Tsunami wave field in the Bay of Bengal one hour after the 2004 Magnitude 9.2 Sumatra earthquake. About 19 hours later, and half a globe away, a significantly smaller wave reached the tip of South America. (Credit: USGS)

By the time the waves of the 2004 Sumatra tsunami swept half way across the globe and reached Drake Passage at the Southern tip of South America, they were just ripples.

But two pressure gauges deep below the surface of the passage reacted to those tiny remnants of the once-towering waves, giving scientists a fortuitous opportunity to study the long-distance behavior of a tsunami. Using the information from the gauges, scientists have helped clear up a tsunami-forecasting mystery, and show that experts are on-track in predicting far off effects of the giant waves. The Drake Passage gauges were in place to study currents around Antarctica, not tsunamis, notes Vasily Titov, a coauthor of the new study and a senior tsunami modeler at the NOAA’s Pacific Marine Environmental Laboratory in Seattle.

So, it was a lucky break that the gauges yielded information about tsunami waves that had traveled different directions around the globe before arriving at Drake Passage. And it was particularly lucky that the gauges were out in the open ocean, Titov explains.  The Drake Passage instruments provided “a direct measurement of the tsunami in the water,” he says, noting that open-ocean gauges don’t pick up interferences from coastline topography, like tidal ones do.

When the waves reached the gauges, 20 hours after a distant earthquake had launched the tsunami, they were a mere 4.9 centimeters (1.9 inches) and 7.4 cm (2.9 in) in height.  In the Indian Ocean, those waves had reached heights of 30 meters (98 feet) and killed more than 230,000 people.  “This was the furthest instrument we could imagine – it was almost on the other side of the globe”– 11,000 kilometers (6,835 miles) from where the tsunami began, Titov says.

The new study by Titov and his colleagues has been accepted for publication in Geophysical Research Letters, an AGU journal.

When a tsunami passes over a gauge it lifts up the column of water, causing a measurable dip in the pressure. The first wave the scientists measured came from the Atlantic, after traveling around the horn of South Africa; the second set traveled past Australia and New Zealand and across the Pacific Ocean, arrived three hours later.

But the “big” waves arrived several hours after travel time models predicted they would – something that commonly happens in tsunami arrival time estimates far from the point of origin. “The discrepancy has always been a mystery within our science,” Titov says.

In this case, however, the researchers were able to figure out why. Using the amplitude forecasting models to figure out where the first wave should have arrived, they took a closer look at the data from the pressure gauges. Because the instruments were out in the ocean, free from coastal interruptions, they were able to pick up tiny signals of that first wave when they knew where to look.

“In the open ocean,” Titov says, “with the help of the models and the data, we could actually see that the first arrival is really much earlier” than when the measured waves passed through.

The second set of waves, however, is larger – and easier to detect, because those waves follow ocean topography and ridges, allowing it to conserve energy and keep a higher amplitude.

The new data helps confirm that the forecasting model works, he said, but that instruments aren’t always sensitive enough to pick up that first set of waves. “It looks like it solves the mystery,” Titov said.

Alexander B. Rabinovich, Philip L. Woodworth, and Vasily V. Titov (2011). Deep-Sea Observations and Modeling of the 2004 Sumatra Tsunami in Drake
Passage Geophysical Research Letters

— Kate Ramsayer, AGU science writer