Lab earthquakes show how squalls at fault boundaries lead to major earthquakes: faults once considered “creeping” but stable may be at risk of large ruptures

By simulating earthquakes in a laboratory, Caltech engineers provided strong experimental support for a form of earthquake propagation now thought to be responsible for the 9.0 magnitude earthquake that devastated the coast of Japan in 2011.

Along certain fault lines, which are the boundaries of tectonic plates, fine-grained gravel forms when the plates rub against each other. The influence of this gravel on earthquakes has long been the subject of scientific speculation. In a new article published in the journal Nature on June 1, Caltech researchers show that fine gravel, known as rock gouge, first stops the propagation of earthquakes, but then triggers the revival of earthquakes to generate powerful ruptures.

“Our new experimental approach allowed us to look closely at the seismic process and uncover key features of fracture propagation and friction evolution in rock gouges,” says researcher and lead author Vito Rubino. of the study Nature paper. “One of the key findings of our study is that fault sections previously thought to act as barriers against dynamic failure may in fact harbor earthquakes, due to the activation of weakening mechanisms by co-seismic friction.”

In the article, Rubino and his co-authors Nadia Lapusta, Lawrence A. Hanson, Jr., Professor of Mechanical Engineering and Geophysics, and Ares Rosakis, Theodore von Kármán Professor of Aeronautics and Mechanical Engineering, show that the se -so-called “stable” or “creeping” faults are not actually immune to major ruptures after all, as previously suspected. Such faults occur when tectonic plates slowly slide past each other, without generating large earthquakes (for example, the currently creeping section of the San Andreas Fault in central California).

Instead, the rock gouge has a complex behavior. It first acts as a break barrier, absorbing energy and blocking its progress. But, when the plates slide past each other with high enough speed, the rock gouge interface weakens and greatly reduces the friction between the two plates, causing the earthquake to re-emerge. This process is known as “renucleation”.

“Based on the rich body of previous experiments on rock friction, we know that rock gouge can either strengthen with fault slip and act as a barrier, or weaken and promote seismic failure,” says Lapusta. “However, these behaviors are generally considered to be spatially separate, with weakening and strengthening occurring on different fault locations. Our experiments show how these behaviors can combine on the same fault locations during the same faulting event. slip, on dynamic failure time scales, leading to intermittent slip and potentially transforming a fault barrier into an earthquake-prone region.”

The Nature The study explores the role and response of rock gouge, a micrometer-sized granular material, to seismic activity. To simulate the rock gouge’s effect on the propagation of an earthquake, the team used Caltech’s so-called seismological wind tunnel, founded by Rosakis and former director of Caltech’s seismology laboratory, Hiroo Kanamori, John E. and Hazel S. Smits, Professor of Geophysics, Emeritus. The facility, which has been around since 1999, allows engineers and scientists to study major earthquakes on a miniature scale.

To simulate an earthquake, the team first cut a meter-sized transparent block of a type of plastic called Homalite in half. The bulk properties of homalite allow dynamic fracture nucleation in samples as small as tens of centimeters in diameter; the study of these effects in the rock would require samples of several tens of meters.

The researchers then placed the two halves of the Homalite together under high pressure and shear (a situation in which the two halves want to slide against each other in opposite directions), simulating slowly building tectonic pressure. along a fault line. Between the pieces, fine-grained quartz powder was embedded to replace the fault gouge. Next, the team put a small fuse wire between the two halves; its location was the “epicenter” of the earthquake they planned to simulate. As the simulated earthquake progressed, high-speed imaging technology was used to record its progress, one millionth of a second at a time.

“In the late 1990s, when we were designing the ‘seismological wind tunnel,’ we could never have imagined its success in uncovering such a rich spectrum of physical phenomena related to frictional earthquake source processes and as such phenomena could be rigorously scaled to explain the natural behavior of earthquakes occurring at vastly different length scales around the globe,” says Rosakis. “It’s a testament to the tremendous power of the discipline of mechanical.”

Next, the team plans to study the effects of fluids, naturally present in the earth’s crust, on the friction behavior of rock gouges.

The Nature the paper is titled “Intermittent Laboratory Earthquakes in a Dynamically Weakening Fault Gouge”. This research was funded by the National Science Foundation (NSF), the US Geological Survey, the NSF-IUCRC program at Caltech’s Center for Geomechanics and Mitigation of Geohazards (GMG), and the Southern California Earthquake Center (SCEC).

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