Image 1 - Japanese Devastation The Result Of Merging Waves
December 6, 2011

Japanese Devastation The Result Of Merging Waves

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Researchers from NASA and Ohio State University have discovered that the deadly tsunami generated from the March 2011 Tohoku-Oki earthquake that devastated northeastern Japan resulted from merging waves, causing the killer tsunami to double in intensity over ocean ridges, and then amplifying its power upon landfall.

Data taken from NASA and European radar satellites captured at least two wave fronts during the events that day. The fronts merged to form a single, more intense wave far out at sea that was capable of traveling great distance without losing power. Ocean ridges and undersea mountain ranges pushed the waves together, but only along certain directions from the tsunami´s origin.

The discovery helps explain how tsunamis can cross ocean basins to cause significant destruction in certain areas while leaving other areas mostly untouched. Scientists hope the new findings will help improve tsunami forecasting.

“Nobody had definitively observed a merging tsunami until now,” Y. Tony Song, a research scientist at NASA´s Jet Propulsion Laboratory (JPL) in Pasadena, California, said during a media briefing on Monday. “It was a one in 10 million chance that we were able to observe this double wave with satellites.” Song is the principal investigator in the NASA-funded study.

“We can use what we learned to make better forecasts of tsunami danger in specific coastal regions anywhere in the world,” added Professor C.K. Shum of Ohio State University.

The earthquake and tsunami together were responsible for more than 15,500 deaths in Japan. The events also led to a nuclear crisis at the Fukushima Daiichi Nuclear Power Plant, causing widespread chaos because of leaking radiation.

“Researchers have suspected for decades that such ℠merging tsunamis´ might have been responsible for the 1960 Chilean tsunami that killed about 200 people in Japan and Hawaii, but nobody had definitively observed a merging tsunami until now,” Song noted. “It was like looking for a ghost. 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.”

The sea floor topography nudges tsunami waves in varying directions and can make the destruction appear to be random. For that reason, hazard maps that try to predict where tsunamis will likely strike rely on sub-sea topography. Previously, these maps considered only topography near the shoreline. But based on the current findings, scientists believe forecasts need to consider all undersea topography, even those far out at sea.

Three separate satellites passed over the region on March 11, during the tsunami. The NASA/Centre National d'Etudes Spaciales Jason-1 satellite, the NASA/European Jason-2 satellite and the European Space Agency´s EnviSAT, all carry a radar altimeter, which measures sea level changes to an accuracy of a few centimeters.

Each satellite crossed the tsunami at a different location. Jason-2 and EnviSAT measured wave heights of 8 inches and 12 inches, respectively. But as Jason-1 passed over the undersea Mid-Pacific Mountains to the east, it captured a wave front measuring 28 inches.

Song and his team were able to verify the data through model simulations based on independent data, including GPS data from Japan and buoy data from the National Oceanic and Atmospheric Administration´s (NOAA) Deep-ocean Assessment and Reporting of Tsunamis program.

“Tools based on this research could help officials forecast the potential for tsunami jets to merge,” Song said. “This, in turn, could lead to more accurate coastal tsunami hazard maps to protect communities and critical infrastructure.”

Song and Shum´s collaborators include Ichiro Fukumori, an oceanographer and supervisor in JPL´s Ocean Circulation Group; and Yuchan Yi, a research scientist in the Division of Geodetic Science, School of Earth Sciences at Ohio State.

The research was supported by NASA.

Results of the study were presented recently at an American Geophysical Union meeting.


Image 2: A 3D ocean model created by Y. Tony Song of NASA´s Jet Propulsion Laboratory and colleagues replicates the March 11, 2011 tsunami. The red line shows the path of the Jason-1 satellite, which crossed the wave front at the site of a double-high wave. Image courtesy of NASA.


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