Systematic evaluation of ETAS forecasting skill across 69 ML ≥ 6 earthquakes in Taiwan (1999–2024)

Operational earthquake forecasting (OEF) increasingly relies on the Epidemic-Type Aftershock Sequence (ETAS) model to provide short-term, time-dependent seismicity rates, yet whether the model performs consistently across Taiwan’s complex tectonic settings has not been systematically evaluated. This study retrospectively assesses the forecasting skill of a 2-D ETAS model across 69 ML ≥ 6 earthquakes in Taiwan between 1999 and 2024, computing forecasts at three model times (immediately before the mainshock, immediately after, and one hour later) and evaluating them against observed earthquakes with ML ≥ 2 , ML ≥ 3 , and ML ≥ 4 using the Molchan diagram. Forecasting skill improves as the target-aftershock magnitude threshold is raised, because larger-magnitude events concentrate more strongly within the high-seismicity-rate region. Pre-event forecasting skill remains limited unless preceding activity has already concentrated the seismicity rate around the impending epicenter, as exemplified by the 2022 Taitung sequence, in which the ML 6.6 event on 17 September produced a concentrated hotspot that encompassed the ML 6.8 event on 18 September. Once a mainshock has occurred, forecasting skill improves within the first hour as each newly recorded aftershock refines the high-seismicity-rate region, although the ML ≥ 2 comparison remains subject to short-term aftershock incompleteness (STAI). These results establish the ETAS model as most valuable during the evolving stage of a sequence and provide a quantitative foundation for extending the framework toward OEF and time-dependent probabilistic seismic hazard analysis (PSHA) in Taiwan.