ANALYSIS OF LOCAL SEISMIC WAVE-FORMS FROM UNDERGROUND NUCLEAR-EXPLOSIONS - FORWARD MODELING FOR EFFECTIVE SOURCE FUNCTIONS AT PAHUTE-MESA

In this study we use forward modeling of surface particle velocity data recorded within 15 km of underground nuclear explosions at Pahute Mesa in the Nevada Test Site. A plane-layered velocity structure model is developed, and effective source functions are obtained for six events with yields of 155-1300 kt (kilotons), recorded at ranges presumed to be well outside the zone of inelastic deformation. We then use the effective source functions determined from well outside the elastic radius to compute synthetic seismograms for comparison with recordings within the spall zone. Prior to the onset of spall the comparison is good, enabling us to estimate effective source functions for two additional events for which only recordings from the spall zone are available. As surface zero is approached, however, the first half cycle of the observed velocity records includes an inflection, which appears as a shoulder in acceleration records. This suggests that, in addition to the elastic arrival, we are seeing secondary arrivals due to spall opening or inelastic shock wave propagation. Elastic wave propagation predicts the observed peak velocities well beyond a scaled slant range of about 175 m/(kt)1/3 but overpredicts the first phase at closer ranges. This limit of applicability of elastic wave propagation provides a useful estimate of the elastic radius for Pahute Mesa explosions. Yield scaling relations are determined using three events of known yield, and these are then used to estimate yields for the eight Pahute Mesa explosions modeled in this study. Finally, implications for the attenuation of teleseismic body waves are discussed.