How do you Perform a DSHA for a Distributed Seismicity Source?

Seismic hazard assessments are conducted to estimate ground motions and establish response spectra for use in engineering evaluations of dams, and many guidance documents recommend or require a deterministic seismic hazard analysis (DSHA) as part of the seismic hazard assessment (e.g., FERC Chapter 13, 2018; CA DSOD, 2018; USACE ER 1110-2-1806, 2016). A fundamental step in the DSHA is to define the deterministic scenarios in terms of the earthquake magnitude, distance, and ground motion level. When the sources capable of producing ground motions at the dam site consist of known faults, the selection of magnitude and distance are unambiguous and the slip rate can be used to inform the selection of a ground motion level; however, when the seismic sources consist of distributed seismicity sources without mapped faults, the selection of a single deterministic magnitude, distance, and ground motion level is ambiguous because the scenarios for a distributed seismicity source are inherently comprised of a broad distribution and the activity of the source is not characterized by a slip rate. With guidance documents geared toward the known fault case, practitioners and owners are left asking: how do you perform a DSHA for a distributed seismicity source? This paper describes different approaches applied in practice, explores whether there is an objective approach, and presents examples from seismic hazard assessments conducted in the Central and Eastern United States (CEUS).