The powerful earthquake that struck off the coast of Japan on Friday triggered tsunami warnings around the Pacific Rim, including in Hawaii and on the West Coast of the United States, but most of the destructive flooding appeared to have occurred in Japan itself, in the area nearest the quake’s epicenter.
Experts said that this pattern was not unusual. With this earthquake as with others, essentially two tsunamis are generated — one that hits the local coastline, often within minutes, and another can travel for thousands of miles in the opposite direction, some of its energy dissipating as it spreads across the open ocean.
With the local tsunami, the first wave is usually the most destructive, said Eric Geist, a scientist with the United States Geological Survey in Menlo Park, Calif. “Between the earthquake and the Japanese coast there is not a huge amount of variation” in the seafloor, he said. “So for the local tsunami, the energy mainly depends on the earthquake parameters.”
With far-off tsunamis, though, some of the energy dissipates as the wave spreads outward, like a ripple from a rock thrown into a pond. Coastal features can also reduce some of the energy as the wave runs up on land. But coastal features can also cause secondary effects that amplify the forces, so that the second or third tsunami wave is often the most severe.
“Once the first wave hits the coastline, it gets very complicated,” Mr. Geist said. “There are reflected waves, scattered waves that propagate up and down the coastline.”
Reports from Sendai, Japan, the city closest to the quake, suggest that wave heights reached more than 12 feet above normal as the first tsunami wave struck.
In North America, the worst waves — worse even than in Hawaii — were forecast for the California-Oregon border, with heights of more than 8 feet above normal sea level. The reason for this, Mr. Geist said, is that there is a feature in the sea floor — a cliff-like rise called the Mendocino fracture, which runs east-west — that serves to guide the tsunami waves toward the area, concentrating them.
The West Coast and Alaska Tsunami Warning Center forecast waves of about 7 feet for Brookings, Ore., for example, an area affected by the Mendocino fracture, but in Eureka, Calif., about 100 miles south, the prediction was for a little more than a foot.
But tsunami experts warned that predicting wave heights was difficult, and that because of the complicated interactions as wave after wave struck the coast, the full effect of the tsunami would not be known until about a day after the earthquake.
“Tsunamis are rare events,” said Paul Huang, a seismologist with the tsunami warning center. “And calibrating the big events is hard. You have no data.”
The 8.9 magnitude earthquake on Friday, the largest ever recorded in Japan, occurred in a subduction zone, where one of the earth’s tectonic plates is sliding beneath another. In this case, the Pacific plate is sliding beneath the Eurasian plate, which the Japanese island of Honshu sits on, at a rate of slightly more than 3 inches per year.
At the actual boundary between the two plates, stresses build up that are held in check by friction. At some point, said Ross S. Stein, a geophysicist with the geological service, “the stress overwhelms that friction,” and an earthquake occurs.
In a subduction earthquake, parts of the fault zone are uplifted, while others dip down. “It’s almost like you took a rug and kind of popped it and watched a ripple roll through it,” Mr. Stein said.
If the quake occurs under water, as this one did, the up-and-down movement displaces an enormous amount of water, triggering the tsunami.
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