One of the most powerful earthquakes in history Sunday triggered a tsunami that swept across thousands of miles of shoreline in southern Asia, swamping fishing villages in India and Sri Lanka as well tourist resorts in Thailand and the Maldives, and killing more than 8,000 people.
This tsunami - a wall of water up to 30 feet high - was spawned by an 8.9 magnitude underwater earthquake off of Sumatra.
While tsunamis are relatively rare in the Indian Ocean, the devastation is likely to prompt calls to extend a 26-nation tsunami warning network in the Pacific, where most tidal waves occur. And strong aftershocks could trigger more tsunamis. The earthquake may also have created new quake zones hundreds of miles away.
"Tsunamis occur in all oceans," and the warning center the US runs in Hawaii "does a good job of monitoring the Pacific," says Harley Benz, chief scientist for the US Geological Survey's National Earthquake Information Center in Golden, Colo. But this event was outside the area monitored by the network.
This tsunami originated where two plates in the Earth's crust - the India and Burma plates - collide. A 620-mile segment of the boundary snapped. Suddenly the Burma plate's margin stood at least 15 meters higher than the India Plate. With a preliminary magnitude of 8.9, the quake was the strongest anywhere in the globe in the last 40 years and perhaps the strongest ever recorded in this region. It followed by three days a magnitude 8.1 quake off of Tasmania. The sudden shift in plate-boundary heights was like slapping the underside of a full pail of water. It triggered a tidal wave that reached an estimated 20 to 30 feet high when it made landfall.
Depending on a location's distance from the undersea quake or landslide, warning times may be short. In the open ocean, a tsunami can travel several hundred miles per hour, with the distance between wave crests stretching for 300 to 400 miles. At sea, a tsunami's height may only reach a foot or two above the surrounding ocean. Only when it begins to feel the sea floor's friction along a coast does it slow down and rear up.
Hardy Bebuch, a German tourist staying on Thailand's idyllic Phi Phi island, was looking out over the bay from his second-floor hotel window when the first wave hit about 10:30 a.m. local time. Moments before, he recalled by phone, "The sea went back. I said to my wife, 'I've never seen those rocks before, they must be hundreds of meters away.' It was incredible," he said. Then the first wave swept into his beachfront hotel, submerging the floor below. Wooden boats were hurled into trees. "We were staying in a strong building. It didn't collapse. But there were many villas washed away."
Associated Press reported that 200 bungalows were destroyed on Phi Phi island, where "The Beach," starring Leonardo DiCaprio, was filmed. It's off the southwest coast of Thailand.
Mr. Bebuch said the first wave was one (3.3 feet) meter high, the second was almost 10 feet high, with a gap of two and a half minutes between them. "People were running everywhere," he says. He joined hundreds of tourists and tourist workers in scrambling to higher ground on the island, as more waves crashed onto the shore.
Kim Yongmi, an oil painter from Korea who was vacationing on a small island off Phuket, Thailand, said by phone that she and half of the island's tourists sought refuge in a Buddhist monastery on a hill on the island. They plan to spend the night there.
On Phi Phi, electricity and phone lines were cut by the tsunami. Army and police helicopters landed on the island later Sunday afternoon to evacuate some 600 tourists.
Bebuch left on a private ferry bound for Phuket and met a Dutchman who was asking if anyone had seen his wife. "They were walking on the beach when it happened, and he said they were running away, but they went different directions. He didn't see her after that," says Bebuch.
Arriving in Phuket, also devastated by high waves, Bebuch joined hundreds of other foreign tourists trying to find rooms for the night away from the coast, where hotels had been evacuated. Many were wary of aftershocks and more waves. "They think there's more on the way."
In Sri Lanka, where the death toll was more than 3,500, thousands fled their homes. Idyllic beaches were turned into fields of debris and destruction. "I think this is the worst-ever natural disaster in Sri Lanka," N.D. Hettiarachchi, director of the National Disaster Management Centre, said. Officials appealed to the world for aid, saying that one million people, or 5 percent of the Sri Lankan population, were affected.
"The real tragedy is that although you cannot predict an earthquake, you can predict a tidal wave that might follow, but the meteorological department failed to inform anyone about this," says Jehan Perera, media director of the National Peace Council, reached by phone in Colombo, Sri Lanka. "The earthquake took place around 7 a.m. our time and the tidal wave came in around 9 a.m. That's enough time to give people some warning."
In the Indian coastal state of Tamil Nadu, a government official said at least 1,625 had been killed. Rescuers were searching for hundreds of missing fishermen, and thousands were homeless.
In Indonesia, 2,583 people were reported killed. The earthquake badly damaged the provincial capital, leaving cracks in major roads. Aceh is currently closed to international media and aid agencies due to a 30-year old conflict between the government and separatist forces. Lhokseumawe, an area badly affected by tidal waves and home to the local operations of US company Exxon Mobil, has also been a site of fighting.
Researchers say that one concern now is the effect this quake may have had on adjoining plate boundaries - indeed, Sumatra is sandwiched between three. Research in other parts of the world along land-based faults have shown that a major earthquake in one fault segment can change patterns of stress in the crust surrounding other faults hundreds of miles away.
"That is a real issue," Dr. Benz says. "What is the failure rate for this fault, and what sort of loading" does this quake place on them?
The problem is more challenging for undersea faults, such as the one that triggered Sunday's tidal wave. Quakes of this magnitude are rare worldwide. Researchers must rely on the historical record and on modeling seismic activity, which has made major strides over the past two decades.
This two-pronged assault on understanding submarine faults and plate boundaries has allowed seismologists to advance from reporting the magnitude and location of an earthquake to reconstructing information on depth, likely displacement, and the transmission of seismic energy through the crust. Though this information comes after the fact, it remains critical to assessing earthquake and tsunami hazards and taking steps to minimize casualties, if not property damage, when these events occur.
"If you could model an earthquake before it happens, that would be great," Benz says. "But information afterward is just as important. It lets you know where you are in the earthquake cycle" of a particular fault system.
Researchers will be monitoring the Sumatra epicenter closely now: There is the potential for other tsunamis if aftershocks top a magnitude of 7.5. More tsunamis may accelerate the call for a more extensive, and better monitoring system.
The Pacific Tsunami Warning System is a network of seismic-monitoring stations and sea-level gauges that detects earthquakes and abnormal changes in sea level. Currently, Japan has the most advanced tsunami system in Asia.
All warnings are based on such information as the scale of the earthquake as measured by special cables on the seabed. A warning takes about 10 minutes to issue, and is flashed on TV screens nationwide in Japan, and sent to disaster-prevention agencies at local government offices that sound evacuation alarms. Many of Japan's coastal towns also have water gates to shut out waves that might head inland via low-lying river networks.
But even this technology has been criticized as slow and able to provide only a rough projection of the height of a wave. After a government study in 2003 showed that a tsunami resulting from an 8.6 magnitude quake in the Pacific south of Japan could kill up to 8,600 people if evacuations were slow, both government and private industry groups renewed efforts to improve warning systems.
A Japanese research agency has since developed a method to accurately predict the height of a tsunami just three seconds after an earthquake hits. More seabed cables are currently being laid to broaden the area this system covers.
The company Hitachi has developed another system using Gobal Positioning System (GPS) satellite technology that can detect the presence of a tsunami several kilometers offshore by measuring how much a giant 13-meter buoy rises or falls on the ocean surface.
• Simon Montlake contributed reporting from Bangkok, Thailand, Tom McCawley from Jakarta, Indonesia, Janaki Kremmer from Delhi, Bennett Richardson from Tokyo, and staff writers Robert Marquand from Beijing.
What is a tsunami?
A tsunami is a series of large waves usually generated by a violent undersea disturbance. The word tsunami (pronounced tsoo-nah'-mee) is composed of the Japanese words "tsu" (which means harbor) and "nami" (which means "wave").
How big - and fast - are they?
In deep ocean, the height of the tsunami may be only a few centimeters to a meter or more. Tsunami waves in deep water can travel at high speeds thousands of miles and lose very little energy. The deeper the water, the greater the speed.
For example, at the deepest ocean depths, the tsunami wave speed will be as much as 497 m.p.h, about the same as that of a jet aircraft. In 1960, great tsunami waves generated in Chile reached Japan, more than 10,439 miles away in fewer than 24 hours, killing hundreds of people.
What role do earthquakes play?
By far, the most destructive tsunamis are generated from large, shallow earthquakes with epicenters or fault lines near or on the ocean floor.
The quakes can disturb the ocean's surface, displace water, and generate destructive waves that can travel great distances. Not all earthquakes generate tsunamis. Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.
What else causes tsunamis?
Less frequently, tsunamis can be generated from displacements of water resulting from rock falls, icefalls, and sudden submarine landslides or slumps. Major earthquakes are suspected of causing underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that a major 1998 tsunami along the northern coast of Papua-New Guinea was generated by a large underwater slump of sediments, triggered by an earthquake.
The largest tsunami wave ever observed was caused by a rock fall in Lituya Bay, Alaska, on July 9, 1958. Triggered by an earthquake along the Fairweather fault, a massive rock fall at the head of the bay generated a wave that reached the incredible height of 1,720 feet on the opposite side of the inlet.
- The International Tsunami Information Center, Honolulu, Hawaii. http://www.prh.noaa.gov/pr/itic/library/about_tsu/faqs.html#7