Long-term influence of giant earthquakes: backward empirical evidence and forward test

We investigate on the capability of the strongest earthquakes to modify significantly the seismicity in a wide spatio-temporal window. In particular, we show that the strongest earthquakes of last century were probably able to influence the seismicity at large spatio-temporal distances, extending their reach over thousand of kilometers and decades later. We report statistically significant differences between worldwide seismicity before and after the occurrence of such strongest earthquakes of the last century, whose perturbation is modeled by means of coand post-seismic stress variations. This long-term coupling has produced time variations in the worldwide seismic activity, that appear related to the physical coupling between the focal mechanism of source earthquakes and the tectonic setting of each zone. These results could provide new important insights on seismic hazard assessment, because they raise doubts on the validity of two paradigms, i.e., the steadiness of the mainshock rate, and the isolation of a seismic region compared to the surrounding areas. Finally, beside this backward analysis, we also provide a formal forward test by forecasting the effects on global seismicity of the recent Sumatra-Andaman earthquakes; this is maybe a unique chance to test the long-term hypothesis with an independent dataset that avoids, by definition, any kind of (often unconscious) ”optimization” of the results, that is an unavoidable possibility in backward analyses.