Validation of Las Vegas basin response to the 1992 Little Skull Mtn. earthquake as predicted by physics-based Nevada ShakeZoning computations

Brady A. Flinchum, William H. Savran, Kenneth D. Smith, John N. Louie, Satish K. Pullammanappallil, & Aasha Pancha

Published October 8, 2012, SCEC Contribution #1537

Over the last two years the “Nevada ShakeZoning” (NSZ) procedures have been developed and refined to characterize earthquake hazards in the Intermountain West. Nevada ShakeZoning relies on physics and geology to estimate earthquake shaking hazards, rather than statistics. To verify the results of the NSZ process and the ground shaking it predicts for Las Vegas Valley (LVV), the ML 5.6 -5.8 Little Skull Mountain (LSM) earthquake was replicated. The NSZ process employs a finite-difference code to compute wave propagation through complex 3d models. The finite difference model and computing power restricts models to have lower frequencies. For the extensive LSM-LVV model the limit is 0.1-0.3 Hz and lower. The Clark County Parcel Map is a critical data set included in NSZ predictions for LVV. The Parcel Map only represents velocities in the upper 30 m of our models but by using the Parcel Map amplifications of 120% to 300% can be seen in LVV, even at low frequencies. A detailed model of the LVV basin-floor depth, and regional basin-thickness models derived by the USGS are also important components of NSZ. Before comparison, the ground-motion time histories that were recorded by accelerometers are integrated to convert to velocity. A band-pass filter with corner frequencies of 0.1 and 0.6 Hz is applied to the derived velocity seismograms, so that the recordings can be compared against the ShakeZoning synthetic seismograms. The Rayleigh-wave minus P-wave (R-P) times and the pulse shapes of Rayleigh waves correlate well between the data and synthetics. Most importantly, NSZ-predicted peak ground velocities matched what was observed, to closer than a factor of two. The basin and velocity models that are used in NSZ still need refinement, since observed seismograms within LVV show longer durations of shaking, caused by horizontally reverberating, 0.2-Hz longitudinal waves beyond 100 sec after Rayleigh-wave arrival. Within the basins, the current velocity models are homogeneous below 30 m depth, causing the LVV synthetics to show insufficient shaking durations of only 30-40 s.

Flinchum, B. A., Savran, W. H., Smith, K. D., Louie, J. N., Pullammanappallil, S. K., & Pancha, A. (2012, 10). Validation of Las Vegas basin response to the 1992 Little Skull Mtn. earthquake as predicted by physics-based Nevada ShakeZoning computations. Presentation at 2012 EGGE Conference.