Sequence-Based Hazard Maps for the United Kingdom

The current practice of probabilistic seismic hazard analysis (PSHA) does not take into account that earthquakes actually occur in time–space clusters. The input for PSHA is based on declustered seismic catalogs, used to characterize only the mainshocks, that is, the largest magnitude events within each cluster. However, the so-called sequence-based PSHA (SPSHA; Iervolino et al., 2014) allows us including the effect of aftershocks in hazard analysis, that is, the events following the mainshock, still conveniently resourcing from declustered catalogs. In the United Kingdom (UK), the seismic source model developed for the national seismic hazard assessment has been recently updated by the British Geological Survey (BGS, 2020). In this study, the source model developed by the BGS (one directly derived from it, in fact) is used to implement SPSHA in the UK. The calibration of the model for the occurrence of aftershocks, that is, the modified Omori’s law, is fitted on a few sequences and under some simplifying assumptions. The results, represented by hazard maps for selected spectral ordinates and exceedance return periods of interest for structural engineering, are compared to the PSHA counterparts to discuss the increase in the design seismic actions when the effects of aftershocks are considered. The maps show that, based on the modeling of aftershock sequences considered in the study, in the UK this increase can be up to 14%, at least for the spectral ordinates and exceedance return periods herein investigated. The discussed maps are provided as supplemental material to this article.