Estimating Fault Distances Without a Fault

Abstract eng:
Most ground motion prediction equations (GMPEs) require finite-fault distance metrics, such as the distance to the surface projection of the rupture (RJB) or the closest distance to the rupture plane (RRUP). There are a number of situations in which GMPEs are used where it is either necessary or advantageous to compute the finite-fault distances from point-source distances, such as hypocentral (RHYP) or epicentral distances (REPI). For ShakeMap [1], it is necessary to provide an estimate of the shaking levels for events without finite fault models, and before finite faults are available for events that eventually do have finite fault models. In probabilistic seismic hazard analysis (PSHA), it is often convenient to use point-source distances for gridded seismicity sources, particularly if a preferred orientation is unknown. This avoids the computationally cumbersome task of computing fault-based distances for virtual faults across all strikes for each source. As recommended by Bommer and Akkar [2], it would be ideal if GMPE developers provided coefficients for point-source distances as an alternative to fault-based distances (as done by Akkar et al. [3]), but these are rarely available. In the 2008 version of the U.S. national seismic hazard model [4], equations were derived for the average finite fault distance as a function of REPI for vertical faults as a function of earthquake magnitude. Here, we follow the same method but extend it to allow for dipping faults. Since the dip is also not known in many cases, we provide correction factors that assume an average dip associated with each mechanism. Additionally, we derive adjustment factors for the inter- and intra-event standard deviations of GMPEs that reflect the added uncertainty in the ground motion estimation when point-source distances are used to estimate the finite-fault distances.

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