Broadband Ground Motion and Variability from 3D Dynamic Rupture Simulations along the Wasatch Fault, Utah, incorporating both Stochastic Fault Roughness and Deterministic Long-wavelength Geometry

Kyle B. Withers, Morgan P. Moschetti, & Kenneth Duru

Submitted August 9, 2018, SCEC Contribution #8359, 2018 SCEC Annual Meeting Poster #004

We numerically model earthquake rupture on a fault surface that includes long-wavelength geometry matching that of the Salt Lake City segment of the Wasatch fault, Utah, in addition to superimposed stochastic fault roughness. We seek to better understand the rupture process and assess broadband ground motion and variability along dipping faults by extending deterministic ground motion prediction to higher frequencies (>1 Hz). Our approach utilizes dynamic rupture simulations using a summation by parts method with the recently developed finite difference code, Waveqlab3d. We generate a suite of ruptures ~Mw 7.0 (40 x 20 km along strike and width, respectively), with varying hypocenter locations and initial stress conditions. The fault roughness generates ground motion with flat acceleration spectra across a wide frequency bandwidth (from 0.1 up to 5-10 Hz). Long-wavelength geometry affects the initial stress distribution on the fault (along both strike and dip), creating regions where rupture is either inhibited or promoted. We also investigate the significance of features such as free-surface topography and off-fault plasticity (via a Drucker-Prager approach) and their influence on the features specific to dipping faults. For example, a dipping fault plane is known to cause asymmetries in the rupture across the hanging wall/footwall boundary. We find that plastic strain accumulation is mainly generated on the hanging wall, in agreement with previous studies, and that there are minimal effects of the free-surface topography to the rupture process. However, both of these features significantly affect the level of ground motion, by reducing peak amplitudes from shallow near-surface scattering. Our synthetically generated ground motion is near that of a median event and the level of intra-event variability slightly lower than that of recent GMPE relations. We expect that the use of a 3D seismic velocity model in future simulations, in place of a 1D velocity profile, will contribute to increasing the simulation-derived variability to values closer to what is observed in recorded ground motions.

Key Words
deterministic ground motion, dynamic rupture

Citation
Withers, K. B., Moschetti, M. P., & Duru, K. (2018, 08). Broadband Ground Motion and Variability from 3D Dynamic Rupture Simulations along the Wasatch Fault, Utah, incorporating both Stochastic Fault Roughness and Deterministic Long-wavelength Geometry. Poster Presentation at 2018 SCEC Annual Meeting.


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