Illuminating a fresh method for earthquake predictions

Scientists have crafted a laboratory earthquake model that associates the genuine contact area between fault surfaces with the likelihood of earthquakes. Published in the Proceedings of the National Academy of Sciences, this innovation sheds light on the relationship between microscopic friction and earthquakes, providing new perspectives on seismic mechanics and prospects for prediction.

“We’ve fundamentally opened a portal into the core of earthquake mechanics,” stated Sylvain Barbot, associate professor of earth sciences at the USC Dornsife College of Letters, Arts and Sciences and lead investigator of the research. “By observing how the authentic contact area between fault surfaces changes throughout the earthquake cycle, we can now clarify both the gradual accumulation of stress in faults and the swift rupture that ensues. In the future, this could foster new strategies for tracking and forecasting earthquake initiation at early phases.”

For years, researchers have depended on empirical “rate-and-state” friction principles to simulate earthquakes — mathematical formulations that function well but fail to elucidate the essential physical processes. “Our model uncovers what is genuinely occurring at the fault interface during an earthquake cycle.”

Barbot asserts that the discovery is a deceptively straightforward idea: “When two coarse surfaces slide against one another, they only touch at tiny, isolated points encompassing a small fraction of the overall surface area.” This “real area of contact,” imperceptible to the eye yet quantifiable through optical methods, emerges as the crucial state variable governing earthquake behavior.

Laboratory earthquakes: Observing earthquakes in real time

The study employs transparent acrylic materials that enabled the researchers to directly witness seismic ruptures as they happened. Utilizing high-speed cameras and optical assessments, the team monitored how LED light transmission varied as contact points formed, expanded, and dissipated during laboratory earthquakes.

“We can actually observe the contact area change as ruptures progress,” Barbot noted. “During swift ruptures, we witness about 30% of the contact area vanish in milliseconds — a significant weakening that propels the earthquake.”

The laboratory findings exposed a previously unnoticed connection: The empirical “state variable” utilized in conventional earthquake models for decades represents the genuine contact area between fault surfaces. This revelation offers the first physical explanation of a mathematical concept that has been fundamental to earthquake science since the 1970s.

From simulation to forecast

The researchers examined 26 distinct simulated earthquake scenarios and discovered that the link between rupture velocity and fracture energy aligns with the predictions of linear elastic fracture mechanics. The team’s computer simulations accurately replicated both slow and rapid laboratory earthquakes, matching not just the rupture velocities and stress reductions but also the quantity of light transmitted across the fault interface during ruptures.

As contact areas fluctuate throughout the earthquake cycle, they influence multiple quantifiable properties including electrical conductivity, hydraulic permeability, and seismic wave transmission. Since the genuine contact area affects various physical characteristics of fault zones, continuous monitoring of these indicators during earthquake cycles could yield novel insights into fault behavior.

The significance extends far beyond theoretical issues and laboratory studies. The investigation indicates that monitoring the physical status of fault contacts could offer new instruments for short-term earthquake systems and potentially for trustworthy earthquake forecasting via the electrical conductivity of the fault.

“If we can continuously observe these properties on natural faults, we may detect the early phases of earthquake initiation,” Barbot elaborated. “This could lead to innovative methods for observing earthquake initiation at early stages, long before seismic waves are emitted.”

Looking forward

The researchers intend to scale their discoveries beyond controlled laboratory settings. Barbot elucidated: The study’s model lays the physical groundwork for comprehending how fault characteristics develop during seismic cycles.

“Envision a future where we can recognize subtle alterations in fault conditions prior to an earthquake occurring,” Barbot remarked. “That’s the long-term promise of this research.”

About the study: In addition to Barbot, Baoning Wu, formerly at USC and now at the University of California, San Diego, contributed to the study.

The research was supported by National Science Foundation award number EAR-1848192 and the Statewide California Earthquake Center proposal number 22105.