(3)The 1978 Earthquake off the Coast of Miyagi Prefecture (June 12, 1978, M 7.4)
The focal region of this interplate earthquake was in the open sea east of the Ojika Peninsula (Ozika Hanto). It generated strong ground motion, primarily near Miyagi Prefecture, and seismic intensity 5 in JMA scale was observed in Sendai and other locations (Fig.4-19). The earthquake occurred at a depth of about 40 km. This is a somewhat deep location; and the tsunami was therefore not very large. The maximum height of the tsunami recorded by the tide station was 49 cm at the new harbor in Sendai, and minimal damage resulted. The damage caused by the ground motion was concentrated in Miyagi Prefecture and resulted in 28 fatalities in total. This damage was concentrated in hilly reclaimed land used for housing (Fig.4-20). Of the fatalities, 18 were killed when they were crushed by a collapsing block wall (Fig.4-21). The new type of damage caused by this earthquake left a lasting impression on the Japanese people. Extensive damage was caused to the infrastructure indispensable for daily life, including gas, water, and electric utilities. This caused great turmoil in the lives of local citizens. In that sense, this was the first earthquake experienced by a modern city in Japan. The largest aftershock M 6.3 occurred two days after the main shock on June 14 on the eastern edge of the aftershock area. Seismic intensity 4 in JMA scale was observed in Fukushima City, and 2 in Sendai, but the damage there was negligible (Fig.4-22, Fig.4-23). Also, a foreshock of M 5.8 was registered near the hypocenter of the main shock about eight minutes before the main shock.
Other earthquakes of about the same magnitude have occurred in roughly 40-year intervals near the area of the sea where the 1978 Earthquake off the Coast of Miyagi Prefecture occurred. These include earthquakes in 1855 (M 7 1/4), 1897 (M 7.4), and 1936 (M 7.5). The 1936 earthquake had a hypocenter nearly identical to that of the 1978 earthquake, and was of roughly the same magnitude, with seismic intensity 5 in JMA scale observed in Sendai. The 1936 earthquake caused much less damage, however, than the 1978 earthquake. This indicates that the nature of the damage caused by earthquakes has changed due to factors such as urbanization, the development of residential areas, and new social conditions. The source area of tsunami generated by the 1936 earthquake is hypothesized to have been south of the focal region of the 1978 earthquake.