Delayed Post-El Niño Seismic Enrichment Across Pacific Margin Tectonic Systems: Evidence for Magnitude-Dependent Responses

This manuscript examines whether major El Niño–Southern Oscillation (ENSO) intervals are associated with delayed and magnitude-dependent patterns of earthquake occurrence across Pacific margin tectonic systems. Using United States Geological Survey earthquake catalogs from California, Japan, Chile, and Alaska spanning 1990–2025, earthquakes were classified into magnitude-specific populations and evaluated relative to Active ENSO, Immediate Post-ENSO, Delayed Post-ENSO, and Neutral intervals. Observational analyses identified recurring elevations in seismic activity following major El Niño events, with the strongest apparent relationships emerging among moderate-magnitude earthquakes rather than among the largest events. To assess whether these patterns exceeded expectations under simple temporal coincidence, inferential approaches specifically designed for clustered seismic catalogs were incorporated. The findings do not demonstrate that ENSO causes earthquakes, nor do they support deterministic earthquake prediction. Rather, they suggest that delayed post-ENSO responses and magnitude-dependent seismic expression warrant continued investigation within an Earth-system framework linking climate variability, surface loading processes, and tectonic stress redistribution.