A Partially Nonergodic Ground-Motion Model for Cascadia Interface Earthquakes

Abstract

ABSTRACT The Abrahamson and Gulerce (2020) ground-motion model (GMM), called AG20, is modified to include the nonergodic effects from the suite of 3D numerical simulations of long-period ground motions from M 9 megathrust earthquakes on the Cascadia subduction zone developed by the M 9 project (Frankel et al., 2018). Three modifications are made to the AG20 model. First, the scaling of the basin effects as a function of the depth to a shear-wave velocity of 2.5 km/s (Z2.5) is modified to be consistent with the Z2.5 scaling from the 3D simulations. Second, the spatial distribution of the nonergodic site terms is estimated using the varying coefficient model for the region covered by the 3D velocity model. The change in the median ground motion due to the nonergodic site terms ranges from factors of 0.25 to 5 at a period of 3 s. Third, a reduction in the aleatory variability for single-station sigma is estimated. On average, including the nonergodic site terms reduces the sigma by 15%–25% compared with an ergodic sigma for Cascadia. In addition, a spatially varying single-station sigma model is developed, showing the highest variability for sites near the basin edges. Application of nonergodic GMMs requires an estimate of the epistemic uncertainty in the nonergodic terms. Without simulation results for different 3D velocity models, we assumed that the epistemic uncertainty due to uncertainty in the 3D velocity model is half of the between-site standard deviation from the simulations. As an example, the seismic hazard for interface events using the nonergodic GMM is compared with the hazard using the original AG20 GMM.