Think of rocks and hard, unyielding structures come to mind.
But Hiroki Sone, an assistant professor of geological engineering at the University of Wisconsin-Madison, says the truth isn’t so straightforward. In fact, rocks are elastic, in certain ways. Press on them with enough force and give enough time, they’ll flow and bend permanently. These characteristics can play out over huge scales along fault lines, where pieces of the earth’s crust grind against each other.
The idea that rocks can flow isn’t new. Since the dawn of computer research, scientists have tried to incorporate the idea of rock elasticity into models to better understand what causes earthquakes as tectonic plates move past each other. However, Sone says rocks, which can be porous aggregates of different materials, may actually also act like a fluid over long periods of time, with changes that aren’t due solely to pressure. Now a $528,000 National Science Foundation CAREER Award will fund a five-year research project to help him study those properties.
“Rocks are a poro-viscoelastic material,” Sone says. “If you press on a rock, if it’s linearly elastic, it shrinks, and then stays that way as long as you don’t change the force you’re applying to it. A viscous material will also deform due to elasticity, but there’s also time-dependent, slow deformation that happens. Rocks are solid, but if you give them enough time, they can behave somewhat like a fluid.”
For his project, Sone will investigate the damage zone around faults—the areas where rocks are already broken due to past slips along the fault line. Damage zones are created because, despite their common depictions as smooth lines, faults have rough, uneven edges. These edges give rise to differing stresses along fault lines, because the forces are concentrated along the rough edges where part of one plate juts into the other. When faults slip and create earthquakes, those edges tear at the surrounding material. That causes cracking and the formation of damage zones.
One positive note in all of this wear and tear is that, because those areas are already fractured, they also can make it easier for researchers to observe rocks’ fluid-like behaviour.
“Looking at damage zones, in and of itself, isn’t new,” Sone says “Structural geologists have done that for decades. But I’m doing this with an additional view from understanding rocks’ viscous properties. I’ll be going out into the field, characterizing these zones, sampling rocks, and then in the lab, measuring how these rocks exhibit poro-viscoplastic properties.”
Sone will work to develop a formulation that describes these viscous properties after conducting field observations. He hopes such a formula could be implemented into numerical models that acknowledge the damage zone’s effect on earthquakes, as current models do not account for it. Sone has recently been working with Assistant Professor Shiva Rudraraju from the Department of Mechanical Engineering to make this leap, taking advantage of their strengths in modelling multi-physics problems.
Sone hopes his work will bolster research on the interseismic periods, or the times between earthquakes, along faults. Most research over the last half-century has focused on what happens in the co-seismic periods, or the brief windows during earthquake events. Acknowledging the damage zone, combined with rocks’ poro-viscoplastic properties, can help us understand how stress accumulates along faults. Understanding how pressure builds and moves the rocks along faults can, ultimately, help us better understand how earthquakes happen.
“Studies to try to understand the interseismic period have been severely lacking,” Sone says. “Part of that is because what governs this phenomenon is this really messy rock behavior that not many have dared to study. It’s a very difficult challenge experimentally and theoretically. But we have to start somewhere.”
Sone says it will likely take far longer than this one project’s duration to fully understand the viscous properties of rocks in damage zones. However, he says that knowledge may prove crucial for industries like those that extract gas from shale gas reservoirs and heat from geothermal reservoirs. It may also help us understand and predict what areas are vulnerable to earthquakes.
“In the broadest sense, we’re trying to understand the mechanics of earthquakes, which we need in order to predict hazards,” he says. “The short term prediction—where you might have something like an earthquake warning go out before one hits—is beyond the prospect of this project. There’s so much more that we need to understand to get to that point. But the fundamental insights gained from this project will help in the long-term outlook or in mid-term predictions, letting us know where to expect earthquakes and why some places produce more earthquakes than others.”
NSF CAREER Awards include a public education component, and Sone is planning to take a focused approach for the geoscience community. Experimental rock mechanics labs are few and far between, and they require extremely specialized equipment. He’s planning to host free workshops, starting at the American Rock Mechanics Association Symposium, as a way to share knowledge with students and new practitioners. He hopes his first steps will build momentum and lay the foundation for a self-sustaining system to facilitate knowledge-sharing among rock mechanics labs.
“The learning curve with this equipment is so steep and there’s hardly any knowledge transfer between labs,” Sone says. “Even within labs, it’s rare to have sustained critical mass needed to pass knowledge between generations. If we can reduce the amount of time we spend troubleshooting equipment from 80% to even 40%, imagine how much more we could accomplish.”
Author: Alex Holloway