Defects once thought ‘creeping’ but stable may be at risk of major rupture – ScienceDaily

By simulating earthquakes in the lab, Caltech engineers have provided robust support for a form of earthquake propagation now believed to be responsible for the 9.0-magnitude earthquake that devastated the coast of Japan in 2011.

Along some fault lines, which are tectonic plate boundaries, fine-grained pebbles are formed when the plates meet each other. The impact of this pebble on earthquakes has long been the subject of scientific speculation. In a new paper published in Nature on June 1, Caltech researchers show that fine gravel, known as rock pits, first stops the spread of earthquakes, but then triggers the rebirth of earthquakes to generate powerful faults.

“Our new approach enabled us to look at the earthquake process up close, revealing key features of rupture propagation and the evolution of friction in rock pits,” says Vito Rubino, research scientist and lead author of the Nature paper. “One of the main findings of our study is that fault sections previously thought to act as barriers against dynamic rupture may in fact host earthquakes, as a result of the activation of joint seismic friction weakening mechanisms.”

In the paper, Rubino and co-authors Nadia Laposta, Professor of Mechanical Engineering and Geophysics Lawrence A. Hanson, and Aris Rosakis, professor of aeronautics and mechanical engineering, argue that so-called “flaw-stable” or “creeping” faults are not actually immune to large ruptures, as previously thought. Such faults occur when tectonic plates slide together slowly, without generating large earthquakes (for example, the currently creeping section of the San Andreas Fault in central California).

Instead, the rock clicking technique has a complex behaviour. It acts first as a barrier to the rupture, absorbing energy and impeding its progression. But when the plates slide over each other at a high enough speed, the rock-drill interface weakens and drastically reduces friction between the two plates, causing the earthquake to reappear. This process is known as “nucleation”.

“Based on the previous rich set of rock friction experiments, we know that rock pits can either strengthen as the fault slips and act as a barrier, or weaken and enhance earthquake rupture,” LaPosta says. “However, these behaviors are usually viewed as separate in space, 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 timescales of dynamic rupture, resulting in It leads to intermittent slip and possibly transforms the fault barrier into an earthquake-prone zone.”

The Nature study explores the role and interaction of rock pits, a micrometer-sized granular material, for seismic activity. To simulate the effect of shale drilling on earthquake propagation, the team used Caltech’s so-called seismic wind tunnel, which was founded by Rosakis and former Caltech Seismological Laboratory director Hiro Kanamori, and the John E and Hazel Smits Professor of Geophysics, Emeritus. The facility, in existence since 1999, allows engineers and scientists to study major earthquakes on a micro-scale.

To simulate the earthquake, the team first cut half a meter-wide translucent block from a type of plastic known as Homalite. The volumetric properties of Homalite allow to intend a dynamic rupture within samples as small as tens of centimeters in diameter; Studying these effects on rocks requires samples tens of meters in size.

Then the researchers put 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 the tectonic pressure slowly accumulating along the fault line. Between the cuts, fine-grained quartz powder was included as a spare fault carrier. Next, the team placed a small wire fuse between the two halves; Its location was the “epicentre” of the earthquake they planned to simulate. As earthquake simulation progressed, high-speed imaging technology was used to record its evolution, at a rate of one millionth of a second at a time.

“Back in the late 1990s, when we were designing the Seismic Wind Tunnel, we could never have imagined that it had succeeded in discovering such a rich spectrum of physical phenomena related to the source processes of frictional earthquakes and that these phenomena could be precisely scaled up to explain them,” Rosakis says. Natural earthquakes occur on vastly different scales of length around the world.” “This is evidence of the tremendous strength of the system 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 pressure on the rocks.

Nature’s paper is titled “Intermittent Laboratory Earthquakes in Dynamically Weakening of Fault Impairment”. 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 Geographic Hazard Mitigation (GMG), and the Southern California Seismic Center (SCEC).

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