A new laboratory model has been developed that connects the actual contact areas between fault surfaces to the dynamics of earthquakes, significantly 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 plays a critical role in seismic activity, thereby deepening our comprehension of earthquake behavior and the possibilities for early prediction.
“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 throughout the earthquake cycle, we can now elucidate both the gradual accumulation of stress in faults and the swift ruptures that follow. In the future, this could lead to innovative methods for monitoring and predicting the early stages of earthquake nucleation.”
For years, scientists have utilized empirical “rate-and-state” friction laws to simulate earthquakes. These mathematical models effectively describe the motion of seismic events but fail to explain the underlying causes. Barbot noted, “Our model illustrates what is actually occurring at the fault interface during an earthquake cycle.” He explained that when two rough surfaces move against each other, they only make contact at tiny, isolated junctions that occupy a minuscule fraction of the total surface area. This “real area of contact,” although not visible to the naked eye, is measurable using optical techniques and serves as a crucial variable influencing earthquake behavior.
The study employed transparent acrylic materials, enabling researchers to observe earthquake ruptures as they occurred in real time. By using high-speed cameras and optical measurements, the team monitored changes in LED light transmission as these contact junctions formed, expanded, and were destroyed during laboratory earthquakes. “We can literally watch the contact area evolve as ruptures propagate,” Barbot remarked. “During rapid ruptures, we observe approximately 30% of the contact area vanish in mere milliseconds—this significant weakening is what drives the earthquake.”
The laboratory findings uncovered a previously unrecognized correlation: the empirical “state variable” utilized in conventional earthquake models for decades corresponds to the actual contact area between fault surfaces. This revelation provides the first physical interpretation of a theoretical concept that has been a cornerstone of earthquake science since the 1970s.
The researchers conducted analyses on 26 different simulated earthquake scenarios and discovered that the relationship between rupture speed and fracture energy aligns with the principles of linear elastic fracture mechanics. Their computer simulations successfully replicated both slow and fast laboratory earthquakes, matching not only the rupture speeds and stress drops but also the light transmission across the fault interface during ruptures. As contact areas fluctuate throughout the earthquake cycle, they impact various measurable properties, including electrical conductivity, hydraulic permeability, and seismic wave transmission.
Given that the real area of contact influences multiple physical characteristics of fault zones, continuous monitoring of these proxies during earthquake cycles could yield novel insights into fault behavior. The implications of this research extend beyond theoretical understanding and laboratory settings. It suggests that monitoring the physical state of fault contacts could pave the way for new tools in short-term earthquake systems and possibly enhance reliable earthquake prediction through the electric conductivity of faults.
“If we can continuously monitor these properties on natural faults, we might be able to detect the early stages of earthquake nucleation,” Barbot explained. “This could lead to innovative methods for monitoring the initial phases of earthquakes, well before seismic waves are emitted.”
Looking ahead, the researchers aim to apply their findings beyond controlled laboratory conditions. Barbot elaborated that the study”s model lays the physical groundwork for understanding how fault properties transform during seismic cycles. “Envision a future where we can identify subtle changes in fault conditions prior to an earthquake occurring,” Barbot said. “That is the long-term potential of this research.”
This work was supported by the National Science Foundation and the Statewide California Earthquake Center.
