New Approach to Nonergodic Path Effects in Ground-Motion Models

ABSTRACT A recent advance in modeling nonergodic path effects by Sung et al. (2023) includes a path term for the 3D velocity structure effects. For the spatial correlation of the path effects, the difference between two paths was parameterized by the vector sum of the distance between the two source locations and the distance between two site locations. This metric works well for very different paths with small correlations (0–0.1) and very similar paths with large correlations (0.9–1.0), but it overestimates the correlation for paths with intermediate correlations. We propose a new metric for measuring the difference between two ray paths based on the difference in site locations, azimuths, and rupture distances. This new metric leads to a correlation model that better captures the observed spatial correlation structure of the ground motions than the existing metric. The new metric has a better physical basis for parameterizing the difference in the path effects and improves the accuracy of the extrapolation of the model to scenarios outside of the range constrained by the data. In addition, we present an efficient numerical method (Scalable Kernel Interpolation for Product Kernels [SKIP]) that can be applied to large data sets (more than 100,000 ground motions), as is common in large simulation data sets. Using SKIP, the forward predictions of path effects for a grid of source locations for a given site can be computed with a calculation time of about a minute using traditional laptop computers with standard memory.