SDSC/NCAR Viz
Ground Motion Prediction From Low-velocity Sediments Including Statistical Models of Inhomogeneities in Southern California Basins
SC12 Movie
- SDSD/NCAR Viz JordanASD2.1920x1080.mov (190MB .mov file)
Explanation of Movie
Problem: State-of-the-art CVMs do not include small-scale (near-surface) amplification effects; no prospects of 10s of meters of resolution from drilling, other direct sampling
Proposal: Incorporate statistical description of near-surface inhomogeneities into the CVMs
As ever-increasing computational resources allow earthquake scientists to push the frequency limits of deterministic ground motion estimates higher, understanding small-scale, near-surface heterogeneities becomes paramount. The variation of the soil amplification over short distances (from tens to hundreds of meters) is important for the design of smaller structures, including lifelines. These small-scale heterogeneities may significantly affect ground motion in geologic basins, and are not included in state-of-the-art Community Velocity Models (CVMs). The density of current (and expected future) direct and indirect measurements of these heterogeneities is insufficient to capture their variation in sedimentary basins.
We have generated statistical models of seismic velocities and densities in agreement with the results of the analysis of near-surface measurements and integrated these models into the most recent SCEC CVMs. The effects of the near-surface heterogeneities on ground motion and scattering are tested using simulations of 0-2.5 Hz wave propagation for the 2008 Mw 5.4 Chino Hills, CA earthquake in a 56 km x 40 km subset of the SCEC CVM 4.0. The 29 July 2008, M 5.4 Chino Hills, California, earthquake, the largest seismic event in the Los Angeles area since the 1994 Mw 6.7 Northridge earthquake. This earthquake was very well recorded on hundreds of seismic stations. The mainshock occurred at a depth of 14.7 km and the epicenter was located in Chino Hills, approximately 28 miles east-southeast of downtown Los Angeles. The earthquake was felt by many people throughout the Los Angeles basin and the surrounding areas although there was very little damage. We assume a minimum Vs of 200 m/s with a grid spacing of 16 m in all directions, calculated using the finite-difference code AWP-ODC. A fractal distribution of near surface heterogeneities with a Hurst exponent of 0.1 and σ=10% is added to the near-surface sediments.
The simulation including the statistical model of the heterogeneities shows several new and interesting results. For example, the wavefield is markedly incoherent, broken up into parts of variable scales, as expected from the fractal statistics. The waves linger for more than one minute in the model area. When compared to strong-motion seismic data from the Chino Hills earthquake, the results tend to predict the duration of ground motion better than the results in the same CVM, but without the statistical model of the heterogeneities, dependent on the relations for the anelastic attenuation. Near-future work includes additional simulations on Yellowstone to refine the parameters of the statistical model.
