(6)The 1983 Central Sea of Japan Earthquake (May 26, 1983, M 7.7)

This earthquake occurred on a reverse fault, and had a large source region extending from an area in the open sea off the Oga Peninsula (Oga Hanto) to the western side of the Tsugaru Straits (Tugaru Kaikyo). Strong ground motion was felt, primarily along the Sea of Japan coast in Akita and Aomori Prefectures. Seismic intensity 5 in JMA scale was observed in Akita City, Mutsu (Mutu) City, and Fukaura Town (Fig.4-33). The earthquake generated a large tsunami whose height was recorded at 6.6 m in Hachiryu in Akita Prefecture in a field investigation by the Japan Meteorological Agency (Fig.4-34). The Faculty of Engineering of Tohoku University also reported that the tsunami reached 14 m at Minehama in Akita Prefecture. The focal region of the earthquake was close to land, so the tsunami struck the coast in some places as quickly as 7 minutes after the earthquake. This resulted in the death of many people who were unable to flee in time. A total of 100 of the 104 earthquake-related fatalities were caused by the tsunami. The damage caused by the tsunami from this earthquake covered a wide area and reached every coastal area of the Sea of Japan, extending as far as the Sanin coast, the Korean Peninsula, and the maritime area of Russia (Fig.4-35). In addition, ground liquefaction occurred in plains and other areas with sand deposit, and in areas where the ground water level was high. This resulted in substantial damage to buildings (Fig.4-36).

The largest aftershock of M 7.1 occurred on June 21, about one month after the main shock, in the western part of the Tsugaru Straits, an area at the northern edge of the main shock's focal region. Seismic intensity 4 in JMA scale was observed for this aftershock in Aomori City and other locations, and 3 in Akita City. The aftershock also generated a small tsunami. The frequency of aftershocks increased for a time until the largest aftershock, but gradually decreased thereafter (Fig.4-37, Fig.4-38). In addition, foreshock activity was observed beginning on May 14 in an area near the epicenter of the main shock. The largest of these foreshocks was measured at M 5.0.

Changes were observed in crustal movement as a result of this earthquake, with the surface extending in an east-west or northwest-southeast direction from the coastline of the Sea of Japan at Akita and Aomori Prefectures to the interior (Fig.4-5B).