Validation of Deterministic Broadband Ground Motion and Variability from Dynamic Rupture Simulations of Buried Thrust Earthquakes

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

Published November 13, 2018, SCEC Contribution #8071

We numerically model broadband ground motion (up to $5-7.5$ Hz) from blind thrust scenario earthquakes matching the fault geometry of the 1994 Mw 6.7 Northridge earthquake. Several realizations are modeled (by varying the hypocenter location in the dynamic rupture simulation) in a 1D layered velocity profile. In addition, we include Q(f), nonlinear effects from Drucker-Prager plasticity, and superimpose small-scale medium complexity in both a 1D layered and 3D velocity model within the subsequent wave propagation. We investigate characteristics of the ground motion and variability up to 50 km from the fault by comparing with ground motion prediction equations (GMPEs), simple proxy metrics, as well as strong ground motion records from the Northridge event. We find that median ground motion closely follows the trend predicted by GMPEs and that the intra-event standard deviation, although varying with hypocenter location, lies near that of GMPE models. Plasticity affects ground motion amplitudes in regions near the source, reducing intra-event variability above $\sim$ $0.5$ Hz. Heterogeneity in the velocity structure on both the regional and small-scale is needed for the simulated data to match two proxy metrics: the period-to-period correlation of spectral acceleration and the ratio of maximum-to-median spectral acceleration. While small-scale heterogeneity has a negligible effect on median spectral acceleration for this style of rupture, it serves to significantly increase the cumulative absolute velocity, better agreeing with observations. When compared with strong motion data, we find that long-wavelength velocity structure within our deterministic simulations reduces bias at both short and long periods. Finally, synthetic ground motion at both footwall and hanging wall sites has no clear dependence on the distance to rupture (at both short and long periods); directivity is likely overpowering any hanging wall effect.

Withers, K. B., Olsen, K. B., Shi, Z., & Day, S. M. (2018). Validation of Deterministic Broadband Ground Motion and Variability from Dynamic Rupture Simulations of Buried Thrust Earthquakes. Bulletin of the Seismological Society of America,. doi: 10.1785/0120180005.