Influence of Earthquake Principal Stress Axes on the Orientation of Maximum Horizontal Intensity in Strike-Slip Earthquakes

ABSTRACT Earthquake ground-motion intensity at a given site varies with orientation, and several studies have investigated the orientations at which the maximum intensity occurs. Maximum horizontal intensities were thought to occur in random orientations beyond the near field until a recent study found that for strike-slip earthquakes, they generally occur close to the epicentral transverse orientation (i.e., perpendicular to a line connecting the station to the epicenter). This article examines the orientation of maximum intensity in both theoretical and recorded strike-slip events. In particular, the influence of source depth and principal stress axes orientations, as quantified by the angle relative to the fault strike, on theoretical maximum displacements in a horizontal plane above a shear dislocation in an isotropic, homogeneous medium is investigated. Using spatial patterns observed in the theoretical case as a guide, the spatial distribution of the orientation of maximum intensity in recorded strike-slip earthquakes at stations without local site effects is qualitatively studied using various types of waveforms, including filtered ground velocities, high-rate Global Navigation Satellite Systems ground displacements, and oscillator responses to strong-motion or physics-based simulated ground accelerations. The results indicate that the orientation of maximum intensity remains close to the epicentral transverse orientation for a wide range of azimuthal angles relative to the fault strike but systematically shifts to the radial orientation in cones near the principal stress axes. These findings indicate that the assumption that maximum intensities for strike-slip earthquakes consistently occur near the epicentral transverse orientation across all source-to-site azimuths may not be valid. Instead, improved estimates for the orientation of maximum intensity can be obtained by also using the angle from strike as a predictor variable to assess whether a particular point is situated close to the principal stress axes. These results could be used in future seismic hazard analyses.