(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.
“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
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.
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
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,
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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
“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.”
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
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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