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Imagine dragging your outstretched fingers through wet beach sand, leaving long grooves behind. Scientists recently discovered similar but enormous grooves buried under the seafloor near Costa Rica. The detailed three-dimensional data they used to uncover these corrugations can help them better understand large subduction zone earthquakes and related tsunamis worldwide.

In an article published online by Nature Geoscience (see “Corrugated mega-thrust revealed offshore from Costa Rica”), researchers reported finding corrugations, or giant grooves, kilometers long, hundreds of meters wide, and tens of meters high, between the Cocos and Caribbean tectonic plates that form part of the Costa Rica subduction zone. Using 3D seismic imaging techniques developed by the oil industry, combined with state-of-the-art computer visualization software, scientists from the USGS, University of California Santa Cruz, University of Texas Austin, and McGill University produced an unprecedented, detailed view of the megathrust fault formed by these colliding and sliding plates.

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“Subduction zones are hugely important, both because of the hazards they pose and because it’s where the earth destroys crust,” said lead author Joel Edwards, a USGS student contractor pursuing a Ph.D. at UC Santa Cruz. “And megathrust faults, within subduction zones, host the largest earthquakes on the planet. They are more likely to generate tsunamis, which as we saw from Japan and Sumatra, can be a bigger hazard than earthquake shaking.” In the United States, subduction zones with megathrust faults exist offshore of Alaska, Washington, Oregon, Northern California, Puerto Rico, the U.S. Virgin Islands, Guam, and the Northern Mariana Islands.

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“Gently rolling hills and valleys that are parallel and stretch for miles.” That’s how Edwards described the scene if you could stand on the fault surface looking across the grooves. “Corrugations are a great record of how blocks are sliding past each other along a fault,” he added. Scientists have found similar grooves at the base of fast moving glaciers and other faults, but not on megathrust faults.

This unique, highly-detailed image revealed other buried secrets. Small changes in groove direction record a history of shifts in plate movement. The rougher areas indicate where the megathrust jumped to form a new fault, and hasn’t had enough time to smooth the surface. Other details in the 3D seismic imaging data imply that the grooves may channel fluids that could lubricate the fault and affect earthquake size and frequency.

Petroleum companies developed 3D seismic imaging several decades ago to help find oil and gas in areas with complex geology. For offshore 3D exploration, ships tow several streamers, kilometers long, containing hundreds of waterproof microphones floating just beneath the waves. These hydrophones record sound bounced off the rock layers below the seafloor. Extensive computer processing and advanced visualization techniques help researchers uncover hidden geologic details in three dimensions.

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“It’s like the Hubble Telescope for geologists,” said co-author Jared Kluesner, a USGS geophysicist. “But collecting and processing this type of data can be really expensive and time consuming.” For this study, data collected in 2011 took several years to analyze and visualize. Kluesner and study co-author professor emeritus Eli Silver of UC Santa Cruz are the Ph.D. advisors of Edwards.

“It’s the first time we’ve been able to image the megathrust surface itself in such detail,” said Janet Watt, a USGS geophysicist, who was not involved with the study. “I think there could be a lot more papers to come from this.”

Now that the technique has proven to be so valuable, USGS scientists and academic partners are eager to use 3D seismic imaging closer to home: the Cascadia subduction zone just offshore of Washington, Oregon, and Northern California. In February 2018, researchers met to plan a cooperative Cascadia research program, which could lead to a new 3D seismic survey and improved knowledge of the hazards.

“Cascadia has a history of large earthquakes and tsunamis,” said Kluesner, “yet we still don’t know that much about the megathrust there. It’s an obvious choice.”

The article, titled “Corrugated megathrust revealed offshore from Costa Rica,” is available from Nature Geoscience. The National Science Foundation funded data collection.