Consistent Source-to-Site Distance Metrics in Ground-Motion Prediction Equations and Seismic Source Models for PSHA

Most modern ground-motion prediction equations (GMPE) use definitions of the source-to-site distance that reflect the dimensions of the fault rupture for larger earthquakes rather than using point-source measures relative to the epicenter or hypocenter. This is a positive development since it more realistically reflects the fact that energy is released from the crust around the entire fault rupture during a large earthquake. However, seismic source configurations defined for probabilistic seismic hazard analysis (PSHA) almost invariably include areas of distributed point-source seismicity in addition to linear fault sources, particularly in regions of lower earthquake activity. Herein, two GMPEs are derived from the same dataset to demonstrate the errors that can result from combining point-source simulations and extended-source distance metrics. The case is made for all ground-motion model developers to consider deriving pairs of equations, one using an extended-source distance metric, the other a point-source measure.