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How it’s done

Predicting tsunamis with sound waves

Author Jacqueline Detwiler Illustration James Provost

howitsdone

After a large earthquake, the last thing a devastated area needs is to be pummeled by a tsunami, and yet, physics being what it is, that’s often exactly what happens. The 2011 Tōhoku earthquake that ravaged the east coast of Japan, for example, spawned a tsunami that reached 133 feet high in places and added further stress to a country struggling to cope with the first disaster. While there’s little humans can do to fully predict quakes, Stanford geophysicist Eric Dunham and his team believe they have found a way to give victims a few more minutes to prepare for an impending tsunami. His plan involves using sound waves generated by some undersea earthquakes to predict wave heights and destinations tens of minutes earlier than was previously possible. Here’s how they’ll do it.

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1. First, Dunham studied very low-frequency ocean sound waves called PL waves. After an undersea earthquake causes the sea floor        to rapidly lift, these propagate toward the shore, with a tsunami following behind. The PL waves reach land 30 times faster than the tsunami itself, providing an opportunity for advance warning.

2. With a lot to gain from early prediction of tsunamis, Japan has already begun installing seismometers and pressure meters on the seafloor between the Japan Trench and the islands. Currently, the plan is to detect the tsunami itself as it passes over, but Dunham is hoping to use the same equipment to detect the PL waves first.

3. To predict wave height and direction using the PL waves, Dunham will run computer simulations of various sorts of earthquake in the Japan Trench, comparing some of them to actual data from the Tōhoku earthquake. Next, he’ll tackle other undersea quake areas, like the Cascadia subduction zone, which runs from Vancouver Island to Northern California.

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