Spatio-temporal occurrence patterns among the foreshocks preceding the 2007 Noto Hanto earthquake

The 2007 Noto Hanto earthquake was a magnitude 6.8 crustal earthquake (Japan Metrological Agency) that occurred in central Japan. By analyzing continuous seismograms that recorded activity during the 25 days before the earthquake, we detected 36 small seismic events around the mainshock hypocenter that classified as either isolated events or members of three clusters. Three of the detected events are listed in official seismic records. Two clusters included more than ten events and occurred some distance from the mainshock fault, indicating that the clusters did not relate to the mainshock event. Only the third cluster was located along the mainshock fault. This cluster occurred in the vicinity of the mainshock hypocenter and consisted of four foreshocks with identical seismograms. This cluster began twelve minutes before the mainshock. The occurrence pattern of this clustered foreshock sequence is similar to that observed in association with the 2008 Iwate-Miyagi Inland earthquake.