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The Science of the Slow Slip


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Dr. Eran Bouchbinder


We all know about the big earthquakes: The edges of two tectonic plates in the earth’s crust slip against one another, causing shaking and damage as the slip releases huge amounts of pent-up energy within mere seconds. The earth, however, literally moves under our feet much more often than we’re aware. In the last few years researchers have come to realize that there is another kind of earthquake – called a “slow” or “silent” earthquake – which does not entail a strong ground-shaking energy release. Rather, the slip takes place many orders of magnitude more slowly – over hours or even days. Although the ultimate energy release might be similar, the slow pace means such quakes are not felt.

Slow quakes, says Dr. Eran Bouchbinder of the Chemical Physics Department (Faculty of Chemistry), appear to be a common occurrence. It is only the recent development of novel geophysical and laboratory measurement techniques that have enabled scientists to uncover their existence and prevalence. Researchers are eager to understand the physical principles underlying the slow quakes: Are they governed by the same physics as their fast counterparts? What is actually determining their speed? There is some evidence that in certain cases, slow quakes may be precursors of the fast ones, though no one can say, as yet, how to identify those that might presage a major event.
In research recently published in Geophysical Research Letters, Bouchbinder and his team proposed a mathematical model suggesting that slow quakes might be controlled by somewhat different physics than the fast ones, accounting for their leisurely rate of movement.

Both types of earthquake involve frictional interfaces – the surfaces where the plates meet. Such interfaces are present in a great number of physical systems and play a central role in phenomena occurring on widely disparate scales, ranging from nano-machines and tiny biological motors to the geophysical scale of earthquakes. The strength and stability of these interfaces is an issue of fundamental importance; when frictional interfaces fail, it could spell trouble.  
A rupture in frictional interfaces involves the interplay between stored “elastic” energy and frictional resistance. Two bodies – whether tectonic plates in the earth’s crust or car tires and the road – are typically brought into friction-causing contact by normal forces that press them together. Parallel to the interface, a phenomenon called shear force acts to slide the two bodies against each other. In other words, one force pushes the bodies to move; the other resists movement. As long as the frictional resistance balances out the shear force, the interface remains pinned in place. But when shear force overcomes frictional resistance, sliding occurs, and ruptures propagate along the interface. How fast these ruptures progress determines whether the elastic energy – often built up over long periods as the shear force strains against friction – will be released abruptly or a little at a time. In fast, shaking quakes, this propagation is limited by the speed of sound, on the order of kilometers a second through ordinary solids – the speed at which waves can move within a material.
Earthquake damage: occurs when ruptures propagate at the speed of sound


For Bouchbinder, the question was: What makes the slow quakes slow? Is their speed simply a small fraction of the speed of sound or is it really a different physical quantity?
To investigate this, he and his team, research student Yohai Bar Sinai and Dr. Efim Brener, a visiting professor from Peter Grünberg Institut, Forschungszentrum Jülich, Germany, looked at geophysical measurements as well as experimental data from the group of Prof. Jay Fineberg of the Hebrew University of Jerusalem in which the factors involved in various types of earthquake are tested on a lab-sized scale. They then developed a model that offers a new explanation for the dynamics of the slow quakes.

According to conventional models, frictional resistance decreases as the sliding speed rises: The faster something slides the easier the sliding becomes. This is why an interfacial rupture tends to accelerate until it approaches the limit – the speed of sound. But the new model Bouchbinder and his team proposed allows for a different scenario: Frictional resistance first acts as in the standard model, decreasing with increasing sliding speed; but at some point the effect is reversed, and friction begins to progressively rise again. The slow quake phenomenon, says Bouchbinder, should occur around that point in the model at which friction levels reach their minimum and begin to swing upwards. The speed of the rupture, in this case, is determined by the frictional properties of the interface alone, rather than by the speed of sound, and thus can be orders of magnitude slower.

Bouchbinder and his team now plan to continue testing their mathematical model with more complicated calculations and to compare their predictions to experimental measurements, in hopes of providing a useful tool for better understanding the failure of frictional interfaces, including those that result in earthquakes.
Satellite images from 1992 and 1997 reveal gradual slippage of 2-3 cm along the Hayward fault (thin red line) in California, evidence of a slow earthquake Image: NASA




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