Hierarchical rupture growth evidenced by the initial seismic waveforms

The ability to predict the eventual size of an earthquake during its early growth stage is a crucial component of earthquake early warning systems. Recent studies have revealed that the onsets of small and large earthquakes are variable but statistically indistinguishable. However, it is unknown whether small and large earthquakes can share the same processes at the same location. Here we show clear evidence of almost identical growth processes shared by repeating earthquakes of various sizes that have occurred in the Naka region, eastern Japan. Our results indicate that a large earthquake is a failure with a large characteristic spatial scale that is initially triggered by a failure with a small characteristic scale, which may also occur independently controlled by subtle differences in the physical conditions, suggesting the existence of a hierarchical structure on the plate interface. Earthquakes are random, but they may also be controlled by such structures.