A rupture directivity adjustment model and its application in seismic hazard

Abstract

This article contains a rupture directivity adjustment model for strike-slip earthquakes that can be applied to a traditional ground-motion model (GMM; one without explicit treatment of rupture directivity) to incorporate rupture directivity effects in either deterministic or probabilistic seismic hazard analyses. Application of the directivity model requires adjustments to both the GMM median and aleatory variability. The model described herein supersedes the previous models for strike-slip earthquakes developed by the authors of this article (Abrahamson, 2000; Bayless et al., 2020; Somerville, 2003; and Somerville et al., 1997; chapter 2 of Spudich et al., 2013). A key feature of the directivity model is that it is centered because it does not alter the magnitude and distance scaling of the GMM when averaged over uniformly distributed sites at a given rupture distance. Additionally, we address a longstanding issue regarding the directivity condition contained in the NGA-W2 dataset (Ancheta et al., 2014) and conclude that the biases in the mean and standard deviation of directivity effects are small enough to ignore for the purposes of modeling directivity. We provide guidance on the directivity model implementation, including a review of deterministic and probabilistic seismic hazard applications, and recommend methods for modeling hypocenter locations and multisegment ruptures. The model addresses the RotD50 (Boore et al., 2010) horizontal component of 5% damped spectral acceleration. A future update will address directionality and directivity effects for other styles of faulting. Implementations of the model are provided in the electronic supplement.