Seismic sea waves, generally referred to by the Japanese word “tsunami”, occur after strong seaquakes or large submarine landslides often induced by earthquakes or volcanic eruptions in the sea or on the coast. Theses waves spread out in all directions at great speed which depends on the depth of the water. In the great oceanic basins the mean speed is about 700 km/h. Although the waves are hardly noticeable in the open sea, they reach gigantic proportions in deep coastal waters, especially in narrow bays (in shallow waters they die before they even reach the coast). In Hawaii and Japan for example, waves of this kind suddenly hitting the coastline have been known to reach 30m in height, destroying long sections of the coast. As the waves can travel 10,000 km or more without much attenuation, regions that have not experienced any direct earthquake effects can be affected (e.g. Japan by the Chile earthquake in 1960). This is why a tsunami early warning service has been set up for the whole circum pacific zone. Exposure is limited to regions directly on the coast, but under worst case conditions may extend several km inland. With a rapidly increasing number of major industrial areas and large hotels being built along coastal regions, the tsunami risk has become considerably higher.
