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Lecture 6

EPSC201 Lecture 6 Notes.doc

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC201 - Lecture 6 Notes Tsunamis – huge waves caused by earthquakes Japan Earthquake (2011): The pacific plate is plunging under Asia, which causes lots of earthquakes at the interface of the Asian and pacific plates. Its an ongoing process. The movement is jerky, never smooth, which causes tension to build up, and eventually overcomes friction. The net result is an earthquake. The longer time in between earthquakes, the greater in magnitude they will be. Disturbances of the plates on the ocean floor will cause waves. Not just one wave, but possibly many. It took nine hours from the wave from Japan to hit North America. It takes 13 hours by plane, which means the waves are traveling faster then the plane (over 1000km/h). Extremely fast when moving in the open ocean. In the open ocean, you would hardly notice a wave. The picture below show the height of the waves in the open ocean. Closer to land, the wave is amplified. The wave slows down from 1000 km/h to about 60 km/h but it is much greater in amplitude. Fisherman will notice a tiny wave and not think much of it, and return back to shore to find devastation. The incoming wave is of average height of 11 meters at shore. However, it could be much higher then this, for example 30 meters. The devastation is enormous. Many lives are usually lost. Coastal countries are extremely vulnerable. In Japan’s case, the added problem was that Japan is a nuclear dependent country. The Fukushima reactor was close to the coast, and there was a meltdown. In a hundred meter radius, people were affected by the radiation. A large number of people were affected by radiation sickness or poisoning. There was a slow response due the tsunami occurring simultaneously. 9.3 Richter scale earthquake in the South Pacific. Large number of people killed in Sri Lanka and Indonesia. Happened on Boxing Day, 2004. 50000 killed in Sri Lanka and 200000 in Indonesia. The pacific plate is again diving under the Asian plate, causing earthquakes. Major damage and loss of life, over a large area. Thailand was also affected. The water from the beaches goes out to sea. The water recedes out very far. This is a sign that a tsunami is incoming. If people knew, they would have more time to get to safety. A 13- year-old British schoolgirl was on vacation in Thailand, and had learned about tsunamis in geography. She recognized that a tsunami was incoming because the rapid recession of water out to sea. This girl saved about 300 lives. What about warning systems? Education – first line of defense. You have several minutes after the water recedes before the incoming wave(s) hit shore. Banda Aceh took the most damage. This change occurred in a manner of minutes. By looking at plate movement and past earthquakes, one can predict where the next earthquake will occur. Sea floor displacement propagates tsunami waves. Sri Lanka had about a 1.5 hours before the waves hit. If a warning system was in place, 50000 lives could have been saved. Stuck area will eventually slip and release all the energy at once. Will cause sea floor to bounce up rapidly, causing tsunami waves. Tsunamis in open water have a wavelength of 100,000 meters and are small in amplitude. As they come into shallow water, wavelength shortens, and amplitude increases. As the wave gets pushed up, it slows down, which causes the waves behind it to catch up. The waves pile up, and will hit shore in succession. Sri Lanka had approx 2 hours. Std – standard time in hours The coast of East Africa got hit several hours afterwards. Well developed countries have good early warning systems. The West coast of North America is earthquake prone. The small island on the coast of B.C. provide a barrier for the mainland (example – Vancouver Island shelters Vancouver City) Within seconds, people will know a tsunami is imminent. Its better to error on the side of caution. Indian Ocean did not have an early warning system. In the last year, one has finally been installed, due to recent tsunamis and awareness. The pacific ocean has had a warning system for a long time. Earthquakes Are A
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