Analysis of Site Response at U1A Hole at the Nevada Test Site From Weak Motion Readings

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We utilize weak motion recordings to evaluate the site response at the U1A hole, Nevada Test site to determine the effect on potential ground motion at the drift of the U1A hole 962 ft deep. We estimated the site response amplification of ground motion at the surface relative to the drift with the spectral ratio method. We utilized Fourier amplitude and absolute acceleration response spectra, and confined our study to frequencies of 0.5 to 25.0 Hz (.04 to 2.0 s periods). We identified 8 earthquakes in the area that were recorded at the bottom and top of the hole that were used for spectral ratios. We calculated the average and one standard deviation of ratios from all the events. Examining the data, we found that: (1) Fourier amplitude spectral ratios provided more detailed information on the site response than the absolute acceleration response that can be directly related to the effect of large earthquakes. (2) plots of the Fourier amplitude spectra for most of the recorded earthquakes show evidence for a spectral hole in the downhole recordings. This is due to downward reflected energy from the surface. This is not evident in absolute acceleration response records. (3) Fourier amplitude spectral ratios show a relative amplification at the surface of about a factor of eight for frequencies between about 9 to 15 Hz (.07 to .ll s periods) due to the spectral hole. (4) The free surface results in an amplification of about a factor of 2 for frequencies of about 13.0 to 25.0 Hz (.04 to .08 s periods). (5) The geology results in an amplification of about a factor 2 of the surface relative to the bottom for frequencies 1.0 to 25.0 Hz (0.04 to 1.0 s period). (6) A full site response function is provided as a function of frequency from the Fourier amplitude spectral ratios. This includes the effect of the spectral hole, free surface effect, and geologic amplification. It shows that strong ground motion would be diminished at the bottom of the U1A hole by a factor of .5 to 0.07 (as a function of frequency) for frequencies from 1.0 to 25.0 Hz (.04 to .5 s periods). This is an estimation based upon linear ground motion response. More sophisticated soils modeling is necessary to determine whether non-linear ground motion may occur. More sophisticated ground motion modeling is necessary to determine under what conditions the spectral hole can diminish ground motions at the drift of the U1A hole 962 ft deep.