Are Strong Earthquakes Possible in Low-Seismic Areas?

Abstract

Many earthquakes occur unexpectedly: their impact exceeds the expected level based on standard seismic hazard estimates adopted for the region. For strong earthquakes (magnitude 6 and greater), current hazard assessments rely primarily on historical records, paleoseismological data, and information on active faults. In contrast, data on smaller earthquakes are largely disregarded, even though moderate earthquakes are assessed precisely on the basis of such smaller events. This study aims to determine whether a high level of seismicity, expressed through the activity of weak earthquakes, can—accounting for the Gutenberg–Richter law—serve as an indicator of possible future strong earthquakes, and conversely, whether a low level of seismicity implies a low probability of a strong event. We analyzed the strongest earthquakes worldwide from 2001 to 2025, including those for which the hazard was underestimated by national (regional) seismic hazard maps. For each earthquake, we estimated the 50‑year probability of an event of that magnitude within a region containing its rupture zone. Assuming a stationary Poisson process, we used the number of seismic events of magnitude of completeness and greater during the 20 years preceding the target earthquake, scaled to the number of events of magnitude equal to or larger than that of the target earthquake over 50 years. For the vast majority of events, the estimated probability exceeded 10%. This threshold—a 10% probability over 50 years—is the typical benchmark used in seismic hazard mapping. Thus, our results demonstrate that seismic hazard assessment, particularly in delineating potential source zones for strong earthquakes, must include a statistical analysis of weaker earthquakes. Such an approach will reduce the number of “unexpected” earthquakes in the future and improve the accuracy of probabilistic seismic hazard assessments.