Sizing Up Tsunamis By Their Sound Waves

Scientists at Stanford may have found a way to build a better warning system for tsunamis. The key is listening, for the earthquake’s sonic signature.

When earthquakes rumble below the ocean floor, they can trigger the big killer waves we call tsunamis. But they also make sound waves. And those race ahead at ten times the speed of the ocean waves.

“These sound waves are very sensitive to vertical sea floor uplift, exactly the same thing that’s exciting the tsunamis,” says Eric Dunham, the Stanford geophysicist whose team at Stanford stumbled on that connection when they were modeling the 2011 tsunami that claimed more than 15,000 lives in Japan.

“When making plots of that, we just saw incredibly huge-amplitude sound waves, which is not something we were really expecting, Dunham told me in a phone interview. “It is,” he added, “something that’s obvious in retrospect.”

Those sound waves are the sonic signature that, if captured in real time, could give scientists a critical piece of information that’s been missing: the height of the waves likely to make landfall. Current systems can estimate the magnitude of an undersea quake, but not whether the slip of the fault occurred far beneath the ocean floor, or close to it. And that’s important.

“Shallow slip is what causes sea floor uplift and hence, large tsunamis,” explains Dunham.

Dunham says that knowing the sonic signature in real time could have signalled to people in Japan the enormity of the waves heading for them 10 or 15 minutes sooner.

This kind of sonic monitoring wouldn’t be cheap or simple. It would require a network of underwater microphones called hydrophones, listening for sub-sea quakes, and fast computers to quickly analyze the data.

Dunham thinks a warning system based on the concept would be most helpful in areas near subduction zones, such as Japan and the U.S. Pacific Northwest, less so along the California coast. The 2011 Tohoku quake erupted in the Japan Trench, a subduction zone just 43 miles off the coast of Japan. Hours later, it did nearly $60 million in damage along the California coast, but by then Californians had a pretty good idea of what was coming. The Japanese had only about 30 minutes warning and were caught off guard by the height of the waves, which overwhelmed defenses at the Fukushima nuclear power plant.

Dunham’s work, developed with Jeremy Kozdon, now an assistant professor at the Navel Postgraduate School, is described in the current issue of The Bulletin of the Seismological Society of America.