Mega-Earthquakes Occur In Regions Where Faults Are Flat

The geometry of faults at subduction zones — regions of Earth's crust where two tectonic plates meet — plays a key role in determining the intensity of earthquakes, a new study has revealed.

According to the study published Friday in the journal Science, regions with flat faults are more likely than those with curvier faults to experience earthquakes of magnitude 8.5 or higher.

"The way people in the science community think about earthquakes is that some fault areas resist failure more than others, and when they break they generate large earthquakes," lead author Quentin Bletery, a postdoctoral researcher at the University of Oregon, said in a statement. "The reason they resist failure longer is often debated. I thought variations in fault geometry could be responsible, so I looked for changes in the slope of the major subduction faults of the world."

Bletery and his colleagues examined the geometry of subduction faults around the world and variations in their slope gradients. When they compared this data with the historical distribution of very large earthquakes, they found a clear and strong correlation — the likelihood that mega-earthquakes are linked to fault curvatures is more than 99 percent.

"What I found is the opposite of what I expected: Very large earthquakes occur on fault areas where the slope is the most regular, or flat," Bletery said.

Earthquakes take place once the energy accumulated due to a "slip deficit" — created as a result of the movement of tectonic plates being blocked along subduction zones for hundreds, or even thousands, of years — exceeds a certain threshold. The study shows that the curvier a fault is, the higher the variation in this threshold along the subduction zone is. And, although such a heterogeneous threshold produces more frequent earthquakes than a homogenous one, they affect a smaller area and are of lower magnitude.

"Our findings backstop the idea that if you are at a location that hasn't had evidence for large earthquakes in the past and your location is on a curvy plate, then maybe mega-quake will never happen," Alan Rempel, also from the University of Oregon, said in the statement. "Not all subduction zones can have really large earthquakes is the implication of this study."

According to this analysis, flat faults capable of producing mega-quakes are present in South America, Indonesia and Japan. On the other hand, faults bordering the Philippines, Solomon Islands and Vanuatu are not favorable to mega-earthquakes.