The main source of seismic hazard information in the Tokyo region is the accounts of the effects of historical earthquakes –accounts that potentially extend the 110-year instrumental record of earthquakes back an additional 500+ years. Damage and other effects of earthquakes in Japan are codified by the Japan Meteorological Agency (JMA) in an intensity scale. There are different techniques to infer the location and magnitude of historical earthqukes from intensity data. Bakun and Wentworth’s (Bull. Seismol. Soc. Amer., 1997) technique is adapted to Japan because it is designed to provide objective estimates of the location and magnitude of historical earthquakes even if only a few intensity assignments are available. The technique also provides objective estimates of the uncertainties in location and magnitude.

JMA intensity assignments IJMA are used to derive intensity attenuation models suitable for estimating the location and an intensity magnitude Mjma of historical earthquakes in Japan. The intensity for shallow crustal earthquakes is equal to -1.89 +1.42*MJMA – 0.00887 *∆h -1.66* log(∆h), where MJMA is the JMA magnitude, ∆h = (∆2 +h2)1/2, and ∆ and h are epicentral distance and focal depth (km), respectively. Mjma is less than MJMA for events near magmatic intrusions, consistent with strong attenuation in near-source magmatic rock. Four earthquakes located near the Japan Trench were used to develop a subduction-plate intensity attenuation model where intensity is equal to -8.33 +2.19*MJMA –0.00550 *∆h -1.14* log(∆h). The IJMA assignments for the MJMA7.9 1923 Great Kanto earthquake on the Philippine Sea-Honshu plate interface are consistent with the subduction-plate model. Using the subduction-plate model, the location of the intensity center is 16 kilometers southeast of the epicenter, Mjma is 7.6, and MJMA is 7.3 to 7.9 at the 1σ confidence level. Intensity assignments and reported aftershock activity for the enigmatic 11 November 1855 Ansei Edo earthquake are consistent with an MJMA 7.2 Philippine Sea-Honshu interplate source (or a Philippine Sea intraslab source) at about 30 kilometers depth. If the 1855 earthquake was a Philippine Sea-Honshu interplate event, the intensity center was adjacent to and downdip of the rupture area of the 1923 Great Kanto earthquake, suggesting that the 1855 and 1923 events ruptured adjoining sections of the Philippine Sea–Honshu plate interface.