HU researchers find way to predict serious quakes

Israeli physicists discover basic assumptions about earthquakes are wrong and suggest a new way to replicate how real ones develop.

November 29, 2010 04:36
3 minute read.
SEISMOGRAPHS LIKE THIS measure motions of the grou

Seismograph 311. (photo credit: Yamaguchi)

Since Leonardo da Vinci described the involvement of friction in earthquakes six centuries ago, this model has been thought to be true. But Hebrew University of Jerusalem physicists who created “model earthquakes” in the lab have discovered that these basic assumptions are wrong and suggested a new way to replicate how real ones develop.

Writing in the prestigious journal Science, they said they may be able to predict severe earthquakes.

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“The findings have a wide variety of implications for materials science and engineering and could help researchers understand how earthquakes occur and how severely they may develop along a fault line,” said Prof. Jay Fineberg of the Racah Institute of Physics. The work by Fineberg, graduate student Oded Ben-David and fellow researcher Gil Cohen, has also has been published online in Wired magazine.

For centuries, physicists have thought that the amount of force needed push an object to make it sliding across a surface is determined by the coefficient of friction, which is the ratio between the forces pushing sideways and pushing down (basically, how much the object weighs). Da Vinci’s description of the phenomenon – redefined a few hundred years later – was so widely accepted that it has consistently appeared in introductory physics textbooks.

The experiments studied two contacting blocks as they just start to slide apart. When Ben-David tried to check whether these “laws” work at different points along a block’s contact surface, the laws fell apart. In carefully controlled lab experiments, Ben-David found that points along the contact surface could sustain up to five times as much sideways force as the coefficient of friction predicted, and the object still wouldn’t budge.

Although the blocks look like they are smoothly touching, in reality they are only connected by numerous, discrete, tiny contact points, whose total area is hundreds of times less than the blocks’ apparent contact area. Performing sensitive measurement of the stresses at contacting points, the researchers noted that the strength at each point along the contact surface could be much larger than the coefficient of friction allows before the contacts ruptured and the block began to slide.

Furthermore, the contacts don’t all break at the same time. Instead, they rupture one after another in a rhythmic sequence that sets the rupture speed. These rapidly moving ruptures are closely related to earthquakes, Fineberg said.

The blocks in effect represent two tectonic plates pushing one against each other, and when the force between them is enough to disengage the plates, the resulting contact- surface rupture sends shock waves through the blocks, exactly as in an earthquake.

The team found that ruptures come in three distinct modes: slow ruptures that move at speeds well below the speed of sound; ruptures that travel at the speed of sound; and “supershear” ruptures that surpass the speed of sound.

Which type of wave occurs is determined by the stresses at the contact points, which provide a measure of how much energy would be released if an actual earthquake were to occur. These different types of earthquakes have all been seen in underground quakes, but these experiments provide the first clue on how the earth “chooses” how to let go.

“An earthquake is the same system as in the HU experiments, just scaled up by factors of thousands,” Fineberg said. “We can watch how these things unfold in the lab and measure all of the variables that might be actually relevant in a way that you could never observe under the earth.”

How an earthquake “chooses” to rupture is not simply an academic question. Each type of rupture mode determines how the earth releases the enormous pressures that are locking tectonic motion and is intimately related to the hazards embodied within an earthquake.

While sonic earthquakes are destructive, their supersonic cousins are potentially much more dangerous as they release the enormous stored energy within the earth as a shock wave. In contrast, slow ruptures create negligible damage for the same amount of stress release.

And while it is still impossible to make detailed measurements of the stresses along a real fault, the research results suggest a method by which stresses can be tracked as an earthquake is under way and how one earthquake can set the stage for the initial conditions for the next one. This new understanding has the potential to provide unprecedented predictive power, estimating both the rupture mode and extent of a future earthquake.

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