Determination of Low-Strain Site Amplification Factors in the Salt Lake Valley, Utah, Using ANSS Data

Using data from the Advanced National Seismic System (ANSS) network in and near the Salt Lake Valley (SLV), Utah, we measured average, frequency-dependent, low-strain site amplification factors for site response units mapped by others on the basis of geology and near-surface shear-wave velocity. The site amplification factors were determined using distance-corrected spectral ratios between horizontal-component ground-motion recordings from soil sites and reference rock sites. To test various models for the distance correction terms, we measured spectral ratios between recordings at 12 Paleozoic rock sites. These spectral ratios indicate that the ground motions decrease with hypocentral distance, r, at rates of r^-1.5 in the period range 0.4 to 2.0 sec and r^-2.0 in the period range 0.1 to 0.5 sec. We calculated the soil/rock spectral ratios using two different reference stations on Paleozoic rock. Geometric mean site amplification terms for three SLV site response units were obtained by combining data from both reference stations. Comparing the resultant site amplification factors to those of previous studies indicates that empirically-based predictions better fit the observed data. Specifically, the empirically-based site amplification factors of Borcherdt (1994) and Boore and others (1997) fit the data better than the theoretically-based factors of Wong and others (2002), even though the latter were developed specifically for the SLV site response units.