Validating Nevada ShakeZoning Predictions of Las Vegas Basin Response against 1992 Little Skull Mountain Earthquake Records

Abstract

Over the last two years, the Nevada Seismological Laboratory has developed and refined Nevada ShakeZoning (NSZ) procedures to characterize earthquake hazards in the Intermountain West. Simulating the M L 5.6-5.8 Little Skull Mountain (LSM) earthquake validates the results of the NSZ process and the ground shaking it predicts for Las Vegas Valley (LVV). The NSZ process employs a physics-based finitedifference code from Lawrence Livermore Laboratory to compute wave propagation through complex 3D earth models. Computing limitations restrict the results to low frequencies of shaking. For this LSM regional model the limitation is to frequencies of 0.12 Hz, and below. The Clark County Parcel Map, completed in 2011, is a critical and unique geotechnical data set included in NSZ predictions for LVV. Replacing default geotechnical velocities with the Parcel Map velocities in a sensitivity test produced peak ground velocity amplifications of 5%-11% in places, even at low frequencies of 0.1 Hz. A detailed model of LVV basin-floor depth and regional basin-thickness models derived from gravity surveys by the U.S. Geological Survey are also important components of NSZ velocity-model building. In the NSZ-predicted seismograms at 0.1 Hz, Rayleigh-wave minus P-wave (R − P) differential arrival times and the pulse shapes of Rayleigh waves correlate well with the low-pass filtered LSM recordings. Importantly, peak ground velocities predicted by NSZ matched what was recorded, to be closer than a factor of two. Observed seismograms within LVV show longer durations of shaking than the synthetics, appearing as horizontally reverberating, 0.2 Hz longitudinal waves beyond 60 s after Rayleigh-wave arrival. Within the basins, the current velocity models are laterally homogeneous below 300 m depth, leading the 0.1 Hz NSZ synthetics to show insufficient shaking durations of only 30-40 s.

Large‐Scale Earthquake‐Hazard Class Mapping by Parcel in Las Vegas Valley, Nevada

“…The Parcel Map V S30 velocity values replaced default NEHRP site class geotechnical velocities to test the sensitivity of the ground motions. Parcel Map V S30 geotechnical details were essential for correctly simulating the data, even for low-frequency computations, as demonstrated by the comparisons made by Flinchum et al (2014). Results from Flinchum et al (2014) confirm the findings of Anderson et al (1996), demonstrating the upper 30 m have a large influence on observed ground motions.…”

“…Despite the limitations of the single-value V S30 parameter in defining the effects of soil structure on ground motion, applications utilizing V S30 still clearly demonstrate its significance for describing the overall site effect on ground-motion estimation (e.g., Sandikkaya et al, 2013). Flinchum et al (2014) validated deterministic computations of ground motions and amplifications against earthquake recordings of the 1992 M L 5.6-5.8 Little Skull Mountain event (Smith et al, 2001) across Las Vegas Valley, matching amplitudes and time histories of shaking at 0.2 Hz and below. This work included the CCDDS and CoH Parcel Map results from this study in building the 3D geological and geotechnical model for wave propagation.…”

“…The municipalities are using the resultant Parcel Map in planning for development and disaster response, in addition to its direct use for building code implementation and enforcement. Further, the V S30 measurements have been incorporated into 3D numerical earthquake simulations toward a more comprehensive seismic-hazard assessment of the city (Flinchum et al, 2014). In this article, we present the methodology used to accomplish the Parcel Mapping within a three-year project period, details of the resultant site-class Parcel Map, and the benefits of the V S mapping.…”

Development and Testing of a 3D Seismic Velocity Model of the Po Plain Sedimentary Basin, Italy

“…A highly detailed description of the shallow velocity structure (shallower than 1-3 km), beyond even the reach of seismic tomography with the existing seismograph stations, is, however, needed to decrease the minimum period to T ∼ 1 s (Kagawa et al, 2004). This includes the necessity to model the shallow geotechnical layer (often modeled by the V S30 parameter), but the sparsity of available information, confronted with the expected rapid spatial variability of the geological structure, makes the incorporation of such constraints into large 3D models problematic (Flinchum et al, 2014;Taborda and Bielak, 2014). Stochastic synthesis is required to reach even higher frequencies, with engineering interest for low-rise residential buildings, and a hybrid deterministicstochastic approach could be used (Mai et al, 2010).…”

“…However, the sparsity of available information, confronted with the expected rapid spatial variability of the geological structure, makes the incorporation of geotechnical constraints into large 3D models problematic. Recent examples include the studies by Taborda and Bielak (2014) on the Los Angeles region, and by Flinchum et al (2014) on the Las Vegas area.…”