Davide Conte — Università degli Studi della Campania "Luigi Vanvitelli" #
Towards a Predictive Physics-Based Framework for Foreshocks: Moving Beyond Hindsight Bias in Seismic Forecasting. #
Statistical seismology classifies earthquakes into three main categories: mainshocks, foreshocks, and aftershocks. A mainshock is typically defined as the largest earthquake within a seismic sequence or region, aftershocks are the events triggered by it, and foreshocks are the earthquakes that occur before the mainshock. This definition immediately highlights a fundamental problem: unlike aftershocks, foreshocks can only be identified retrospectively, once the mainshock has already occurred. As a result, it remains unclear whether foreshocks carry genuine predictive information or simply reflect ordinary seismic clustering viewed in hindsight.
The Epidemic-Type Aftershock Sequence (ETAS) model is the standard framework used to describe seismicity as a branching process in which earthquakes trigger subsequent events. Although ETAS successfully reproduces many features of aftershock clustering, it systematically underestimates the number of foreshocks observed in real seismic catalogs and does not provide a physical mechanism for earthquake nucleation.
To address this limitation, we introduce the ETAS-BC model, an extension of ETAS in which earthquakes can generate magnitude-ordered chains of “connector” events that progressively activate the fault system and culminate in a mainshock. Within this framework, foreshocks emerge as the manifestation of a preparatory process rather than as a purely retrospective classification. The model remains analytically tractable, allowing us to derive predictions for the magnitude distribution, productivity, and spatio-temporal organization of connector sequences.
Comparison with Southern California seismicity shows that ETAS-BC reproduces the observed number of foreshocks significantly better than standard ETAS while preserving its successful description of aftershock statistics. These results suggest that introducing connector-driven nucleation processes provides a promising physics-based framework for describing earthquake preparation and improving statistical models of seismicity.