Earthquake simulations show how major earthquakes begin at grains in a fault boundary

Laboratory earthquakes illustrate how grains at fault boundaries cause large earthquakes.

Caltech engineers provided significant support for the type of seismic propagation currently presumed to be responsible for the 9.0-magnitude earthquake that devastated a Japanese beach in 2011.

(Photo: LOUISA GOULIAMAKI/AFP must read via Getty Images)

seismic simulation

Caltech engineers provided significant support for the type of seismic propagation currently presumed to be responsible for the 9.0-magnitude earthquake that devastated a Japanese beach in 2011.

When tectonic plates grind against each other, fine pebbles form along fault lines, which are the boundaries of tectonic plates. The impact of these pebbles on earthquakes has long been a source of debate among scientists.

Caltech researchers explain in a new report published in the journal Nature June 1 that small pebbles, known as rock pits, initially stop the spread of earthquakes but later trigger seismic rebirth, leading to severe ruptures.

“We have been able to look at the earthquake process up close and find the key elements of rupture propagation and friction growth in rock pits thanks to our new method,” explains Vito Rubino, research scientist and lead author of the Nature Study.

“As a result of the activation of the joint seismic friction weakening processes, fault sections previously assumed to act as dynamic rupture barriers may host earthquakes,” says one of the study’s preliminary findings.

Also Read: Magnitude 4.2 earthquake shaking area near Yellowstone National Park

intensive study

(Image: VALERY HACHE/AFP via Getty Images)

Rubino and Associates Nadia LaPosta, Lawrence A. Hanson, Jr., professor of mechanical engineering and geophysics, and Aris Rosakis, professor of aeronautics and mechanical engineering, Aris Rosakis, explain in the paper that so-called “stable” or “creeping” faults are not as immune to major ruptures as previously thought.

These faults form when tectonic plates slide gently over each other without causing major earthquakes (for example, the currently creeping section of the San Andreas Fault in central California).

On the other hand, the rock gouge has a complex behaviour. It acts as the first line of defense against rupture, collecting energy and preventing it from spreading. The rock-drill interface degrades and the friction between the two plates is greatly reduced when the plates slide over each other at a high enough speed, allowing the earthquake to reappear. Regeneration is the term for this process.

“We know that rock pits may strengthen as the fault slips and act as a buffer or weaken and encourage earthquake rupture,” adds LaPosta, citing a large body of previous rock friction tests.

“However, these behaviors are typically thought to be space-discrete, with weakening and strengthening at different fault sites. Our experiments demonstrate how these behaviors can combine at the same fault sites during the same slip event, over dynamic rupture timescales, resulting in discontinuous slip and the potential to turn the fault barrier into an earthquake-prone zone.”

The impact of rock pits, micrometre-sized granular debris, on seismic activity is investigated in the Nature article. The researchers used Caltech’s so-called seismic wind tunnel, built by Rosakis and former Caltech Seismological Laboratory director Hiro Kanamori, to study the effect of rock drilling on earthquake propagation. Emeritus Professor of Geophysics Hazel S. Smits Engineers and scientists can search for large, small-scale earthquakes at the facility, which has been in operation since 1999.

The team first cut a meter-sized translucent piece of Homalite in two to create an earthquake simulation. Dynamic rupture nucleation can occur in samples as small as tens of centimeters in diameter, while examination of the same phenomena in rocks requires samples of tens of meters in size.

Next, the researchers applied massive pressure and shear to the two halves of the Homalite, replicating the tectonic pressure that builds up along the fault line. A fine-grained quartz powder was inserted between the pieces as a back-up carrier for the error.

The scientists then placed a short wick of wire between the two pieces, the “epicenter” they were simulating. High-speed imaging technology was used to document the evolution of the simulated earthquake in a millionth of a second at a time as it developed.

“We could never have imagined that they had discovered such a rich spectrum of physical phenomena related to frictional seismic source processes and that these phenomena could be precisely scaled to explain the natural seismic behavior that occurs on vastly different scales of length around the world when we were designing wind tunnels for seismic monitoring.” It goes back to the late ’90s,” says Rosakis. “This is an appreciation for the discipline’s tremendous strength of mechanics.”

Needs more research

(Image: STR/JIJI PRESS/AFP via Getty Images)

The researchers will then investigate the effect of fluids naturally present in the Earth’s crust on the frictional behavior of drilling rocks.

Related article: Experts study ‘the largest earthquake in human history’

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