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Mega-Quakes Happen Where Faults Are Usually Flat, Study Claims

First Posted: Nov 29, 2016 03:50 AM EST
Earthquake Aftershocks Rattle North Sulawesi Province
A Meteorology and Geophysics Agency (BMG) officer points to a screen graphic at the BMG office of a 6.5-magnitude earthquake that struck North Sulawesi province.
(Photo : Dimas Ardian/Getty Images)

There have been a number of strong earthquakes that have rocked different parts of the world. But did you know that most major earthquakes with magnitude 8.5 and stronger happen where faults are mostly flat? This is the result of a study done by the University of Oregon and French geologists. The report says that curvier faults are less likely to experience earthquakes exceeding that strength.

This was revealed in a new study at the University of Oregon. This also spared previous beliefs that strong earthquakes originate mostly at the boundary of young tectonic plates. This invalidates a previous theory regarding the magnitude 9.4 quake in Indonesia in 2004 and the magnitude 9.0 quake in Japan in 2011.

The results of the geologists from the University of Oregon have been published in the journal Science and promoted the fact that mega-quakes can happen on any huge faults.

"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," said lead author Quentin Bletery, a postdoctoral researcher at the UO. "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."

According to Science Daily, Bletery had arrived at the university with the idea that geometry could give clues, based on his doctoral work at the Universite Nice, Sophia Antipolis. He developed a mechanical model to study his theory with University of Oregon co-authors Amanda Thomas, Alan Rempel and Leif Karlstrom, all in the Department of Earth Sciences.

For the National Science Foundation-supported research, Bletery examined the geometry of subduction faults around the world to find the slope gradients, not the steepness of dipping itself, but its variations.

"I calculated the gradient of the slope (or curvature) curvature along the main faults and compared it with the distribution of very large earthquakes that happened in the past," he said. "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," he further added, as reported by Tech Times.

Meanwhile, the Cascadia fault, which last experienced a mega-quake in 1700, lies along such a flat region, Rempel and Thomas said. "Earthquakes like the one that happened in Sumatra are mind-bogglingly large," Thomas mentioned. "The rupture was 1,600 kilometers (994 miles) long. When Cascadia goes, it could be 1,000 kilometers (621 miles) if it ruptures completely."

However, based on the average curvature inside the giant earthquake rupture areas, the researchers concluded that the likelihood that mega-earthquakes are linked to fault curvatures is more than 99 percent.

The discovery is not expected to have a direct impact on the ability of scientists to predict when an earthquake will occur, Thomas said.

"Instead, 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," Rempel said. "Not all subduction zones can have really large earthquakes is the implication of this study."

It is also important to note that the study does not say that a 7.5 quake cannot cause significant damage, Thomas said. "The next step in the research is asking why having a flat plate is more amenable to a large earthquake than a curvy plate," she said. The information eventually, she said, could lead to improved hazard maps for earthquake-prone areas around the world.

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