3D Ground‐Motion Simulations of M w 7 Earthquakes on the Salt Lake City Segment of the Wasatch Fault Zone: Variability of Long‐Period ( T ≥1 s) Ground Motions and Sensitivity to Kinematic Rupture Parameters

Abstract

Abstract We examine the variability of long‐period ( T ≥1 s) earthquake ground motions from 3D simulations of M w 7 earthquakes on the Salt Lake City segment of the Wasatch fault zone, Utah, from a set of 96 rupture models with varying slip distributions, rupture speeds, slip velocities, and hypocenter locations. Earthquake ruptures were prescribed on a 3D fault representation that satisfies geologic constraints and maintained distinct strands for the Warm Springs and for the East Bench and Cottonwood faults. Response spectral accelerations (SA; 1.5–10 s; 5% damping) were measured, and average distance scaling was well fit by a simple functional form that depends on the near‐source intensity level SA 0 ( T ) and a corner distance R c :SA( R , T )=SA 0 ( T )(1+( R / R c )) −1 . Period‐dependent hanging‐wall effects manifested and increased the ground motions by factors of about 2–3, though the effects appeared partially attributable to differences in shallow site response for sites on the hanging wall and footwall of the fault. Comparisons with modern ground‐motion prediction equations (GMPEs) found that the simulated ground motions were generally consistent, except within deep sedimentary basins, where simulated ground motions were greatly underpredicted. Ground‐motion variability exhibited strong lateral variations and, at some sites, exceeded the ground‐motion variability indicated by GMPEs. The effects on the ground motions of changing the values of the five kinematic rupture parameters can largely be explained by three predominant factors: distance to high‐slip subevents, dynamic stress drop, and changes in the contributions from directivity. These results emphasize the need for further characterization of the underlying distributions and covariances of the kinematic rupture parameters used in 3D ground‐motion simulations employed in probabilistic seismic‐hazard analyses. Electronic Supplement: Description of seismic‐velocity model and wave propagation code and interpretation of directivity effects, and figures of fault geometry, Z1 and Z2.5 depths, effects of rupture speed variations on directivity effects and amplifications.