Underwater landslides tallied near Puerto Rico

An oceanographic survey off the northern coast of Puerto Rico has found remnants of many past underwater landslides, a handful of which were large enough to have caused deadly tsunamis.

Although most tsunamis are caused by earthquakes, a small percentage of the destructive waves are triggered by seafloor slumping (SN: 3/6/04, p. 152: Killer Waves). Sonar revealed the landslide remnants off Puerto Rico, says Uri S. ten Brink, a geophysicist with the U.S. Geological Survey in Woods Hole, Mass. He and his colleagues report their findings in the June 16 Geophysical Research Letters.

In the 12,000-square-kilometer area that the team surveyed, there was evidence of 160 underwater landslides that had each moved at least 70 million cubic meters of material—enough to fill the New Orleans Superdome 20 times—at some time in the past.

The largest landslide, which displaced 22 cubic kilometers of material, occurred 35 km north of the city of Arecibo. Computer models suggest that that landslide, like eight others that the team discovered, would have triggered a tsunami measuring 2.5 meters or higher. That height matches the size of a killer tsunami that struck Puerto Rico after an earthquake in 1918, says ten Brink.

Researchers don’t yet know when any of the landslides occurred. Samples of the sediments that have accumulated on the slumped material since their occurrence may enable scientists to estimate how frequently killer landslides can strike in the region, says ten Brink.

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