As well as being highly destructive in their own right, earthquakes can also trigger two other very destructive natural hazards. One of these is a landslide. This is a rapid movement of earth materials down a slope, the materials ranging from huge boulders to soil. Landslides can involve the movement of just a small amount of material or enough to bury whole towns in their path. They can have a number of causes, of which earthquakes are just one. The shock of an earthquake may be sufficient to start the slide. One of the most destructive earthquake-induced landslides occurred in Peru in 1970, as described below, The 1970 Mount Huascaran landslide.
. It swept along the valley at the foot of the mountain, filling it with rock, mud and ice, and partially destroying the town of Ranrahirca, 12 km from the mountain. Part of the landslide branched off to one side, swept over a ridge and roared through the village of Yungay. The village was obliterated; only a few of its inhabitants managed to escape by running to higher ground as the landslide approached. Survivors described the landslide as like a gigantic ocean wave with a deafening roar and rumble. The earthquake also triggered many other smaller landslides in the region, destroying thousands of buildings and causing even more deaths. The final toll was 67 000 dead and 800 000 homeless, making this the worst earthquake-induced disaster in the Western Hemisphere.
The second natural hazard that can be triggered by earthquakes is that of tsunami (pronounced 'tsoo-nam-ee'). Tsunami are ocean waves caused by movement of the ocean floor by an earthquake beneath the ocean. The water is moved as if it were being pushed by a giant paddle, producing powerful waves that spread out from the region of the earthquake across the ocean. Tsunami are hardly detectable in the open ocean, having only a low wave height, 1 m or less, but when they reach shallow water at a coastline their wave height increases significantly, reaching over 10 m, with disastrous effects.
Tsunami is a Japanese word meaning bay or harbour wave, and is particularly apt as it is only along the shore that they become noticeable or destructive; in the open ocean they do no harm to ships. The term 'tidal wave' is sometimes used in newspaper reports of tsunami but this is inaccurate as they are not related to tides (which are generated on the Earth's surface by the gravitational attraction of the Moon and the Sun) so scientists use the term 'tsunami'. The Japanese word is particularly appropriate because Japan has suffered greatly from the destructive effects of tsunami. The 1964 'Good Friday' tsunami, describes the effects of one of the most damaging tsunami, triggered by an earthquake near Alaska.
Figure 2.2.1 Wreckage in Crescent City, California, from the 1964 Good Friday tsunami.