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Surveying the damage: Locals walk past a boat swept ashore by the force of a tsunami at Nagapatti

The deadly wall of water that devastated the coastal communities of nearly a dozen countries in South Asia on Dec. 26 was the ocean’s natural response to the massive underwater earthquake, UCLA geophysicists say.

“As far as I know, it behaved itself as it should have,” said John Vidale, professor of geophysics at UCLA. “The only surprise was the earthquake. Nobody expects an earthquake that big.”

Tsunamis can occur following any earthquake on an underwater fault-line, provided the earthquake is big enough, said Emily Brodsky, assistant professor of geophysics at UCLA.

“The earthquake moves the ground underneath the ocean,” Brodsky said. “Imagine a big bucket of water. If you punch the bottom of it, it’s going to create a big wave.”

Though the earthquake released more than 20,000 times as much energy as an atomic bomb, according to National Geographic, it was the tsunami that caused the most damage, said Heidi Houston, associate professor of geophysics at UCLA.

“I don’t think the shaking caused much damage,” Vidale said. “I’m not sure they even felt the earthquake.”

Vidale explained that the wave would have been barely noticeable at the epicenter of the earthquake, which took place in the middle of the Indian Ocean, roughly 200 kilometers away from Sumatra.

In the open ocean, the energy put into the water by the earthquake is spread over a wide area. But as the ground becomes shallower, the wave becomes larger and more powerful as its energy becomes concentrated into a smaller volume of water.

“The weird thing about a tsunami is you don’t see a very large swell out on the ocean,” Brodsky said. “It builds up to a higher and higher front that crashes onto the shore.”

The height of each wave of the tsunami also depends on how shallow coastal waters are, Brodsky said, leading to 50-foot waves in some areas, according to the National Geographic.

“The tsunami is as big as the earthquake,” Vidale said, pointing out that this one “moved the sea floor a couple meters up and down.”

Tsunamis are usually caused by what Brodsky calls “shallow” underwater earthquakes.

Earthquakes occur when two of the plates that make up the earth’s crust rub against one another as they move. But the point at which the two plates rub together might occur nearly at the surface, or hundreds of kilometers beneath the earth’s crust. The closer the earthquake occurs to the surface, the more likely it is to cause a tsunami. “This earthquake happened right at the top,” Brodsky said.

Vidale adds that the two plates converging at the Sumatra faultline are also meeting head-on in a “subduction zone,” with one plate being pushed on top of the other – a situation which is conducive to producing tsunamis.

Because of its abundance of subduction zones, the Pacific Ocean is thought to be at an even greater risk for tsunamis than the Indian Ocean, Houston said. But although it is possible for a tsunami to hit the coast of Los Angeles, Brodsky, Houston, and Vidale say it is unlikely.

“The nearest fault is on land,” Vidale said, “so it’s not likely to shake the water.”

In addition, Vidale said, the fault lines near Los Angeles are of a different type from the fault lines which tend to produce tsunamis. “The faults here are more sideways,” Vidale said. “They don’t push the water up or pull it down.”

Houston adds that Los Angeles is unlikely to have an earthquake with a magnitude as high as 9.0 because the plates converging under Los Angeles do not rub against one another over as broad or deep an area as they did in Sumatra. “There’s no techtonic structure that will produce a big enough fault plane,” Houston said.

Despite the unlikelihood of a tsunami occurring locally, Houston warns everyone to be aware of the signs of an impending tsunami. “If you’re at the beach and the water withdraws or you feel strong shaking, leave the beach and get onto higher ground – even in Los Angeles,” Houston said.

Although some of the waves did not reach shore until six hours after the earthquake, Houston said, most victims were taken by surprise.

“One of the reasons there was no warning system in place was that there had not been that many tsunamis,” Houston said. “They’ve had them, they just haven’t had them sufficiently close to that region.”

Unlike the Indian Ocean, the Pacific Ocean has a tsunami warning system in place, with seismometers to detect earthquakes, as well as sensors to identify unusually large waves in the open ocean and the receding tide characteristic of a coming tsunami, Vidale said. A “call-down” system is also in place for contacting all the countries in the area efficiently, Vidale said. “That sort of structure is not in place in the Indian Ocean,” Brodsky said. “It is not a particularly well-instrumented part of the world.”

Brodsky hopes that the disaster will bring people together to create a similar tsunami warning system in the Indian Ocean. “The most important thing societally to come out of it is a better interaction between scientists and society,” Brodsky said. “It will improve communication between scientists and policy makers, so we’re not just doing our science in a box.”