Double Tsunami

When is a tsunami not the worse case: when it becomes a double tsunami. Researchers have discovered that the destructive tsunami generated by the March 2011 Tōhoku-Oki earthquake was a long-hypothesized merging tsunami that doubled in intensity due to passing over rugged ocean ridges, amplifying its destructive power before reaching shore. Satellites captured not just one wave front that day, but at least two, which merged to form a single double-high wave far out at sea – one capable of traveling much longer distances without losing its power. Ocean ridges and undersea mountain chains pushed the waves together, but only along certain directions from the tsunami’s origin.

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A tsunami earthquake is an earthquake that triggers a tsunami of a magnitude that is very much larger than the magnitude of the earthquake as measured by shorter-period seismic waves. The term was introduced by Hiroo Kanamori in 1972.

The discovery helps explain how tsunamis can cross ocean basins to cause massive destruction at some locations while leaving others unscathed, and raises hope that scientists may be able to improve tsunami forecasts.

At a news conference Monday at the American Geophysical Union meeting in San Francisco, Y. Tony Song, a research scientist at NASA’s Jet Propulsion Laboratory (JPL); and C.K. Shum, professor and Distinguished University Scholar in the Division of Geodetic Science, School of Earth Sciences at Ohio State University, discussed the satellite data and simulations that enabled them to piece the story together.

"It was a one-in-ten-million chance that we were able to observe this double wave with satellites," said Song, the study’s principal investigator. "Researchers have suspected for decades that such merging tsunamis might have been responsible for the 1960 Chilean tsunami that killed many in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now."

"It was like looking for a ghost," he continued. "A NASA/French Space Agency satellite altimeter happened to be in the right place at the right time to capture the double wave and verify its existence."

Shum agreed. "We were very lucky, not only in the timing of the satellite, but also to have access to such detailed GPS-observed ground motion data from Japan to initiate Tony’s tsunami model, and to validate the model results using the satellite data. Now we can use what we learned to make better forecasts of tsunami danger in specific coastal regions anywhere in the world, depending on the location and the mechanism of an undersea quake."

The researchers conjectured ridges and undersea mountain chains on the ocean floor deflected parts of the initial tsunami wave away from each other to form independent jets shooting off in different directions, each with its own wave front.

The sea floor topography nudges tsunami waves in varying directions and can make a tsunami’s destruction appear random. For that reason, hazard maps that try to predict where tsunamis will strike rely on sub-sea topography. Previously, these maps only considered topography near a particular shoreline. This study suggests scientists may be able to create maps that take into account all undersea topography, even sub-sea ridges and mountains far from shore.

Other tsunami earthquakes include:

1605 Keichō Nankaidō earthquake
1946 Aleutian Islands earthquake
1960 Peru earthquake
1963 Kuril Islands earthquake
1975 Kuril Islands earthquake
1994 Java earthquake
1996 Chimbote earthquake
July 2006 Java earthquake

For further information and photo: http://researchnews.osu.edu/archive/doubletsunami.htm