It was recognized shortly afterwards that the wave was very unusual. It was a “double” tsunami; two large, distinct waves were recorded by satellites and buoys throughout the Pacific, separated in time by several minutes. The presence of two waves greatly increased both the height of the waters onshore and the damage.
Consequently, scientists have spent the last few years trying to understand why there were 2 waves, in order to better prepare for future earthquakes. New research from the British Geological Survey presented at last week’s meeting of the American Geophysical Union seems to have the answer.
The earthquake set off a landslide below the waters. Not just any landslide…an enormous one…almost beyond imagination in its scale.
Using sonar images, the scientists identified a piece of the ocean floor 40 kilometers wide, 20 kilometers long and 2 kilometers thick which slid downhill during the earthquake. That estimate means that 500 cubic kilometers of rock moved in this avalanche.
It’s actually hard to find anything to compare this volume to. The largest recorded slide on land was at the eruption of Mt. St. Helens, and that was about 3 cubic kilometers. An earthquake off of Newfoundland set off a slide with a volume of 200 cubic kilometers in 1929; that slide set of a tsunami that killed several dozen people.
500 cubic kilometers is roughly the amount of material erupted from the Long Valley caldera eruption 760,000 years ago. This slide was comparable in volume to a supervolcano that knocked a hole in the Sierra Nevada mountain range.
The scientists also used seismic data recorded during the earthquake to confirm the presence of this slide. Avalanches give a peculiar seismic signature and, within the data available on this earthquake, they located that fingerprint.
It’s impossible to say how much worse the devastation in Japan was because of this slide; there is no way to separate its damage from the other wave. However, over the last century, there are multiple examples of tsunami waves generated from submarine landslides, with the one in Japan likely being largest.
These mega landslides aren’t common, but they occur often enough that they are a major threat to civilization. They can occur in places thought safe from tsunami, making them a particular threat. These submarine landslides are in need of better scientific characterization soon, before another one contributes to or causes a mega-disaster in a populated area.
AGU Abstract: Tappin et al., 2013.