Ground Motion and Variability from 3-D Deterministic Broadband (0-8 Hz) Simulations
Kyle Withers

Kyle Withers
PhD Candidate, Geophysics
Advisor: Dr. Kim Bak Olsen

Monday, August 29nd, 2016
CSL 422 – 10 am

     The accuracy of earthquake source descriptions is a major limitation in high-frequency ($>1$ Hz) deterministic ground motion prediction, which is critical for performance-based design by building engineers. With the recent addition of realistic fault topography in 3D simulations of earthquake source models, ground motion can be deterministically calculated more realistically up to higher frequencies. We first introduce a technique to model frequency-dependent attenuation and compare its impact on strong ground motions recorded for the 2008 Chino Hills earthquake. Then, we model dynamic rupture propagation for both a generic strike-slip event and blind thrust scenario earthquakes matching the fault geometry of the 1994 Mw 6.7 Northridge earthquake along rough faults up to 8 Hz. We incorporate frequency-dependent attenuation via a power law above a reference frequency in the form $Q_0f^n$ ,with high accuracy down to Q values of 15, and include nonlinear effects via Drucker-Prager plasticity. We model the region surrounding the fault with and without small-scale medium complexity in both a 1D layered model characteristic of southern California rock and a 3D medium extracted from the SCEC CVMSi.426 including a near-surface geotechnical layer.
     Before simulations can be used for engineering applications, validation is required to demonstrate that synthetics have similar characteristics with observed ground motions. The decay of energy at high frequencies, decay as a function of distance, source model, and complementary features between scattering and apparent attenuation are all important features that simulations need to accurately capture to be used for engineering purposes. We find that the spectral acceleration from our models are within 1-2 interevent standard deviations from recent ground motion prediction equations (GMPEs) and compare well with that of recordings from strong ground motion stations at both short and long periods. At periods shorter than 1 second, Q(f) is needed to match the decay of spectral acceleration seen in the GMPEs as a function of distance from the fault. We find that the similarity between the intraevent variability of our simulations and observations increases when small-scale heterogeneity and plasticity are included, extremely important as uncertainty in ground motion estimates dominates the overall uncertainty in seismic risk. In addition to GMPEs, we compare with simple proxy metrics to evaluate the performance of our deterministic models and to determine the importance of different complexities within our model. We find that 3D heterogeneity, at both the long and short scale-lengths, is necessary to agree with data, and should be included in future simulations to best model the ground motion from earthquakes.