Global frequency of magnitude 9 earthquakes

For decades seismologists have sought causal relationships between maximum earthquake sizes and other properties of subduction zones, with the underlying notion that some subduction zones may never produce a magnitude ~9 or larger event. The 2004 Andaman Mw 9.2 earthquake called into question such ideas. Given multicentury return times of the greatest earthquakes, ignorance of those return times and our very limited observation span, I suggest that we cannot yet make such determinations. Present evidence cannot rule out that any subduction zone may produce a magnitude 9 or larger earthquake. Based on theoretical recurrence times, I estimate that one to three M9 earthquakes should occur globally per century, and the past half century with fi ve M9 events refl ects temporal clustering and not the long-term average.