Recurrence and Long-Term Evaluation of Kanto Earthquakes

The 1923 Taisho Kanto earthquake—an interplate event along the Sagami trough where the Philippine Sea plate subducts beneath the Kanto region—produced ground shaking, crustal deformation, landslides, and tsunamis, and caused the worst earthquake disaster in Japan. Based on seismological and geodetic data, many fault models have been proposed, extending ∼100 km from the epicenter, with a moment magnitude (Mw) of 7.8–8.2, and large slips of ∼8 m located near the epicenter and beneath the Miura Peninsula. The penultimate 1703 Genroku Kanto earthquake produced similar macroseismic effects around Sagami Bay and the Miura Peninsula, but larger coastal uplift and tsunami in the Boso Peninsula. The proposed fault models extend off the Boso Peninsula with Mw of 8.1–8.5. In 2004, the Earthquake Research Committee (ERC) classified the Kanto earthquakes as “Taisho type” and “Genroku type” with recurrence intervals of 200–400 yr and 2300 yr, respectively. In 2014, the ERC revised the long-term evaluation to a recurrence interval of 180–590 yr and a 30 yr probability of 0%–5% based on the Brownian passage time model. With the Cabinet Office, the ERC considered the source area of the maximum possible earthquake of Mw 8.6–8.7. The recent historiographical and paleoseismological studies have identified other candidates for the past Kanto earthquakes in 1495, 1433, 1293, and 878. Various combinations of these candidates give a mean recurrence interval of 210–315 yr, an aperiodicity parameter of 0.04–0.76, and a 30 yr probability of 0.0%–19%. The Cabinet Office has calculated the seismic intensity and tsunami heights of various types of Kanto earthquakes. National and local governments estimate the damage from these hazards. For the Tokyo metropolitan area, the estimated damage and occurrence probability are more significant for M ∼7 earthquakes with various types and depths, and most mitigation efforts are directed at such events.