Caltech engineers provided strong experimental support for a form of earthquake propagation that is thought to have caused the magnitude-9.0 earthquake in Japan in 2011.
Gravel is formed as the plates grind against one another along some fault lines. The gravel's influence on earthquakes has been studied. In a new paper appearing in the journal Nature on June 1, the Caltech researchers show that the fine gravel, known as rock gouge, first halts earthquake propagation, but then causes the rebirth of earthquakes.
"Our novel experimental approach has allowed us to look into the earthquake process up close, and to uncover key features of rupture propagation and friction evolution in rock," says Rubino. One of the main findings of our study is that fault sections that were thought to act as barriers against dynamic rupturing may actually host earthquakes, as a result of the activation of co-seismic Friction Weakening mechanisms.
Rubino and his co-authors show in the paper that "stable" is a term used in mechanical engineering. Such faults occur when the plates slide past one another slowly without generating large earthquakes.
Rocks have a complex behavior. It absorbs energy and blocks the progress of the rupture. When the plates slide past each other with high speed, the rock gouge interface weakens and causes the re-emergence of the earthquake. Renucleation is a process.
"Based on the previous rich body of rock-friction experiments, we know that rock can either strengthen with fault slip and act as a barrier, or weaken and promote earthquake rupture." "However, these behaviors are separated in space and weaken and strengthen on different fault locations." Our experiments show how these behaviors can combine on the same fault locations during the same slip event and lead to intermittent slip and potentially turning a fault barrier into an earthquake-prone region.
The Nature study looked at the role and reaction of rock gouge to earthquakes. The team used Caltech's seismological wind tunnel founded by Rosakis and former Caltech Seismological Laboratory director Hiroo Kanamori. Engineers and scientists can study earthquakes on a small scale at the facility.
The team cut in half a block of Homalite, a type of plastic, in order to make a simulation of an earthquake. The bulk properties of Homalite allow for the nucleation of samples as small as tens of centimeters in diameter, which makes it easier to study in rock.
The Homalite was placed under high pressure and shear, which is a situation in which the two halves want to slide against each other in opposite directions. There was a stand-in between the pieces. The epicenter of the earthquake they planned to simulation was where the small wire fuse was placed. One millionth of a second at a time is how long it took to record the evolution of the simulation.
In the late 1990s, when we were designing the'seismological wind tunnel', we could never have imagined its success in discovering such a rich spectrum of physical phenomena relating to frictional earthquake source processes. The discipline of mechanics is shown in this example.
The team wants to study the effects of fluids on the behavior of rock.
There are intermittent lab earthquakes that weaken the fault.
More information: V. Rubino et al, Intermittent lab earthquakes in dynamically weakening fault gouge, Nature (2022). DOI: 10.1038/s41586-022-04749-3 Journal information: Nature
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