Sensitivity of long-period ground motion simulations to seismic velocity perturbations in the Wasatch Front community velocity model
Abstract
Earthquake hazard in the Salt Lake City, Utah region is strongly dominated by the potential for a M7 event on the Salt Lake segment of the Wasatch fault. The lack of instrumental records due to the long recurrence intervals on the fault and the inability of ground motion prediction equations to capture the effects of the basin structure make the estimation of earthquake ground motions from 3-D deterministic simulations an attractive approach to characterizing hazard in the region. Our simulations investigate the effect of velocity perturbations in the seismic material model on earthquake ground motions for a M7 event on the Wasatch fault. The reference seismic velocity model is the Wasatch Front community velocity model (WCVM). Earthquake simulations are carried out with the reference seismic material model and for models in which we make perturbations to the regional seismic velocity model and to the seismic velocities of the deep sedimentary basins. We construct a kinematic fault model that defines the slip amplitudes, rupture velocities and rise times and model seismic wave propagation using the Hercules finite element tool-chain (Tu et al., 2006). We obtain displacement time histories and compare ground motion parameters from the reference and perturbed velocity models. Ground motion parameters that are of greatest importance in engineering seismology (e.g., peak ground motions, spectral accelerations, etc.) are selected for the comparisons. Differential wave propagation through the reference and perturbed seismic velocity models explains the variations in earthquake ground motions. We also present recent results from a comprehensive validation of the WCVM and the perturbed velocity models to characterize the fit between observed and synthetic seismograms from small magnitude (M3-4.5) earthquakes in the region. Overall, we find that the WCVM can reproduce the waveform parameters that are of greatest interest in seismic hazard studies to a fair degree, up to a high-frequency corner of 0.5 Hz for two of the simulated earthquakes and to 0.1 Hz for one of the events. Spatial variation in the GOF is high but strong-motion records generally show poorer fits at the seismographs located atop thick sediments and near basin-edges. In addition, we find a correlation between the discrepancies in peak ground motions (velocity and acceleration) for the observed and synthetic seismograms and distance for two of the modeled earthquakes for the higher frequency band measurements. The effect of velocity perturbations in the sedimentary basins on ground motions increases with the frequency band of the measurement and suggests that knowledge about the velocity structure of the sedimentary basins has important consequences for accurately predicting earthquake ground motions. Our results suggest that future modifications to the WCVM focus on improving the seismic velocity structure of the deeper parts of the sedimentary basins.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.S51B2211M
- Keywords:
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- 7212 SEISMOLOGY / Earthquake ground motions and engineering seismology