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Ground Motion and Intraevent Variability from 3D Deterministic Broadband (0–7.5 Hz) Simulations along a Nonplanar Strike‐Slip Fault

Kyle B. Withers, Kim B. Olsen, Steven M. Day, & Zheqiang Shi

Published November 13, 2018, SCEC Contribution #8068

We model deterministic broadband (0-7.5 Hz) ground motion from a $M_w$ $7.1$ bi-lateral strike-slip earthquake scenario with dynamic rupture propagation along a rough-fault topography embedded in a medium including small-scale velocity and density perturbations. Spectral accelerations at periods 0.2-3 seconds and Arias Intensity durations show a similar distance decay when compared to NGA-West2 GMPEs using a Q(f) power-law exponent of $0.6-0.8$ above 1 Hz in models with a minimum Vs of $750$ m/s. With a trade-off from Q(f), the median ground motion is slightly increased by scattering from statistical models of small-scale heterogeneity with standard deviation (sigma) of the perturbations at the lower end of the observed range (5\%), but reduced by scattering attenuation at the upper end (10\%) when using a realistic 3D background velocity model. The ground motion variability is strongly affected by the addition of small-scale media heterogeneity, reducing otherwise large values of intra-event standard deviation closer to those of empirical observations. These simulations generally have intra-event standard deviations for SAs lower than the GMPEs for the modeled bandwidth, with an increasing trend with distance (most pronounced in low-moderate scattering media) near the level of observations at distances greater than 35 km from the fault. Durations for the models follow the same increasing trend with distance, where $\sigma \sim 5\%$ produces the best match to GMPE values. We find that a 3D background velocity model reduces the pulse period into the expected range by breaking up coherent waves from directivity, generating a log-normal distribution of ground motion residuals. These results indicate that a strongly heterogeneous medium is needed to produce realistic deterministic broadband ground motions. Finally, the addition of a thin surficial layer with low, frequency-independent Q in the model (with a minimum Vs of 750 m/s) controls the high-frequency decay in energy as measured by the parameter $\kappa$, that may be necessary to include as simulations continue to extend to higher frequencies.

Withers, K. B., Olsen, K. B., Day, S. M., & Shi, Z. (2018). Ground Motion and Intraevent Variability from 3D Deterministic Broadband (0–7.5 Hz) Simulations along a Nonplanar Strike‐Slip Fault. Bulletin of the Seismological Society of America,. doi: 10.1785/0120180006.