“Major earthquakes occur somewhere in the world every year or two. Catastrophic tsunamis – giant waves generated by undersea earthquakes or landslides – strike less often, and some of the largest of tsunamis originate in places that do not, at first glance, appear particularly treacherous.”

“Tsunamis follow the same laws of physics as ordinary surf waves generated by wind. The difference is size. For wind-driven waves, the distance between wave crests – the wavelength – is at most a few hundred yards. For tsunamis, that wavelength can be a hundred miles or more. Because the wavelength is so much greater than the ocean depth, the speed of the wave depends on that depth. In water 2.5 miles deep, the average depth in the Pacific, a tsunami travels almost as fast as a jetliner, 440 miles an hour.

Ships at sea notice nothing. As a tsunami races past, the ocean surface rises and falls slightly, a few feet at most, over a period of several minutes to a couple of hours. Underwater, the effects are more pronounced. The downward pressure of a surf wave dissipates a few hundred yards below the surface, while the pressure force of a tsunami extends to the ocean bottom. “

“Videos captured of the tsunami seemed to pale next to the cataclysmic imaginings of Hollywood movies, but “looking at the videos, you would be fooled,” said Dr. Synolakis of U.S.C.

For one, those who tried to videotape more imposing waves might not have survived. But also, unlike an ordinary wave, which quickly dissipates and rolls back out, a tsunami is a long sheet of water. “Behind the wave is a change in sea level coming in,” Dr. Synolakis said. “The wave is coming and coming and coming. A three- or four-meter tsunami can be quite devastating.”

One cubic yard of water weighs nearly a ton, and a tsunamis come ashore at speeds of about 30 miles an hour. An oncoming tsunami can hit a building with millions of pounds of force, said Dr. Peter E. Raad, a professor of mechanical engineering at Southern Methodist University in Dallas.

“And that’s before you put anything in the water,” he said.

Trees, automobiles and pieces of concrete all become lethal projectiles as they are swept along by the rushing water.”

“Others, including Dr. Steven N. Ward of the University of California, Santa Cruz, have warned that the volcano Cumbre Vieja in the Canary Islands off northwestern Africa could be nearing one of its periodic collapses. As the volcano grows through eruptions, the sides become unstable and eventually fall into the ocean. During the last eruption in 1949, a two-mile-long crack opened up and one side of the volcano slid 10 feet.

“Geologically, we’re getting close to the end,” Dr. Ward said. “It’s really the cycle of life for these volcanoes. They grow too big, they collapse.”

In Dr. Ward’s computer models, when Cumbre Vieja collapses – and that may not happen for hundreds of thousands of years – about 100 cubic miles of rock will slide into the ocean at speeds greater than 200 miles per hour, and the splash will generate tsunamis 300 feet high crashing into the northwestern coast of Africa. Waves 40 feet high will reach New York.”