Earthquakes can cause deadly ocean waves called tsunami, although tsunami can be caused by any shock to ocean water, including a meteorite impact, landslide, or a nuclear explosion. When ocean water is displaced by the sharp jolt of an undersea earthquake, the seismic energy forms a set of waves. The waves travel through the sea entirely unnoticed since they have low amplitudes and long wavelengths. When these waves come onto shore, they can grow to enormous heights and cause tremendous destruction and loss of life. Fortunately, few undersea earthquakes generate tsunamis.
The Boxing Day Tsunami of December 26, 2004 was by far the deadliest of all time. The tsunami was caused by the 2004 Indian Ocean Earthquake, also called the Great Sumatra-Andaman earthquake. This earthquake, with a magnitude of 9.2, was the second largest earthquake ever recorded. The energy that reached the planet’s surface was 1,502 times the amount released by the atomic bomb dropped on Hiroshima, but the total amount of energy released was estimated at 550 million times Hiroshima.
The Indian Ocean Earthquake struck 160 kilometers (100 miles) off of Sumatra, Indonesia. In this region the Indian plate is subducting beneath the Burma plate. Slip along the earthquake fault was an incredible 15 meters (50 feet), about two-thirds of that in a horizontal direction and one-third in a vertical direction. The fault ruptured over about 1,600 kilometers (1,000 miles). Faulting went on for up to 10 minutes, the longest duration ever witnessed.
The extreme movement of the crust displaced trillions of tons of water. Water displacement occurred along the entire length of the rupture. This means that tsunami waves formed along a great distance, which increased the area that the killer waves traveled to. Several tsunami were created, with about 30 minutes between the peaks of each one.
The water traveled rapidly across the Indian Ocean outward from the fault. As is typical for tsunami, the waves were not noticeable in open water. Satellites measured the height of the waves across the sea at just 50 centimeters (20”). The first wave hit the northern regions of Sumatra in about 15 minutes. At its worst, the waves rose to around to 10 meters (33 feet) in height. Within 1.5 to 2 hours, waves were striking Sri Lanka and the eastern coast of India. Thailand was battered two hours after the earthquake. Somalia was hit seven hours after the earthquake. The size of the waves decreased with distance from the earthquake so that the waves in Sri Lanka, Thailand, and Somalia were relatively small, about 4 meters (13 feet) in height.
Like other waves, a tsunami wave has a crest and a trough. What people see when the tsunami hits the beach depends on whether the crest or the trough hits first. In some locations, the trough of the wave hit the beach first. When this happens, water is sucked out to sea and the seafloor just offshore from the beach is exposed. Curiosity is often fatal in this instance, since people who go out to the beach to see the unusual sight are drowned when the wave crest hits.
One amazing story was that of Tilly Smith, a 10 year old British girl who was visiting Maikhao Beach in Thailand with her parents. About two weeks before the earthquake, Tilly had learned about tsunamis in school. She knew that the receding water and the frothy bubbles at the sea surface indicated an approaching tsunami. As the trough of the tsunami wave hit the beach, she pointed these features out to her parents. They told other tourists and the staff at their hotel and the beach was evacuated. No one on Maikhao Beach died and Tilly is credited with saving nearly 100 people.
On other beaches, people were not so lucky. In all, the tsunami struck eight countries, with Indonesia, Sri Lanka, India and Thailand the hardest hit. About 230,000 people died, with fatalities even as far away as South Africa, nearly ,000 kilometers (5,000 miles) from the earthquake epicenter. More than 1.2 million people lost their homes and many more lost their ways of making a living. For example, fishermen lost their boats, and business people lost their restaurants and shops. Many marine animals were washed inland, including dolphins, turtles, and sharks.
Only a few scientists had thought that a massive tsunami would strike the Indian Ocean so no warning system had been in place. Tsunami are much more common in the Pacific due to the enormous number of subduction zones that line the Pacific basin, and communities around the Pacific have had a tsunami warning system in operation since 1948. As a result of the 2004 tsunami, an Indian Ocean warning system was put into operation in June 2006.
Warning systems are of limited use. They base their warnings on the location of earthquakes within an ocean basin. Unfortunately, communities that are very close to the earthquake do not receive the warning in time to move inland or uphill since the wave hits too fast. Still evacuation of low-lying areas could save many people in a large tsunami that is further from the earthquake.
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