Big earthquakes measuring more than 8.5 in magnitude originate mostly in flat faults such as the West Coast's Cascadia Subduction Zone.

This was revealed in a new study at the University of Oregon. It also scotched the earlier belief that mega-quakes emanate mostly at the boundary of young tectonic plates.

The previous theory stands invalid in the light of the magnitude 9.4 quake at Indonesia in 2004 and the magnitude 9.0 quake in Japan in 2011.

The findings of UO geologists have been published in the Nov. 25 issue of the journal Science and upholds the fact that mega-earthquakes can happen on any large fault.

Fault Curvature As Key Parameter

The paper argues that the key to predicting big quakes is the topography of the fault curvature.

"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 at the Department of Earth Sciences.

Noting that more than the reasons of resisting failure, fault geometry must be targeted, the lead researcher studied the slopes of major subduction faults.

Armed with a mechanical model, Bletery aligned with other UO researchers Alan Rempel, Amanda Thomas and Leif Karlstrom for a detailed study of the subduction faults with a greater focus on slope gradients.

Subduction zones are regions of the Earth's crust where tectonic plates meet. While comparing the data on variations of slope gradients with the historical distribution of large earthquakes, the researchers found a clear correlation.

Menace Of Flat Faults

Bletery made calculations of the gradient of the slope curvature of main faults.

"What I found was very large earthquakes occur on fault areas where the slope is the most regular, or flat," Bletery said.

Top examples of huge earthquakes in flat faults included the one in the Cascadia fault in 1700.

Also, the rupture thresholds varied along curved faults, showing that the length of a rupture depends on the presence of curvy sections that resist failure. The rupture threshold is more uniform along flat faults, an indication that these faults are more likely to result in a simultaneous rupture and generate a big earthquake.

The clear correlation between the curvatures and mega-quakes was asserted by researcher Thomas, who said the link between mega-earthquakes and fault curvatures was almost 99 percent.

Rempel said the findings were significant as they reinforced the idea that a place on a curvy plate may not experience a mega-quake at all.

In the next phase of the research, the question why a flat plate is more vulnerable than a curvy plate to large earthquakes will be taken up.

Hope is that the new information on flat faults would help in improving hazard mapping in earthquake-prone areas across the world.