The Hokkaido-Nansei-Oki earthquake

At 10.17 pm on the evening of 12 July 1993 an earthquake with a magnitude of 7.8 occurred in the Sea of Japan off the south west coast of Hokkaido, the most northerly of the Japanese islands. The earthquake was located at a depth of 34 km with its epicentre about 55 km from the west coast of Hokkaido and some 75 km north of the small offshore island of Okushiri, according to the Japan Meteorological Agency (JMA). At this point the depth of water is estimated to be 3500 m. The location of the earthquake is shown in Figure 1. The earthquake caused extensive damage and loss of life on Okushiri Island and to a lesser extent on the Hokkaido mainland. Some loss of life and damage was reported on the Russian Pacific coast and in addition fishing boats were lost off the east coast of South Korea. Most of the damage and loss of life was caused by tsunamis generated by the earthquake. In addition major ground damage consisting principally of liquefaction, rockslides and landslides occurred in southern Hokkaido and on Okushiri Island. The damage to structures as a direct result of ground shaking was minimal apart from the collapse of a recently completed snow shelter over a road on Okushiri. Most damage to structures occurred as a result of ground damage. After reviewing reports of damage received from Japan and after seeking information from people in the region, the NZNSEE decided to despatch a reconnaissance team to investigate the earthquake and to identify lessons of value to New Zealand. From the early reports it appeared that the areas of particular interest would be ground damage, damage to lifelines, fire following earthquake and tsunami, performance of time houses and the effects of tsunami. Accordingly, a team of four members of the Society spent eight days in Japan during late July observing the effects of the earthquake. Two days were taken up with briefings at universities in Tokyo and Sapporo on the latest situation in the damaged region and six days were spent in the field. The aim of this Preliminary Report is to provide readers of the Bulletin with up-to-date first-hand information on the event and a brief account of the initial impressions of team members. A detailed Final Report will be published in the December issue of the Bulletin.