Lab earthquakes show how grains at fault boundaries lead to large earthquakes: Faults once thought to be ‘creeping’ but stable may be at risk of large faults

Along some fault lines, which are the boundaries of tectonic plates, a fine-grained gravel is formed as 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 to appear in the magazine Nature on June 1, the Caltech researchers show that the fine gravel, known as rock gouge, first stops the spread of earthquakes, but then triggers the rebirth of earthquakes to create powerful ruptures.

“Our new experimental approach has allowed us to take a closer look at the earthquake process and discover key features of fracture propagation and frictional evolution in rock gouges,” said Vito Rubino, research scientist and lead author of the study. Nature paper. “One of the key findings of our study is that fault sections previously thought to act as barriers to dynamic faults can in fact trigger earthquakes, due to the activation of co-seismic friction attenuating mechanisms.”

In the paper, Rubino and his co-authors Nadia Lapusta, the Lawrence A. Hanson, Jr., professor of mechanical engineering and geophysics, and Ares Rosakis, Theodore von Kármán professor of aeronautics and mechanical engineering, show that so-called “stable” or “creeping” faults are actually not immune to large fractures, as previously suspected. Such faults occur when tectonic plates slide slowly past each other, without triggering major earthquakes (for example, the currently creeping portion of the San Andreas Fault in central California).

Instead, rock gouge has a complex behavior. It first acts as a barrier to the fracture, absorbing energy and blocking its progress. But when the plates slide past each other at a high enough rate, the rock gouge interface weakens and the friction between the two plates decreases drastically, causing the earthquake to resurface. This process is known as ‘renucleation’.

“Based on previous extensive rock friction experiments, we know that rock gouges can either strengthen through fracture and act as a barrier or weaken and promote earthquake rupture,” says Lapusta. “However, these behaviors are typically considered to be separated in space, with attenuation and reinforcement occurring at different fracture sites. Our experiments demonstrate how these behaviors can be combined at the same fracture sites during the same slip event, across the timescales of dynamic fracture, leading to intermittent slip and possibly a rupture barrier turns into an earthquake-prone area.”

The Nature study examines the role and response of rock gouge, a micrometer-sized granular material, to seismicity. To simulate the effect of rock gouges on the propagation of an earthquake, the team used Caltech’s so-called seismological wind tunnel, founded by Rosakis and former director of the Caltech Seismological Laboratory, Hiroo Kanamori, John E. and Hazel S. Smits, professor emeritus. Geophysics. The facility, which has been in existence since 1999, allows engineers and scientists to study large earthquakes on a miniature scale.

To simulate an earthquake, the team first cut a half-transparent one-meter block of a type of plastic known as Homalite. Homalite’s bulk properties allow dynamic fracture nucleation in samples as small as tens of centimeters in diameter; to study these effects in rock, samples tens of meters in size are needed.

The researchers then placed the two halves of the Homalite against each other under high pressure and shear (a situation where the two halves want to slide against each other in opposite directions), simulating tectonic pressure that slowly builds up along a fault line. Fine-grained quartz powder was embedded between the pieces as a substitute for fracture gouge. Next, the team placed a small wire fuse between the two halves; the location was the “epicenter” of the earthquake they wanted to simulate. As the simulated earthquake progressed, high-speed imaging technology was used to record its evolution, one millionth of a second at a time.

“Back in the late 1990s, when we designed the ‘seismological wind tunnel,’ we could never have imagined that it would discover such a rich spectrum of physical phenomena related to friction earthquake source processes and that such phenomena could be rigorously scaled to explain natural earthquake behavior that occurs at vastly different length scales around the world,” says Rosakis. “This is a testament to the tremendous power of the discipline of mechanics.”

Next, the team plans to study the effects of fluids, which are naturally present in the Earth’s crust, on the frictional behavior of rock gouges.

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