Seismic Source Zones for Site-Specific Probabilistic Seismic Hazard Analysis: The Very Real Questions Raised by Virtual Fault Ruptures

Abstract Site-specific probabilistic seismic hazard analyses (PSHAs) very often include areal source zones to represent diffuse seismicity that cannot be associated with known geological faults. Most modern ground-motion prediction models use distance metrics that are defined relative to the extended fault rupture rather than the epicenter or hypocenter. For these distances to be calculated correctly, virtual fault ruptures are generated, having dimensions consistent with the earthquake magnitude, within source zones when performing PSHA calculations. Although the generation of these virtual ruptures is necessary to achieve compatibility between the seismic source and ground-motion models within the hazard calculations, the ruptures should, by definition, represent potentially realizable seismogenic structures within the crust. Frequently, algorithms for the generation of these virtual ruptures are embedded within the PSHA code as an interim calculation without generating any outputs to enable visualization of the location and extension of the resulting ruptures. Such visualizations can reveal features of these hypothetical ruptures that may challenge the assumptions underlying the definition of the source zone boundaries that separate and enclose distinct regions of diffuse seismicity, as well as raising questions regarding the recurrence parameters within each source, especially in terms of the assumed maximum magnitudes. Visualizing the virtual ruptures generated in PSHA calculations and ensuring their consistency with the criteria established, explicitly or otherwise, for the definition of seismic source zones, could lead to important improvements in the modeling of diffuse seismicity in PSHA. We propose that this visualization should become a standard step in any PSHA study that includes source zones of diffuse seismicity. In addition, the choice of strict or leaky source zone boundaries relative to these hypothetical ruptures should always be explained and justified rather than simply stated.