Superquakes and Supercycles

The recent 2011 Mw 9.0 Tohoku, Japan, and the 2004 Mw 9.15 Sumatra–Andaman superquakes have humbled many in earthquake research. Neither region was thought capable of earthquakes exceeding Mw ∼ 8:4. Appealing proposed relationships to predict the size of earthquakes in subduction zones, such as that between earthquake magnitude and parameters such as lower plate age and convergence rate (Ruff and Kanamori, 1980) and plate coupling based on anchored slabs (Scholz and Campos, 1995), at least have many exceptions and may not be valid. Both earthquakes occurred where the subducting plate edge was quite old, ∼50–130 Ma. The role of thick sediments smoothing the plate interface and maximizing rupture area has been considered a contributing factor, and it seems to influence many recent great earthquakes (Ruff, 1989). The Tohoku event is also contrary to this hypothesis. Clearly, much remains to be learned about these great events, so much so that most previous estimates of maximum earthquake size in subduction plate boundaries should be considered suspect, and perhaps other fault systems as well (McCaffrey, 2007, 2008). Our perspective on this issue is clearly hampered by short historical and even shorter instrumental records. The examples noted earlier indicate that basing estimations of maximum earthquake size or models of earthquake recurrence on such short-term records alone clearly cannot encompass the range of fault behavior, even when historical records may be >1000 years long as in Japan. Here, we present several examples of areas where long geologic and paleoseismic records can illuminate a much wider range of seismic behaviors compared with those deduced from historical and instrumental data, and speculate on models of long-term fault behavior based on very long records.