Magnitude and location of historical earthquakes in Japan and implications for the 1855 Ansei Edo earthquake

Japan Meteorological Agency (JMA) intensity assignments IJMA are used to derive intensity attenuation models suitable for estimating the location and an intensity magnitude Mjma for historical earthquakes in Japan. The intensity for shallow crustal earthquakes on Honshu is equal to −1.89 + 1.42MJMA − 0.00887Δh − 1.66logΔh, where MJMA is the JMA magnitude, Δh = (Δ2 + h2)1/2, and Δ and h are epicentral distance and focal depth (km), respectively. Four earthquakes located near the Japan Trench were used to develop a subducting plate intensity attenuation model where intensity is equal to −8.33 + 2.19MJMA −0.00550Δh − 1.14 log Δh. The IJMA assignments for the MJMA7.9 great 1923 Kanto earthquake on the Philippine Sea–Eurasian plate interface are consistent with the subducting plate model; Using the subducting plate model and 226 IJMA IV–VI assignments, the location of the intensity center is 25 km north of the epicenter, Mjma is 7.7, and MJMA is 7.3–8.0 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–Eurasian interplate source or Philippine Sea intraslab source at about 30 km depth. If the 1855 earthquake was a Philippine Sea–Eurasian interplate event, the intensity center was adjacent to and downdip of the rupture area of the great 1923 Kanto earthquake, suggesting that the 1855 and 1923 events ruptured adjoining sections of the Philippine Sea–Eurasian plate interface.