The study, published in the journal Nature, indicates that the alignment of fault networks plays a crucial role in determining earthquake locations and strengths. This challenges the longstanding belief that friction type at fault lines is the primary factor governing earthquake occurrence, potentially enhancing the understanding of seismic activities.
"Our paper paints this very different sort of picture about why earthquakes happen, said Brown geophysicist Victor Tsai, one of the paper's lead authors. "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.
Fault lines are boundaries where Earth's lithosphere plates interact. Traditionally, geophysicists have attributed earthquakes to stress accumulation at these faults, leading to sudden slips or breaks known as stick-slip behavior. It was believed that unstable friction caused these rapid slips, while stable friction resulted in slow, smooth movements called creep.
"People have been trying to measure these frictional properties, like whether the fault zone has unstable friction or stable friction and then, based on laboratory measurements of that, they try to predict if are you going to have an earthquake there or not, 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.
The study highlights that complexities in rock structures, such as bends, gaps, and stepovers, are critical. Based on mathematical modeling and data from the U.S. Geological Survey's Quaternary Fault Database and the California Geological Survey, the research examined fault zones in California.
The research team, including Brown graduate student Jaeseok Lee and Brown geophysicist Greg Hirth, illustrated how earthquakes occur using a detailed example. They compared faults with serrated teeth edges, explaining that fewer or smoother teeth allow for creep, while complex, jagged structures lead to stuck faults, building pressure and causing earthquakes.
This study builds on earlier work regarding ground motion variations in earthquakes of similar magnitude in different regions. It suggested that geometrical complexity in fault zones contributes to high-frequency vibrations and earthquake occurrence.
Analyzing California faults, including the San Andreas fault, researchers found that misaligned fault zones had stronger ground motions and earthquakes, while aligned zones experienced smooth creep with no earthquakes. The misalignment ratio, measuring fault alignment, indicated that misaligned zones caused stick-slip earthquakes, while aligned zones did not.
"Understanding how faults behave as a system is essential to grasp why and how earthquakes happen, said Lee. "Our research indicates that the complexity of fault network geometry is the key factor and establishes meaningful connections between sets of independent observations and integrates them into a novel framework.
More work is needed to validate the model, but initial findings are promising. If confirmed, this approach could be integrated into earthquake prediction models.
"The most obvious thing that comes next is trying to go beyond California and see how this model holds up, Tsai said. "This is potentially a new way of understanding how earthquakes happen.
Research Report:Fault-network geometry influences earthquake frictional behaviour
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