Animals and plants are not the only things that form fossils. Tsunamis-the huge waves created by some submarine earthquakes-do so, too. A tsunami generated in January 1700, off the Pacific coast of North America, for example, has left abundant traces in local rocks, as well as in the art of Japan (it was the inspiration for Kat-sushika Hokusai's woodcut, "The Great Wave of Kanagawa"). That should give pause to coastal dwellers in what is now Oregon, Washington state and British Columbia. They might reasonably wonder when the next big wave will arrive-as might residents of other earthquake-prone coastlines around the world. Harvey Kelsey, of Humboldt State University, in California, has done more than wonder. He and his team have been looking at the Indian Ocean coast of Aceh province, in northern Sumatra. This was the origin, in December 2004, of a powerful submarine earthquake and subsequent tsunami that killed around 230,000 people. The quake in question, as is commonly the case for quakes of this magnitude, was caused by one of the Earth's crustal plates sliding under another. Plate movement at such boundaries tends to be episodic, with strain that has built up over the years being released suddenly as a tremor, when it becomes too much for the rocks to sustain. The rate at which the quake-causing strain accumulates, however, is constant. This means earthquakes of this type can be expected at reasonably regular intervals.
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