5.1 Overview of Spatial and Temporal Models

The heart of any SSC model for PSHA is a description of the future spatial and temporal distribution of earthquakes. The earliest PSHA models identified seismic source zones within which the spatial distribution of earthquakes was assumed to be uniform. The temporal distribution of earthquakes was assumed to follow a Poisson process, and the sizes of earthquakes within the zones were assumed to follow an exponential distribution. In more tectonically active areas such as the WUS, the recognition of active faults allowed the spatial models to evolve such that they were able to include the fault sources as well as the background source zones within which they lie. Further, the development of fault-specific paleoseismic data regarding the timing, amount of slip per earthquake, and geologic slip rate provided a new class of temporal models that could use this information directly in the PSHA. 

In the meantime, the evolution of spatial and temporal models within stable continental regions (SCRs) such as the CEUS has taken place differently and more slowly. Despite continued geologic investigations, the search for the causative faults giving rise to observed seismicity has yielded very few cases where a definitive argument can be made for having identified an active fault in the sense that this is commonly identified in the WUS. With notable exceptions such as the Meers fault and the Cheraw fault, the best-defined cases of localized seismicity and deformation occur in the New Madrid region, where individual fault-like sources can be called out and characterized. Even in locales such as Charleston, where geologic and geophysical studies have continued over the decades, uncertainties persist regarding the unique association of the observed historical seismicity and the one or more causative faults that are responsible. 

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