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The Hokkaido-Nansei-Oki earthquake on July 12 produced one of the largest tsunamis in Japan's history. At 2217 local time (1317 UTC), the Ms-7.8 quake rocked the west coast of Hokkaido and the small, offshore island of Okushiri in the Sea of Japan, generating a major tsunami. Within 2–5 minutes, extremely large waves engulfed the Okushiri coastline and the central west coast of Hokkaido. Extensive damage occurred on the southern tip of Okushiri Island at the town of Aonae (Figures 1 and 2). Figure 3 shows the location of the study area.

Tsunami vertical runup measurements varied between 15 and 30 m over a 20-km portion of the southern part of Okushiri Island, with several 10-m values on the northern portion of the island (Figure 4). Along the west coast of Hokkaido, no survey values exceeded 10 m, but damage was extensive at several coastal towns (Figure 5). Given the sudden onset of the tsunami and its high energy, it is amazing that more people were not killed.