An analysis of simulated and observed blast records in the Salt Lake Basin

We have simulated 0.2 to 1.2-Hz 3D elastic wave propagation in the Salt Lake Basin from a blast at a nearby open-pit mine. A fourth-order staggeredgrid finite-difference method was used to simulate the blast in a two-layer basin model (58 × 43 × 9 km) consisting of semiconsolidated sediments up to 1.3-km thick surrounded by bedrock. Data from four blasts in the mine pit, recorded by a network of 10 digital three-component instruments, were compared to the results of the simulation. The simulation reproduces the overall pattern of ground-motion amplification at basin sites relative to a rock site, as measured by ratios of peak particle velocities, cumulative kinetic energies, and spectral magnitudes. Considering the simple two-layer basin model used in the 3D simulation, this finding suggests that the deep 3D basin structure significantly contributes to low-frequency ground-motion amplification in the Salt Lake Basin. Order-of-magnitude discrepancies exist between some of the observed and pre-dicted ground-motion parameters, and the simulations underpredict the signal durations at most stations. We use 2D simulations along a profile through the southern part of the basin model to investigate the causes of these discrepancies. These causes may be summarized, in order of their importance along this profile, as follows: 1. Effects of a near-surface layer of low-velocity unconsolidated sediments (P-and S-wave velocities of 1.65 and 0.41 km/sec, respectively) that at soil sites along the profile increase the peak particle velocities by up to a factor of 3 and significantly increase the ground-motion durations. 2. Attenuation in the sediments, which greatly diminishes the ground-motion durations on the synthetic seismograms when parameterized by realistic values of the quality factor, Q (20 for soil sites and 35 for bedrock sites). 3. 2D topographic scattering, which increases the peak particle velocities by up to a factor of 2 and increases the signal durations for sites along the profile. Compared to the records from the simple two-layer 3D simulation, the records from a 2D P/SV-wave simulation that includes processes (1) through (3) provide a better match to the blast data - especially the observed durations of shaking. At five of the six stations along the profile, the 2D simulation reproduces the normalized radial and vertical peak particle velocities to within a factor of 2 and the normalized cumulative kinetic energies and spectral amplitudes on these components to within generally a factor of 3. Our results suggest that deep-basin resonance, reverberations in the near-surface low-velocity layer, attenuation, and topographic scattering significantly influence site amplification in the Salt Lake Basin. Future studies of site amplification in the Salt Lake Basin should include the effects of all of these mechanisms.