Next Generation Attenuation (NGA) West2 ground motion models (GMMs) include regional path adjustments for broad jurisdictional regions, which necessarily averages spatially variable path effects within those regions. We extend that framework to account for systematic variations in attenuation within subregions defined in consideration of geologic differences. In recent years, cell-based methods which systematically account for spatial variations by summing the attenuation effects over a fine discretization of uniform-rectangular cells (e.g., Dawood and Rodriquez-Marek 2013; Kuehn et al. 2019) have been shown to be an effective alternative to regionalization and a step towards modelling non-ergodic path effects. The main drawbacks of these models, however, are their increased computational complexity, poorly informed coefficients for cells in which few paths travel, and unoptimized boundaries that may span across the limits of geologic domains. The framework presented here considers physio-geological differences to form subregional boundaries. Broad jurisdictional regions are divided into a number of subregions that are orders of magnitude greater in size than the uniform cells of cell-based methods, but smaller than regions corresponding to the NGA-West2 adjustments. Subregional boundaries are informed by geological differences and empirical observations to create domains with internally similar properties. The total attenuation effect for a given path that traverses multiple subregions is obtained by weighting the individual subregional effects by the proportion of the path length within each subregion. This approach has been successfully applied in California, where it was shown to achieve a reduction in bias and within-event and single-station variability relative to an NGA-West2 GMM for ground motions at large distance (RJB > 100 km). The framework presented here can readily be adapted for other GMMs and regions.
