1983年日本海中部地震による地盤災害と表層地質  [in Japanese] Ground failure caused by the Nihonkai Chubu Earthquake, 1983 in relation to subsurface geology  [in Japanese]

A severe earthquake with magnitude 7.7 occurred in the Sea of Japan off Akita and Aomori Prefectures, Japan, at 12:00 a.m. on the 26th of May, 1983. It was named the 1983 Nihonkai Chubu Earthquake by the Japan Meteorological Agency. The coastal region of Akita and Aomori Prefectures, located on the western side of Tohoku District of Japan, sustained a great damage due to the Tsunami and ground failure caused by the Nihonkai Chubu Earthquake, 104 persons were killed and more than 300 people were injured by this Earthquake: most of the casualties were caused by the Tsunami. Ground failure by the earthquake caused considerable damage to houses, rice fields, roads and railways. Most of the damage were caused by the liquefaction of subsurface layers in the coastal alluvial plain of Akita Prefecture. Geological investigations related to the damage conditions were carried out in the damaged area of Akita Prefecture. This paper describes the order and distribution pattern of the damage to the man-made structures as well as the phenomena of ground failures such as sand and water eruption, rock fall, land slide and slump, then discusses the relation between the damage, ground deformation and subsurface geology. Damages by the Tsunami are excluded. In the coastal region of Akita prefecture, slight damage to man-made structures was observed in the hilly land where pre-Pleistocene formations are distributed. On the other hand, serious damage occurred to various artificial structures such as houses, roads, railways, embankments and quays concentrated in the lowland area in the coastal alluvial plain. In Akita and Noshiro Cities, many houses on the ancient backmarsh, lowland between sand dunes, and artificial fills were damaged by subsidence or cracking of ground together with sand eruption. The damage to roads were widely distributed in the coastal areas. Most of this damage occurred in the roads on artificial fills around the Hachirogata lagoon, as well as in the roads in rice fields and swamp areas. Many embankments along the Yoneshiro River in Noshiro City and the Omono River in Akita City, and the area around the Hachirogata were damaged by subsidence and cracking of ground. In the Akita Port, various facilities such as quays and aprons were severely damaged by the liquefaction of filled sands. Accompanied with the liquefaction, sand and water erupted in the areas along the former streams and channels, on the marginal part of sand dune, and along the boundary between sand dune and backmarsh. A lot of the sand and water eruption were observed on the various artificial ground in addition to natural ground. The artificial ground is distributed in the area of previously existed rivers, lakes and backmarshes which are now filled up with Holocene unconsolidated dune sands. These man-made fills amplified the damage to the structures. In the coastal region of Akita Prefecture, there are some active faulting and folding structures generally having a NNE-SSW trend. Observation of the trend of crackings of ground and sand eruptions in Noshiro City indicates approximately the same NNE-SSW direction as that of the active structures such as the Noshiro Thrust Faults. Furthermore, the NNE-SSW trend in distribution of seismic intensity anomaly is indicated by the percentage of fallen tombstones. The damage caused by the Nihonkai Chubu Earthquake is, therefore, closely related to those active structures in addition to the subsurface geology in the coastal alluvial plian.