Distributions of Seismic Intensities of the 1930s M∼7 Miyagi-oki Earthquakes

Abstract

Distributions of seismic intensities of the 1933 M_JMA 7.1, 1936 M_JMA 7.4 and 1937 M_JMA 7.1 Miyagi-oki earthquakes are investigated and compared with that of instrumental seismic intensities of the 2005 M 7.2 Miyagi-oki earthquake. Residuals of seismic intensities are estimated from an empirical regression equation of seismic intensity attenuations in NE Japan. Large positive residuals of seismic intensities, namely overdetermined seismic intensities, are deduced at several commission stations of the Central Meteorological Observatory (CMO) in the central and western portions of Fukushima prefecture and in northern Kanto District. Based on local newspaper articles reporting seismic damages by the 1930s M∼7 Miyagi-oki earthquakes, seismic intensities are estimated in Iwate, Miyagi, Fukushima, Ibaraki, Tochigi, and Gunma prefectures. Since there were no newspaper articles of seismic damages in the central and western portions of Fukushima prefecture and in northern Kanto District, seismic intensities there are estimated to be less than 4. Locations with seismic intensity 5 are estimated at several places close to the main shock of the 1930s M∼7 Miyagi-oki earthquakes. Distribution of seismic intensity 5 estimated from the seismic damages of the 1936 M 7.4 Miyagi-oki earthquake are very similar to that of the instrumental seismic intensity of the 2005 M 7.2 Miyagi-oki earthquake: these two earthquakes seem to be caused by an slip of the same asperity on the upper surface of the Pacific plate.