Laboratory implanted earthquakes reveal the frictional forces operating beneath our feet

A new study provides a comprehensive look at the motions and laws of friction that drive devastating earthquakes caused by thrust fault

Simulating a mini-scale earthquake in a laboratory informally known as a “seismic wind tunnel,” engineers and seismologists have produced an overview so far into the complex physics of friction that leads to earthquakes with a devastating thrust fault.

Thrust fault earthquakes occur when one side of the fault slides above or below the other side. Thrust faults have been the site of some of the largest earthquakes in the world, including the 2011 Tohoku earthquake off the coast of Japan, which caused tsunamis that destroyed the Fukushima nuclear power plant.

However, the movement or forces that cause these earthquakes cannot be measured directly from the source, as much of the action takes place deep in the Earth. To get more information about it, a team of researchers created and monitored thrust fault earthquakes at the unique “laboratory earthquake” facility at the California Institute of Technology.

High-speed cameras and the dynamic correlation of digital images reveal movements of change and forces along an earthquake simulated by a crack. The arrows indicate the direction and, according to their length, the force of the ground velocity and the fault slip as the earthquake rupture travels up the oblique fault from the epicenter; While the colors indicate the velocity of the particles across the simulated Earth. Notice how the rupture reflects off the simulated ground surface again below the fault line.

Credit: Courtesy of Ares Rosakis

“Simulating earthquakes in the laboratory allows us to observe how these short and violent events grow and evolve by” slowing “their movement through high-speed photography and optics, says Aris Rusakis, professor of aeronautics and mechanical engineering at Theodor von Carman, who runs the facility and has entered Laboratory seismic concept with former Caltech Seismology Laboratory Director Hiro Kanamori, and John E and Hazel Smits Professor of Geophysics, Emeritus.

Rosakis is the corresponding author of a new research paper that Proceedings of the National Academy of Sciences published on August 25. This paper was co-authored with Nadia Labosta, Lawrence A. Hanson, Jr., Professor of Mechanics, Engineering and Geophysics, longtime assistant in problem solving in the interaction between engineering and seismology; California Institute of Technology postdoctoral scientist Yuval Tal, currently assistant professor at Ben Gurion University of the Negev in Israel; And California Institute of Technology research scientist Vito Rubino.

To create an earthquake in the lab, the team first cut a half-transparent block from a type of plastic known as Homalite, which has friction properties similar to rocks. Then they put the two pieces together under pressure and shear, to simulate tectonic pressure building slowly along the fault line. Then, they placed a small wire fuse at the location below the simulated Earth’s surface where they wanted the earthquake to originate. The operation of the fuse reduced friction at that location and allowed for a very rapid rupture to initiate and propagate the miniature fault towards the free surface of the earth, producing intense vibration. Meanwhile, high-speed imaging technology recorded the development of stresses, and thus the coefficient of friction, along the fault as the rupture approached the free surface – a millionth of a second at a time.

The “seismic wind tunnel” has been in existence since 1999, Rosakis says, but the addition of the Digital Image Correlation (DIC) in 2015 gave engineers “two new eyes”. DIC measures the subtle shifts in the location of individual points across the material over time, indicating how dynamically stress and pressure evolve throughout the material during earthquake simulations. Using this information, Rosakis and colleagues were able to map how the rift moved upwards, interacting dynamically with the Earth’s surface, and even influencing itself through the dynamic propagation waves generated by each movement.

They notice a very rapid change in “normal” fault stress, which is the compressive force that keeps the fault closed. There are a number of reasons why the normal pressure of a fault may vary when the fault is slipped. In the case of earthquakes with impulsive faults, the researchers observed that the false natural stress undergoes a rapid cycle of increasing and decreasing amplitude because the waves emitted from the rupture are then reflected off the simulated Earth’s surface like an echo.

Since this pressure, which usually kept the fault locked in place, was rapidly changing in force, it altered the fault’s resistance to slip, known as shear motion. When the normal pressure of the fault subsides, the fault is held less tightly in place and is more prone to slip, causing an earthquake.

Importantly, the researchers were able to challenge the generally accepted (but also contested) assumption that the friction holding the plates in place along the fault is always proportional to the normal stress of the fault. What they find instead is that when the rupture interacts with the earth’s surface, there is a significant time lag between the changes in the normal stress of the fault and the resulting shear resistance, both of which are not proportional to the time scale of the rupture process.

“This indicates the existence of a complex, date-dependent mechanism controlling friction in the presence of a natural rapid fault pressure that characterizes thrust fault configurations,” says Rosakis.

“While previous studies indicated a contradiction between changes in natural stress and friction, it was not clear how important this effect was for driving earthquakes,” Labosta adds. “Our measurements showed that the effect is much greater than expected based on previous studies and allowed us to improve the current laws of friction.”

The team hopes that these physical insights into earthquake dynamics will help Earth scientists create more accurate computer models of earthquake faults that propagate along thrust faults in the real world.

“Obtaining the friction resistance, and thus the simulated movement directly next to the ground surface, is particularly important, because it greatly affects the ground vibration as well as the tsunami generation if the fault effect occurs underwater,” Labosta says. “In fact, many devastating earthquakes in the form of rupture of thrust occur in subduction zones, sometimes resulting in devastating tsunamis such as the 2011-strong Thoku 9.0 earthquake.”

“The law of friction based on the history of the error, which is difficult to define, is the greatest assumption of any symptom,” says Rosakis. “Now we have another piece of the puzzle installed.”

The title of the paper “Shedding light on the physics of dynamic friction through laboratory earthquakes on thrust faults”. This research was supported by the National Science Foundation, the US Geological Survey, the Big Ideas Fund of the Caltech Division of Mechanical and Civil Engineering (MCE), the Department of Geological and Planetary Sciences at the California Institute of Technology, and the Seismic Center in Southern California.

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