The 1703 (Genroku) and 1923 (Taisho) earthquakes in Japan's Kanto region (M 8.2 and M 7.9, respectively) caused severe damage in the Tokyo metropolitan area. These great earthquakes occurred along the Sagami Trough, where the Philippine Sea slab is subducting beneath Japan. Historical records, paleoseismological research, and geophysical/geodetic monitoring in the region indicate that such great earthquakes will repeat in the future.
A dense regional array of geodetic monitoring stations recorded several slow-slip events (or “silent earthquakes”) off the Boso Peninsula in 1996, 2002, and 2007. These are thought to have occurred in the zone between coupled and stable-sliding areas. The shape and extent of these great earthquake asperities (regions of maximum slip along fault planes during earthquakes) and source areas of slow-slip events are still unclear. Geophysical monitoring and understanding the distribution of seismicity in areas exhibiting distinctly different seismotectonic characteristics will help to define regions along these plate boundaries characterized by slow-slip events, repeating events, locked plate interfaces, and regions of maximum coseismic slip.