7.6.10: Special Problems with Tsunamis

Overview

If you live on the coast, you will have the same problems everybody else has with shaking and unstable ground. But you’ll have an additional problem: the threat of inundation from a large wave from the ocean.

In the case of a distant tsunami, such as the one that originated in Alaska and struck Port Alberni, B.C., Seaside, Oregon, and Crescent City, California, in 1964, a warning will be issued by the Tsunami Warning Center in Alaska, including an expected arrival time of the tsunami. You will have time to evacuate to high ground. It’s critical that you have a portable radio turned on to listen for tsunami warning updates. Most of the people who got into trouble in the Easter weekend tsunami of 1964 were just enjoying a normal spring holiday, without enough concern for events in the rest of the world to keep up with the news. With satellite communication and tsunami warning centers throughout much of the Pacific, the warning of a distant tsunami should be taken seriously, but you have to have your radio on to hear it. A coastal community is well advised to have a siren to warn those who aren’t tuned in to their radio or television. This siren should be maintained by emergency-services or fire department personnel and should have its own generator.

In the case of an earthquake on the Cascadia Subduction Zone, you’ll have a much shorter time to react—twenty minutes or less. For this reason, if your area is subjected to very strong shaking lasting twenty seconds or more, don’t wait for a tsunami warning. Leave immediately for high ground and stay there for an hour or so until you’re sure there is no local tsunami.

There is no direct correlation between tsunami height and magnitude of the earthquake. A subduction-zone earthquake off the Pacific coast of Nicaragua on September 2, 1992, generated an unusually large tsunami for the size of the earthquake. It was found later that fault rupture was much closer to the surface, and fault motion took place much more slowly than for most subduction-zone earthquakes. In Papua New Guinea, coastal villagers were swept away by a tsunami generated by a landslide and by sea-floor deformation. Earthquakes like this are sometimes called tsunami earthquakes; the tsunami is much more extreme than the seismic shaking would predict.

The other problem in coping with tsunamis from a distant source is the period of the waves. Frequently, the first wave is not the largest one. The people of Crescent City, California found this out the hard way. The first and second waves were small and caused little damage and people returned to the shoreline, only to be struck by much larger waves that crashed through the town.

Unlike ordinary storm waves, the period of a tsunami wave can be as long as an hour. So when the first wave rushes up and then recedes, for the next half hour or so you will notice only the ordinary surf. But don’t think the tsunami is over. Wait at least two hours before you return. And, just as a tsunami rises higher than ordinary waves, causing great damage, the tsunami also causes the water to recede much farther out to sea, exposing ocean floor not ordinarily seen even at the lowest tides (Fig. 9-9). The temptation to rush to the beach at that time could be fatal.