Brown University

06/05/2024 | Press release | Distributed by Public on 06/05/2024 10:22

Breaking ground: Could geometry offer a new explanation for why earthquakes happen

PROVIDENCE, R.I. [Brown University] - By taking a close look at the geometrical makeup of rocks where earthquakes originate, researchers at Brown University are adding a new wrinkle to a long-held belief about what causes seismic quakes in the first place.

The work, described in the journal Nature, reveals that the way fault networks are aligned plays a critical role in determining where an earthquake will happen and its strength. The findings challenge the more traditional notion that it is primarily the type of friction happening at these faults that governs whether earthquakes happen or not, and they could improve current understandings of how earthquakes work.

"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 the visible boundaries on the planet's surface where the rigid plates that make up the Earth's lithosphere brush against each another. Tsai says that for decades, geophysicists have explained earthquakes as happening when stress at faults builds up to the point where the faults rapidly slip or break past each other, releasing pent-up pressure in an action known as stick-slip behavior.

Researchers theorized that the rapid slip and intense ground motions that follow are a result of unstable friction that can happen at the faults. In contrast, the thought is that when friction is stable, the plates then slide against each other slowly without an earthquake. This steady and smooth movement is also known as 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 geometry to consider includes complexities in the underlying rock structures such as bends, gaps and stepovers. The study is based on mathematical modeling and studying fault zones in California using data from the U.S. Geological Survey's Quaternary Fault Database and from the California Geological Survey.

The research team, which also includes Brown graduate student Jaeseok Lee and Brown geophysicist Greg Hirth, offer a more detailed example to illustrate how earthquakes happen. They say to picture the faults that brush up against each other as having serrated teeth like the edge of a saw. When there are fewer teeth or teeth that are not as sharp, the rocks slide past each other more smoothly, allowing for creep. But when the rock structures in these faults are more complex and jagged, these structures catch on to one another and get stuck. When that happens, they build up pressure and eventually as they pull and push harder and harder, they break, jerking away from each other and leading to earthquakes.