Integration and validation of deterministic earthquake simulations in probabilistic seismic hazard analysis

Seismic hazard models based on empirical ground motion prediction equations (GMPEs) employ a model-based factorization to account for source, propagation, and path effects. An alternative is to physically simulate these effects using earthquake source models combined with three-dimensional (3D) models of Earth structure. We generalized the implementation of those hazard models in probabilistic seismic hazard analysis from the seismological perspectives, and developed an averaging-based factorization (ABF) scheme to facilitate the geographically explicit comparison of these two types of seismic hazard models. Through a sequence of averaging and normalization operations over various model components, such as slip distribution, magnitudes, hypocenter locations, we uniquely factorize model residuals into several factors. These residual factors characterize differences in basin effects, distance attenuation, and effects of source directivity and slip variability. We illustrate the ABF scheme by comparing CyberShake model for the Los Angeles region with the Next Generation Attenuation (NGA) GMPEs. Relative to CyberShake, all NGA models underestimate the basin effects. Using the GMEPs with directivity corrections, we quantify the extent to which the empirical directivity model capture the source directivity effects demonstrated by physics-based ground motion prediction model. In particular, empirical directivity corrections for NGA models underestimate source directivity effects in CyberShake, and do not account for the coupling between source directivity and basin excitation that substantially enhance the low-frequency seismic hazards in the sedimentary basins of the Los Angeles region. We then investigate seismologically to what extent the complex rupture processes and conditional hypocenter distributions affect the ground motion predictions and seismic hazard assessment. At last, considering two 3D velocity models for Southern California used in simulations, we use different CyberShake studies to physically understand the basin excitations and directivity-basin coupling effects. To our knowledge, this is the first systematical and quantitative integration and validation of deterministic earthquake simulations in probabilistic seismic hazard analysis.