Powerful tsunamis have struck earthquake-prone Japan many times over the centuries. In 1771, a local tsunami said to be as high as 85 meters hit the Yaeyama Islands in the westernmost part of Japan, killing more than 12,000. In 1896, a tsunami with a maximum height of 24.4 meters swept away more than 22,000 people in the Sanriku district, along the Pacific Ocean coastline of northeastern Honshu. In more recent times, an earthquake off the coast of Chile in 1960 generated a tsunami that killed 140 in Japan, and in 1983 another quake under the central part of the Sea of Japan pushed a wall of water onto land in northeastern Honshu, killing 100.

Other tsunami disasters in Japan are too numerous to list here, but one still etched in many memories struck 12 years ago after an earthquake off southwestern Hokkaido. The huge tsunami (maximum height, 29 meters) hit nearby Okushiri Island a mere three to five minutes later, swallowing houses at the southern tip of the island. This happened soon after 10 p.m. when many people were in bed, and more than 200 were swept into the cruel sea and drowned (see previous pages).

The surest way to prevent destruction is to construct sea walls that are higher than the highest expected tsunami. The sea walls would need gates across river mouths and in other places to permit the passage of boats, and the gates would have to be closed quickly, before a tsunami arrived. This is not practical for a long archipelago with a convoluted coastline. Sea walls with gates that open and close automatically are now being constructed only along coasts where tsunamis are considered most likely to occur. In some places, construction has been delayed for some future date, and as a stopgap measure evacuation towers and two-story houses are being built to get people to higher locations. Breakwaters with gates that close when the sea surface rises have also been proposed recently. These offer low installation costs, and are expected to be feasible for the protection of wharfs and areas along bays that have narrow openings.