Where slow and large earthquakes meet

The Nankai Trough—a 900-km-long trench off the southeast coast of Japan—bears great risk for large earthquakes and tsunamis. In this location, the Philippine Sea Plate is sliding beneath the Eurasia Plate, building immense strain along the surface where the plates slip past each other (the fault plane). Large earthquakes are triggered when the accumulated stress on the locked (halted) fault plane approaches a critical strength over a wide area. A potential contributing factor to this scenario could be slow movement of rocks along the fault plane. Such slow earthquakes, lasting days to years, have taken place adjacent to a locked fault plane (1). On pages 1396 and 1401 of this issue, Edgington et al. (2) and Ozawa et al. (3), respectively, report a connection between slow earthquakes and large earthquakes in the Nankai Trough. The findings point to the importance of considering slow earthquakes when assessing seismic hazards.