Effect of style of faulting on the orientation of maximum horizontal earthquake response spectra

Horizontal earthquake ground motions have intensities that vary with changes in orientation, that is, they depend on the azimuth. Previous studies have found that the maximum horizontal spectral acceleration at mid-to-long periods is more likely to be closer to the strike-normal orientation than to the strike-parallel orientation for ground motions recorded very close to earthquake ruptures. However, this trend disappears relatively quickly as the distance to the rupture increases and becomes almost nonexistent for rupture distances longer than 5 km. Using a database of ground motions from shallow crustal earthquakes in active tectonic regimes, this work studies the orientation of maximum horizontal spectral acceleration of ground motions from earthquakes with strike slip and reverse faulting. For strike-slip earthquakes, these orientations of maximum intensity are found to be close to the transverse orientation, that is, an orientation that is perpendicular to the line segment between the recording station and the earthquake epicenter. Contrary to what occurs with respect to the previously studied strike-normal orientation, the orientation of maximum spectral response remains close to the transverse orientation regardless of source-to-site distance. Moreover, on average, the orientations of maximum spectral response tend to become closer to the transverse orientation as the period increases. On the other hand, ground motions from reverse earthquakes do not show any trend in the orientation of maximum spectral response relative to the transverse orientation. Probability distributions of the angular difference between the orientation of maximum spectral response and the transverse orientation for strike-slip earthquakes were fitted at several periods between 0.01 and 10 s. These directionality effects from strike-slip earthquakes could be considered in future seismic hazard analyses.