Integrating Long and Short‐Term Time Dependencies in Simulation‐Based Seismic Hazard Assessments

Abstract

Conventional probabilistic seismic hazard analysis (PSHA) only considers mainshock events and uses a time‐independent earthquake rupture forecast to describe the occurrence of mainshocks. This approach neglects the long‐term time‐dependency of mainshocks on specific fault segments, the interaction of adjacent faults, and the spatial and temporal clustering of aftershocks. This study integrates a recently proposed advanced fault‐based PSHA framework with an aftershock simulator based on the Epidemic‐Type Aftershock Sequence (ETAS) model, to overcome the aforementioned limitations. Central Italy is used as a case study to illustrate the proposed framework. The case‐study hazard curves show that including state‐of‐the‐art advances in fault‐based PSHA may increase the ground‐motion amplitudes for low return periods, and decrease the ground‐motion amplitudes for high return periods, relative to a conventional PSHA approach. The magnitude of ground‐motion estimate increases due to aftershock inclusion is dependent on the selected site and return period. Sensitivity analyses of the results demonstrate that: (a) close to modeled faults, high‐return‐period hazard estimates are more affected by fault‐related inputs than the aftershock inclusion; (b) close to modeled faults, the influence of aftershock inclusion is higher for low return periods than high ones; (c) away from modeled faults, aftershocks inclusion greatly affects the variance of the hazard results; and (d) fault interaction has a limited effect on the variance of the hazard estimates compared to other analysis inputs. This study could help with the future implementation of complex fault modeling approaches and aftershock hazard in seismic risk models.

Investigating the sensitivity of losses to time-dependent components of seismic risk modeling

“…The time-dependent fault rupture occurrence model leads to lower values of L gu in general; the AAL gu for as À dacc À BPT mid À fi is approximately 9% less than that for as À dacc ÀTI À fi. This is because the time-dependent rupture occurrence probability of the considered fault system in Central Italy is lower than that calculated with a time-independent model (Iacoletti et al, 2022a). Aftershock inclusion, fault interaction, and the inclusion of state-dependent vulnerability calculations (denoted by as À dacc À BPT mid À fi) lead to an increase in AAL gu of around 22% and an increase of approximately 25% in the 200-year-RP L gu , relative to the respective values obtained for as À dacc À BPT mid À fi.…”

“…This is because time-dependent seismic hazard is expected to be notably lower than time-independent hazard at this location, due to the rupture of the Paganica fault in 2009 (e.g. Iacoletti et al, 2022a; Pace et al, 2016). At Teramo, the highest S i , AA L gu values are associated with aftershock modeling and fault rupture occurrence modeling and the highest S i , RP 2500 gu is associated with fault rupture occurrence modeling.…”

“…Case-study portfolio (subset of that presented in Crowley et al (2021a), blue dots within the blue polygon, representing density-weighted centroids). The red polygon is the study area used in Iacoletti et al (2022a) to generate the stochastic event sets. The 43 considered fault segments (Scotti et al, 2021) are shown in black (fault trace) and gray (geometry at depth).…”

“…Fault data required to calibrate the fault-based seismicity modeling component (i.e. slip rates, paleoseismic records, and date of the last event) are taken from the works by Scotti et al (2021) and Valentini et al (2019), and other available data sources (see Iacoletti et al, 2022a, for more details). One time-independent and three time-dependent fault rupture occurrence models are used.…”

“…Five years of historical seismicity before the simulation period (i.e. between 2017 and 2021) are used to account for the aftershocks due to past seismicity (Iacoletti et al, 2022a). The stochastic event sets that include aftershocks contain a slightly lower number of 5 < M W < 5 . 5 events than the undeclustered CPTI15 observed catalog (Iacoletti et al, 2022a).…”