The geometric structure of rocks where earthquakes occur could play a key role in determining their location and strength, researchers have found.
A study, carried out by geologists at Brown University, suggests that the alignment of rock formations within fault zones may help to explain why seismic activity happens.
The research, which is based on an analysis of fault lines in California, challenges the traditional theory that the type of friction at faults determines whether earthquakes occur.
Victor Tsai, a lead author and geophysicist, said in a statement: "Our paper paints this very different sort of picture about why earthquakes happen.
"And this has very important implications for where to expect earthquakes versus where to not expect earthquakes, as well as for predicting where the most damaging earthquakes will be."
A fault is a fracture or zone of fractures between the plates of rock that make up the Earth's lithosphere—the solid, outer part of the planet.
Geophysicists have long thought that earthquakes happen when pressure builds up at these faults, leading to sudden slips or breaks known as stick-slip behavior. Traditionally, it was believed that unstable friction caused these rapid slips, while stable friction resulted in slower, smoother movements called creep.
However, the new study suggests that complexities in rock structures, such as bends and gaps, are as important in predicting earthquakes as the type of friction.
For the study, the team examined fault zones in California using the U.S. Geological Survey's Quaternary Fault Database and the California Geological Survey. This included the San Andreas fault, the boundary between the Pacific Plate and the North American Plate and one of the largest faults in the world.
They found that geometrically complicated fault zones—where the structures weren't aligned and went in different directions—had stronger ground movement than fault zones that were simpler. Zones where the faults were more aligned allowed for creep to happen, with no earthquakes.
Tsai said: "Our findings suggest that it might be more relevant to look at the geometry of the faults in these fault networks, because it may be the complex geometry of the structures around those boundaries that creates this unstable versus stable behavior."
According to the scientists, it is helpful to imagine some faults as having jagged teeth like the teeth of a saw.
When the rocks are smooth, they can slide past each other. However, when the rocks have a serrated edge, they may stick to one another, causing pressure to build up as these structures push and pull.
Eventually, this accumulation of stress leads to the rocks breaking or moving away quickly, which leads to earthquakes.
The researchers believe this may help to explain why some earthquakes generate more ground movement compared with other earthquakes elsewhere, even if they have similar magnitudes.
They hope that their findings may one day lead to more advanced methods to predict earthquakes.
The full findings of the study were published in the journal Nature.
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