Report on the Damages caused by the Tokachi-oki Earthquake in Aomori Prefecture

Abstract

At 9:49, May 16th, 1968 occurred an earthquake of the magnitude 7.8 with the epicenter in 40.7°N and 143.7°E, and caused a great disaster in the eastern part of Aomori Prefecture.
Slope failures mainly occurred in the valleys dissecting the upland between the Mabechi River and the Gonohe River were the most disasterous accidents. Those slope failures and landslides are typical of land-slips influenced by the ground water. The earthquake acted as a trigger to move the ground after a heavy ranifall amounting 160mm in the preceding three days. Moreover, it is characteristic of this earthquake that many of artificial embankments such as railroad basements, highway constructions and earth-fill dams were severely damaged.
On the alluvial lowlands, especially newly reclaimed or graded lands for factory areas or wharfs in coast districts of Aomori, Hachinohe and Mutsu Cities, there occurred many cracks, depressions and sand-jettings. As its result, many houses and factories were slanted in various degrees. But those damages were limited in reclaimed land in old river courses or back-marshes.
The earthquake brought much damage to Hachinohe City. One of the damaged areas on the alluvial lowland is the coastal area to the north of the city between the Takadate coastal terrace and the shoreline. The land in this area was graded for the construction of factories, and a harbor (Second Industrial Harbor). Unconsolidated lands which had been back-marshes between beach ridges and had been iron sand pits were depressed unevenly and sand was jetted out. At a factory, we could see slanted buildings, tanks and reactional towers, road surface waved, and pipes cut.
Another damaged area is the delta of the Mabechi River. Recently the river mouth was changed artificially, and the old channel has been used as a harbor (First Industrial Harbor). The embankment of the new Mabechi River collapsed at several places. The factory area near the old channel, was depressed unevenly, too, but the damage was less than in the above mentioned damaged area.
On the Hachinohe terrace (10-35 m above the sea level), where the civic center of Hachinohe city is, steel-concrete building for public sendees, many houses, water pipes and gas pipes were damaged, especially in the open shallow valleys dissecting the terrace.