'Merging Tsunami' Doubled Japanese Destruction

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

Research scientist Y. Tony Song of NASA's Jet Propulsion Laboratory in Pasadena, CA, and professor C.K. Shum of The Ohio State University discussed the data and simulations that enabled them to piece the story together at a media briefing Monday, 5^th December 2011 at the American Geophysical Union meeting in San Francisco.

"It was a one in 10 million chance that we were able to observe this double wave with satellites," according to Song. He is the principal investigator in the NASA-funded study. 

He continued by saying that 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. 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 NASA-Centre National d'Etudes Spaciales Jason-1 satellite passed over the tsunami on 11^th March 2011, as did two other satellites: the NASA-European Jason-2 and the European Space Agency's EnviSAT. All three carry radar altimeters, which measure sea level changes to an accuracy of a few centimetres. Each satellite crossed the tsunami at a different location, measuring the wave fronts as they occurred.

The researchers think that 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 its destruction appear random. For that reason, hazard maps that try to predict where tsunamis will strike rely on sub-sea topography. Previously, these maps considered only 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.

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

Images: The NASA/French Space Agency Jason-1 satellite (top left) passed over the tsunami 7 hours and 30 minutes after the earthquake and was able to 'see' a large wave resulting from merging tsunami jets. The NASA/European Jason-2 satellite (lower left) passed over the region 8 hours and 20 minutes after the earthquake and observed the normal tsunami wave. In the figures on the right, the satellite-observed tsunami height data (red and pink lines) was confirmed by JPL computer model predictions (black lines) based on 1,200 measurements of ground displacement from Japanese GPS stations. The satellites cannot change their orbit or be directed to view specific events, but may, by chance, be able to observe ocean phenomena such as this.