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A Nonergodic Ground Motion Model in Southern California With Spatially Varying Coefficients Using a SCEC Cybershake Dataset

Session: Physics-Based Earthquake Rupture Modeling and Strong Motion Simulations II

Type: Oral

Date: 4/23/2021

Presentation Time: 02:45 PM Pacific

Description: 

A key input to PSHA is the total standard deviation of the misfits between ground motion observations and the median ground motion models (GMMs, a.k.a GMPEs). The most promising way to reduce hazard is to reduce the standard deviation through the removal of the ergodic assumption, and instead estimate the variability for site-specific ground motions. Although strong motion networks have been rapidly growing in recent decades, in most cases the empirical data are still too sparse to establish a fully nonergodic model. In comparison, numerical simulations can generate large ground motion datasets for controlled earthquake sources and sites, which are optimal to reduce the total standard deviation by identifying and removing repeatable effects in a fully nonergodic model. However, before such PSHA products based on simulations are accepted for engineering applications, it is crucial to validate the simulated ground motions against empirical data and models.

In this study, we evaluate the ability of the physics-based CyberShake platform to capture the repeatable site and path effects from the empirical data, by applying the spatially varying coefficient model (SVCM) as part of the regression analyses. We use ground motion simulation results at 5s spectral period from CyberShake 15.4, which was computed for southern California and generated over 97 million ground motion seismograms at 336 sites. We first conduct a checkerboard test and examine SVCM’s capability of capturing the input source, site and path terms in various settings. We then compute the total residuals from CyberShake datasets using linear regression with a simple GMM functional form. Finally, we assign spatially varying source effects, site effects and cell-specific anelastic attenuation by applying the SVCM technique. Preliminary results show systematically large site effects within the Los Angeles and Ventura basins, which are expected from basin effects. The results are contrasted with those from sparser empirical datasets, using a similar regression technique, and discussion of discrepancy is also presented.

Presenting Author: Christine A. Goulet

Student Presenter: No

Authors