USC researcher shatters long-held beliefs about fault line behavior

LOS ANGELES — A new study by a University of Southern California researcher has upended decades-old assumptions about earthquake fault friction, potentially reshaping our understanding of seismic activity.

Associate Professor Sylvain Barbot of USC Dornsife College of Letters, Arts and Sciences published his findings in the Proceedings of the National Academy of Sciences, revealing that the relationship between fault friction and temperature is more complex than previously thought.

Barbot's research contradicts the long-standing view that fault friction changes continuously with temperature.

"The classical view of fault friction has always been that fault friction evolves continuously with temperature," Barbot said. "But my study shows a direct effect of friction is largely unrelated to temperature up to the point of a rock's brittle to semi-brittle transition, where temperature effects change drastically."

The study analyzed data from experiments on various rock types, including granite, basalt, and olivine, under conditions simulating deep Earth environments. Barbot discovered that a significant change occurs when rocks transition from a brittle to a semi-brittle state, a finding crucial for understanding rock mechanics and geology.

In the brittle state, rocks deform through fracturing and faulting. The semi-brittle state, however, combines both brittle and ductile deformation, where rocks permanently change shape by "flowing" or "bending" under stress. This transition point marks a departure from traditional models of rock friction behavior.

"Contrary to traditional models of rock friction behavior, I found that the direct effect of friction doesn't always correlate with temperature," Barbot explained. He noted abrupt changes in rock friction during the transition to semi-brittle behavior, challenging the fundamental basis of fault mechanics used in earthquake forecasting.

The study's findings have significant implications for seismology. Most friction theories assume that the relative movement speed of surfaces along a fault depends continuously on temperature. Barbot's research questions this thermal activation dependency, suggesting that predictive models for fault friction may need revision to improve earthquake prediction accuracy.

Barbot emphasized the potential impact of these findings on earthquake hazard assessments. "When incorporated into physics-based models of the seismic cycle, every refinement we discover has the potential to improve the accuracy and reliability of earthquake hazard assessments and long-term forecasts," he said.