Izu detachment hypothesis: A proposal of a unified cause for the Miyake-Kozu event and the Tokai slow event

Based on the fact that interseismic deformation of collision zones is generally described by slip along a detachment at depth, I attempt to interpret the deformation of the Izu collision zone in terms of a detachment model. The systematic deviation of the GPS velocities of the Izu Peninsula (Nov. 1998–June 2000) from the Philippine Sea-Eurasian relative plate motions is fitted by the slip on the detachment at a depth of 15–20 km with a rate of 3 cm/yr. On June 26, 2000, seismo-magmatic activity that started near Miyakejima expanded NW by 20 km close to Kozushima in association with dike intrusion over a few months. The horizontal movements associated with this event, however, spread over wide areas in central Honshu. Simple dike intrusion models cannot explain these movements. To explain these, I hypothesize that a 20 cm of rapid slip occurred on the detachment at the time of this event. The abnormal crustal movements in the Tokai-central Honshu-Kanto region then started after the event. I propose that they represent delayed diffusive transfer of the slip on the detachment over surrounding low viscosity layers, such as nearby rupture zones of great earthquakes.