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Basin Effects From 3D Simulations in Southern California: Basin-Depth Scaling and Nonergodic Site Adjustments

Description: 

We develop basin-depth scaling models and nonergodic site adjustments from the synthetic ground motions of the Southern California Earthquake Center (SCEC) CyberShake project. Calculations use the NGA-West-2 ground-motion models (GMMs) and are developed from a large subset (>600,000) of ground-motion residuals. We explore two approaches to the treatment of by using either site-specific values or using a uniform value that is consistent with the minimum shear-wave velocity of the 3D simulations. Overall, basin-depth scaling in the simulations is similar to the empirical GMMs. However, simulated ground motions within southern California basins are uniformly higher than GMM predictions and show weaker scaling at periods less than 4 s. We explore several options for addressing the uncertainty in absolute ground-motion level—by directly using the simulated ground motions, by constraining the ground motions to match empirical GMMs where differential basin depths are zero, and by using empirical adjustments from recent results in southern California; ultimately, we recommend the latter approach due to its agreement with observational data. We implement the simulation-derived basin-depth scaling models into the nshmp-haz code for use in the 2023 U.S. National Seismic Hazard Model. We find minor differences in probabilistic seismic hazard between the simulation-derived and empirical models. Hazard differences can be explained by differences in the strength of basin-depth scaling and maximum basin amplifications. We also explore methods to incorporate spatially varying adjustments to site terms using the simulated data set. The nonergodic adjustments indicate period-dependent differences in the amplification of the Ventura and Los Angeles basins that cannot be explained by basin depth alone and are likely caused by the effects of 3D geometry or complex wave-propagation effects.

Session: Future Directions in Physics-based Ground-motion Modeling in Preparation for the Fall 2023 Meeting [Poster]

Type: Poster

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Morgan P. Moschetti

Student Presenter: No

Invited Presentation: 

Authors