Analysis of Component Surface/Downhole Ground Motions at Yucca Mountain for Underground Nuclear Explosions in Pahute Mesa

The Weapons Test Seismic Investigations (WTSI) project has been working in support of the Yucca Mountain Site Characterization Project (YMP) since 1977. If a waste storage facility is located near the Nevada Test Site (NTS) it will be subjected to ground shaking generated by underground nuclear weapons tests. A knowledge of expected ground motion levels from these tests will enable the designers to provide for the necessary structural support in the designs of the various components of the repository. The primary mission of the WTSI project involves recording and analyzing ground motion data from these underground nuclear explosions (UNEs) and developing a method to predict the amplitude of ground motions generated at the repository site from future weapons tests. Surface and downhole ground motion data have been recorded at Yucca Mountain stations. Peak amplitudes as well as pseudo-relative velocity response spectra (PSRVs) for these data are studied. Surface/downhole behavior of the ground motion is best described as an arithmetic average of the ratios of surface/downhole PSRVs at each station. For the most part, downhole motions are less than surface motions. The one exception to this is station W28, where an anomalous, high frequency signal is present in the horizontal components. Possible explanations are given for this behavior. 11 refs., 28 figs., 15 tabs.