitudes, measured in the time domain in 5-second windows averaged over 50 seconds of P coda, were found to vary by about 0.1 magnitude units across NORSAR as compared with about 0.2 to 0.3 units for P-wave magnitudes. Also, array-averaged estimates of coda magnitude varied more smoothly with time into the coda than single channel estimates. This result indicates that local subarray scattering causes random perturbations in coda levels which are smoothed out by the averaging process. NORSAR P-coda magnitudes, like the Lg measurements of Rindal (1983), are more consistent with network averaged P-wave magnitudes than NORSAR single-channel magnitudes. There is some indication that Lg measurements may be slightly better than P-coda measurements in terms of reducing scatter and bias. In the analysis of P-codas from seismic events north of the Caspian Sea, codas of presumed explosions in the Astrakhan region are more intense and variable as a function of time into the coda than those for events near Azgir. These differences are attributed to lateral variations in geologic structure in the Pri-Caspian salt basin.