Japan's tsunami waves top historic heights
By Dan Vergano
USA TODAYApril 25, 2011
Tsunami waves topped 60 feet or more as they broke onshore following Japan’s earthquake, according to some of the first surveys measuring the impact along the afflicted nation’s entire coast. Some waves grew to more than 100 feet high, breaking historic records, as they squeezed between fingers of land surrounding port towns.
The tsunami was born when the magnitude-9.0 earthquake struck March 11 about 45 miles off Japan’s coast. The U.S. Geological Survey said the quake lifted and then dropped a slab of seafloor 50 miles wide and more than 180 miles long. The force shifted the seafloor nearly 80 feet westward above the quake center.
Within a half-hour, the waves arrived on Japan’s coast, plateaus of water that surged up to six miles inland and unleashed much of the devastation that killed about 14,300 people, with another 12,000 still missing. The new estimates on wave heights from the United Nation’s Intergovernmental Oceanographic Commission, gathered from Japanese university surveys, show the biggest waves hit the hilly harbor towns north of where the quake was centered. The surge grew in between inlet hills to 124 feet high at the fishing port of Koborinai.
“Waves this high are completely predictable after such a large earthquake. But they are still almost unimaginable,” says tsunami geologist Jody Bourgeois of the University of Washington in Seattle, who was in Japan when the quake struck.
Japan’s science ministry has dispatched more than 200 tsunami experts to map where and how high the waves came during the tsunami.
North of the city of Sendai, University of Southern California tsunami expert Costas Synolakis, found that the surge exceeded 40 foot depths a quarter-mile inland, and still reached over 26 feet high about a half-mile inland.
“We have only seen such extremes ‘recently’ in Banda Aceh during the 2004 tsunami,” Costas says, by e-mail. That 2004 Indian Ocean tsunami killed more than 230,000 people in 14 southeast Asian countries.
Building on an analysis released earlier this month, University of Tokyo tsunami expert Yoshinobu Tsuji, reports the wave heights exceeded those of a record 1896 tsunami in Japan.
Although wave heights were lower on the Japanese coast south of the epicenter of the March 11 quake, they were still high enough to top an 18-foot sea wall at Japan’s crippled Fukushima Dai-ichi nuclear plant, sparking an ongoing crisis there.
Although terrible, the preliminary estimate also finds a better-than 92%survival rate for people living in coastal towns hit by the waves, Bourgeois says. “In that sense, given the magnitude of the unexpectedly large earthquake, things could have been even worse,” she says.
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