On Shaky Ground

 

They're all around us - from dust, to sand dunes, to broken rocks in the earth's crust, and even to plant seeds or coffee. Known as granular materials, their definition is simple: a large collection of macroscopic particles. But understanding their behavior is anything but simple - it's strikingly different from any of the familiar forms of matter: solid, liquid, or gas. A better understanding of these minute particles could shed light on incredibly powerful and large-scale processes shaping the Earth, including landslides or even earthquakes.

 

When and where will the next earthquake strike? How big will it be? These and other questions loom in the minds of people who live in earthquake-prone zones. Focusing on granular materials, Dr. Einat Aharonov, a senior scientist in the Weizmann Institute's Department of Environmental Sciences and Energy Research, is working to develop computer simulations to better understand the movement of tectonic plates and other geological processes. 'I was always interested in rocks, even as kid,' she recalls. 'I like going out into the field, splitting a rock open and seeing the crystals. It has a quietness to it, especially in places where you can see that there was once an ocean where now there is a mountain, or a river that is now desert.'

 

Aharonov's earthquake models focus on the behavior of granular material formed when two tectonic plates crush against each other. The initial inter-actions among the huge number of grains formed are essentially simple but give rise to complex dynamics. By constructing a mathematical model accounting for these processes, Aharonov, working with Dave Sparks of Texas University, succeeded in simulating the 'fault gouge' - the crushed-up granular region that forms at the heart of geological fault zones, where earthquakes strike.

 

'Our model is somewhat like traffic control modeling,' explains Aharonov. Granular material moves and rubs together, causing friction. The grains can't penetrate one another but neither are they mutually repellent. They 'want' to steer clear of one another but there's no place to go - much like cars in a traffic jam.'

 

Other models target an understanding of how the oceanic crust is formed, the ways in which rocks change and evolve over time, and what causes part of the earth's surface to suddenly undergo a phase transition, changing its behavior from that of a solid to a liquid, as in landslides or liquefaction. One of the most dreaded by-products of an earthquake, liquefaction occurs when the powerful seismic waves created during an earthquake travel through the soil, causing it to behave like a liquid. Often leaving a trail of devastation in its path, soil liquefaction can easily cause buildings and bridges to collapse, and poses a significant threat to underground pipelines. In the great 1906 San Francisco earthquake, for instance, liquefaction-related damage to water supply pipelines severely hampered attempts to battle the fires that swept through the city, ultimately causing much of the overall damage. Aharonov and her colleagues hope to use mathematical modeling to further understand these phenomena, making it easier to identify zones prone to liquefaction or landslides.

 

But Aharonov cautions that mathematical modeling is not perfect: 'Simulation is just a crude approximation of nature. There are countless variables that can affect the outcome. Particle size and shape as well as chemical reactions all play an important role in grain-scale modeling. Time is another complicated variable, since the Earth's time scale ranges from less than a second to millions of years.'

Having begun with an interest in breaking up and looking inside rocks, today Aharonov develops mathematical models that help explain how rocks form and change over time. Enhanced understanding of such geophysical processes may facilitate engineering measures to better prepare us for the next time the earth suddenly fails beneath us - as it inevitably will.