Smoothing-Based Aftershock Probabilistic Seismic Hazard Assessment Using the Spatiotemporal ETAS Model

Probabilistic seismic hazard analysis (PSHA) is generally based on computing time-invariant occurrence rates of mainshocks using the Poisson process. However, aftershock probabilistic seismic hazard analysis (APSHA) allows for assessing time-varying aftershock occurrence rates within a short-term seismic hazard mitigation framework. Our proposed methodology of APSHA develops a smoothing-based analytical formulation to capture the spatial distribution and temporal evolution of aftershock sequences using the spatiotemporal epidemic- type aftershock sequence model. This approach is tested on case studies of the 2013 Bushehr, 2021 and 2022 Hormozgan seismic events, and characterizes the aftershocks' hierarchical structure to improve the reliability of aftershock hazard assessments. Then, the results of APSHA (aftershock ground-motion hazard at specific sites) based on smoothing are compared with conventional PSHA (pre-mainshock ground-motion hazard at specific sites). This comparative analysis highlights the importance of considering aftershock effects when assessing ground-motion hazards because PSHA does not fully account for aftershock hazard increases following major earthquakes.