Monday, March 14, 2011

Japan Quake and Tsunami


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On March 11, 2011, a massive 8.9 magnitude earthquake that struck north-eastern Japan devastated large swaths of the coast and spawned a powerful tsunami. For those interested to know, this was caused by the Pacific tectonic plate thrusting underneath the country, and forcing the seabed and ocean water upward. Friday's quake actually occurred 80 miles offshore and 15.2 miles underwater in a subduction zone called the Japan Trench, part of that plate boundary. For more than a minute, the crust ruptured, stretching 180 miles long.

And according to the UK Telegraph, the Japan earthquake was 8,000 times bigger than the one that struck Christchurch last month. The earthquake also generated roughly the amount of energy that the US consumes in a year, said David Wald, a seismologist with the National Earthquake Information Center at US Geological Survey.

Japan unfortunately sits on or near the boundary of four tectonic plates: the Pacific, North American, Eurasian and Filipino plates. These massive slabs of earth's crust are endlessly creeping, slipping, locking up and then jolting again. In fact, the Pacific plate has been inching its way under the North American plate at a rate of 80 to 100 mm a year, said John Bellini, a USGS geophysicist. As these plates butt into each other, pressure builds and builds. Then, very unpredictably, that pressure releases. The release of such tension causes earthquakes, and the sudden lifting of the seabed triggers the tsunami.

This upward thrusting of the seafloor contained enough energy to displace an enormous amount of water, churning up huge waves that move as fast as 400 miles per hour. "Out in the open ocean, the waves can be hundreds or even thousands of miles long," said Bruce Parker, a Stevens Institute of Technology Professor, and author of the book, "The Power of the Sea". "The deeper the water, the longer the wavelength, and as soon as the water gets shallower, the wave gets shorter and higher".

Unlike a normal surfing wave, which curls over from the top, but is quiet underneath, a tsunami moves from top to bottom toward the shore, bulldozing everything in its path. As it moves toward the land, it shortens in length, but rises in height. "It's only when it gets into shallow water that it steepens and lifts up, and that's when it turns into a tsunami," Parker said.
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