Abstract

The level of horizontal ground motion intensity can have large variations from one orientation to another. This study examines the orientation of the maximum response of 5%-damped linear oscillators and its geographical distribution for ground motions recorded during the December 20, 2022, Ferndale, CA earthquake. The orientation of the maximum intensity is found to generally occur close to the transverse orientation for epicentral distances greater than 30 km. At epicentral distances less than 30 km, the angular difference between the orientation of the maximum intensity and the transverse orientation does not show a clear predominant orientation. The angular difference with respect to the transverse orientation is found to be strongly correlated with the level of intensity. The ground motion intensity at the transverse orientation was, on average, between 85% and 95% of the maximum intensity and was 1.0-1.26 times greater than the RotD50 intensity (i.e., the median intensity from all horizontal orientations). The level of polarization increases with the period of the oscillator and ground motions remain fairly polarized up to 350 km from the epicenter. The results support a case for future incorporation of directionality effects, and in particular, the use of orientation relative to the transverse orientation, when estimating response spectral ordinates from strike-slip earthquakes.