Muffled Quakes

A study published March 2, 2020, in Nature Geoscience has found that when mountains on the seafloor are pulled into subduction zones — places where one tectonic plate dives beneath another — they help set the stage for powerful earthquakes but also create conditions that end up dampening them.

“The earth ahead of the subducting seamount becomes brittle, favoring powerful earthquakes, while the material behind it remains soft and weak, allowing stress to be released more gently,” said co-author Demian Saffer, director of the University of Texas Institute for Geophysics. The findings mean that scientists should more carefully monitor particular areas around subducting seamounts, researchers said. The practice could help scientists better understand and predict where future earthquakes are most likely to occur.

The study was led by Tian Sun, a research scientist at the Geological Survey of Canada. Other co-authors include Susan Ellis, a scientist at the New Zealand research institute GNS Science. Saffer supervised the project and was Sun’s postdoctoral adviser at Penn State when they began the study. The researchers made the findings using a computer model of a subducting seamount. The model measured the effects on the surrounding rock and sediment, including the complex interactions between stresses in the earth and fluid pressure in the surrounding material. Getting realistic data for the model involved conducting experiments on rock samples collected from subduction zones by a scientific drilling mission off the coast of Japan. The scientists said the model’s results took them completely by surprise. They had expected water pressure and stress to break up material at the head of the seamount and thus weaken the rocks, not strengthen them.

While the weakened rock left in the wake of seamounts may dampen large earthquakes, the researchers believe that it could be an important factor in a type of earthquake known as a slow slip event. These slow-motion quakes are unique because they can take days, weeks and even months to unfold.