Effects of Lateral Velocity Heterogeneity Under the Nevada Test Site on Short-Period P Wave Amplitudes and Travel Times

摘要

Short-period teleseismic P waves from the Nevada Test Site (NTS) show systematicvariations in amplitudes and travel times, with low amplitudes corresponding to fast travel times, suggesting elastic focusing-defocussing effects. Also, the azimuthal amplitude and travel time patterns for events at the Pahute Mesa subsite are systematically different from those at the Yucca Flat subsite, indicating the presence of a near-source component in both the amplitude and travel-time variations. This component is isolated by removing the mean station pattern for all of NTS from the observations. A very-near-source component in the Pahute Mesa observations is also isolated by removing subsite station means from the measurements, whereas the Yucca Flat observations exhibited no coherent very-near-source component. These anomalies are back-projected through laterally homogeneous structure to form thin lens models at various depths. Travel-time delays are predicted from the amplitude variations using the equation for wavefront curvature. Reprints. (jhd)