Ground Motion Prediction Equations in the San Jacinto Fault Zone: Significant Effects of Rupture Directivity and Fault Zone Amplification

We present a new set of Ground Motion Prediction Equations (GMPEs) for horizontal Peak Ground Acceleration, Peak Ground Velocity, and 5 % damped pseudo-spectral acceleration (PSA), developed for the San Jacinto Fault Zone (SJFZ) area. Besides using these equations to quantify seismic shaking in the area, the results allow us to examine the physics and local properties controlling the observed ground motions. The analyzed dataset includes similar to 30,000 observations from similar to 800 events spanning a magnitude range of 1.5 < M < 6.0 and recorded by up to 140 stations at epicentral distances ranging from essentially zero to 150 km. The local GMPE is developed for the SJFZ by applying classical regression techniques with predictive variables that include first distance and magnitude, and then site characteristics, rupture directivity, and fault zone amplification. The significance of these effects is determined by measuring the uncertainty-reduction of the GMPE due to each factor. The results show that, in contrast to many regional studies, traditional site characteristic has a relatively minor effect on peak amplitudes in our study area. However, rupture directivity is a significant factor controlling the amplitudes of ground motion even for small events. The dense seismic network and newly developed directivity tool enable us to extract efficiently directivity effects with statistical significance, using the ground-motion dataset during the regression analysis process. The obtained rupture directivities are consistent with the main focal mechanism orientations and surface trace orientations, known from other studies, and predictions for bimaterial ruptures in the trifurcation area of the SJFZ. Fault zone amplification is a second important factor, showing strong impact on the peak ground motion values, with increasing role for the lower frequency range (< 10 Hz) examined in the 5 % damped PSA values. We also observe signatures of large amplitude-variances, which indicate additional source-related control on the distribution of amplitudes (besides rupture directivity) for aftershocks close in time and location to the M (L) 5.1 earthquake of March 2013. Using the full set of records we present the most complete set of GMPEs for the SJFZ area, including a higher-amplitude prediction for regions in the direction of rupture.