The devastating earthquake and subsequent tsunami that struck Japan in March of 2011 has just been revealed to be a long-hypothesized but never proved ‘merging tsunami’ that doubled in intensity after passing over rugged sub-sea topography, which amplified its destructive power prior to reaching the shore.
Searching for a Ghost
“It was a one-in-ten-million chance that we were able to observe this double wave with satellites,” said Y. Tony Song, a research scientist at NASA’s Jet Propulsion Laboratory (JPL) and 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.”
That was, until this year, when the NASA/Centre National d’Etudes Spaciales Jason-1 satellite passed over the tsunami on March 11, as did two other satellites: the NASA/European Jason-2 and the European Space Agency’s EnviSAT.
“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.”
“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,” C.K. Shum, professor and Distinguished University Scholar in the Division of Geodetic Science, School of Earth Sciences at Ohio State University. “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.”
Switching Directions
The satellites captured not just one, but two wave fronts that day, which then merged into a single double-high wave far out at sea, thus allowing it to travel longer distances without any loss in power.
The fascinating thing was that the ocean ridges and undersea mountains pushed the waves together in certain directions, but not in others. This explains why tsunamis can cross ocean basins to cause massive destruction at location a and c, for example, while leaving location b entirely unscathed.
Each of the three satellites crossed the tsunami at a different location. Jason-2 and EnviSAT measured wave heights of 20 cm (8 inches) and 30 cm (12 inches), respectively. But as Jason-1 passed over the undersea Mid-Pacific Mountains to the east, it captured a wave front measuring 70 cm (28 inches).
Song and Shum suspect that ridges and undersea mountains that rest on the ocean floor worked to deflect different parts of the tsunami away from each other, thus creating independent sections shooting off in different directions.
Predicting the Future
Research such as this is yet another tool in the toolbox of scientists and researchers who are attempting to forecast tsunami damage and impact.
“Tools based on this research could help officials forecast the potential for tsunami jets to merge,” said Song. “This, in turn, could lead to more accurate coastal tsunami hazard maps to protect communities and critical infrastructure.”
Source: Ohio State University
