A groundbreaking laboratory model has been developed that connects the actual contact areas between fault surfaces to earthquake dynamics, enhancing the potential for improved prediction and early warning systems. This research, published in the Proceedings of the National Academy of Sciences, reveals how microscopic friction impacts seismic activity, providing a deeper insight into earthquake behavior and the possibility for early forecasting.
“We”ve essentially opened a window into the heart of earthquake mechanics,” stated Sylvain Barbot, an associate professor of earth sciences at the USC Dornsife College of Letters, Arts and Sciences and the principal investigator of the study. “By observing how the real contact area between fault surfaces evolves during the earthquake cycle, we can now elucidate both the gradual accumulation of stress in faults and the swift rupture that ensues. In the future, this could lead to innovative methods for monitoring and predicting the early stages of earthquake nucleation.”
Historically, scientists have relied on empirical “rate-and-state” friction laws to model seismic events. While these mathematical frameworks effectively describe seismic motion, they do not address the fundamental cause. Barbot emphasized, “Our model uncovers what is genuinely occurring at the fault interface throughout an earthquake cycle.” He explained that the concept is deceptively straightforward: “When two rough surfaces slide against one another, they only engage at tiny, isolated junctions that cover a minute fraction of the total surface area.” This “real area of contact,” which is imperceptible to the naked eye yet measurable with optical techniques, is a crucial state variable influencing earthquake behavior.
The study employed transparent acrylic materials, enabling researchers to observe earthquake ruptures in real time. High-speed cameras and optical measurements allowed the team to monitor changes in LED light transmission as contact junctions formed, expanded, and ultimately failed during laboratory earthquakes. “We can literally watch the contact area evolve as ruptures propagate,” Barbot noted. “During rapid ruptures, we observe nearly 30% of the contact area vanish within milliseconds—a significant weakening that triggers the earthquake.”
The laboratory findings unveiled a previously obscured correlation: the empirical “state variable” utilized in conventional earthquake models has a physical interpretation related to the real area of contact between fault surfaces. This marks the first time a mathematical concept central to earthquake science since the 1970s has gained a physical basis.
By analyzing 26 distinct simulated earthquake scenarios, the researchers discovered that the link between rupture speed and fracture energy aligns with the principles of linear elastic fracture mechanics. Their computer simulations successfully replicated both slow and rapid laboratory earthquakes, accurately reflecting rupture speeds, stress reductions, and the volume of light transmitted across the fault interface during ruptures. As contact areas fluctuate during the earthquake cycle, they influence various measurable characteristics, including electrical conductivity, hydraulic permeability, and seismic wave transmission.
The implications of this research extend beyond academic theory and controlled laboratory settings. Monitoring the physical state of fault contacts could yield new methodologies for short-term earthquake systems and possibly enhance reliable earthquake prediction through the fault”s electrical conductivity. “If we can continuously monitor these properties on natural faults, we might identify the early signs of earthquake nucleation,” Barbot explained. “This could pave the way for novel strategies to observe earthquake onset long before seismic waves are emitted.”
Looking to the future, the researchers aim to apply their findings beyond laboratory settings. Barbot elaborated, “The model provides a physical framework for understanding how fault properties change during seismic cycles. Imagine a world where we can detect subtle shifts in fault conditions before an earthquake occurs. That is the long-term promise of this research.”
This study was funded by the National Science Foundation and the Statewide California Earthquake Center.
