Fig. 4

Fast earthquakes potentially indicative of slow earthquake occurrence. a Observed waveforms of repeating earthquakes. These repeating earthquakes were observed off Kamaishi in the northern Japan Trench (Igarashi et al. 2003). b Magnitude–time diagram for an earthquake swarm accompanying an SSE. This earthquake swarm was observed during the period of the 2007 SSE off the Boso Peninsula in the Sagami Trough, eastern Japan (Fukuda 2018)