Earthquake-triggering demonstration
Introduction
Today we are going to explore earthquakes. Earthquakes happen every day of every year.
Sometimes the earthquakes are large and destructive; other times the earthquakes are so small
that we can barely detect them. Earthquakes tend to occur in certain parts of the Earth.
The rigid outer layer (lithosphere) of the Earth is divided into separate pieces (plates)
The plates of the lithosphere are separated by faults, which are like giant cracks. Faults also
separate smaller blocks of rock within a plate.
The plates move with respect to one another. The relative plate motions are generally a few
centimeters per year. This constant relative motion causes a build-up of stress along plate
boundaries that are locked together. An earthquake is a periodic unlocking and release of the
built-up stress. Most earthquakes occur along the plate boundaries. However, faults also separate
smaller blocks of rock within a plate and earthquakes occasionally occur within a plate.
How earthquakes are triggered
The brick and the board represent two plates moving with respect to one another. The sandpaper
simulates the friction of a rock surface. Turning the crank generates a stress caused by one plate,
represented by the brick, as it tries to slide past the other plate, represented by the board. The
spring, which represents the elastic rock surrounding the brick, allows the slow build up of stress.
As the crank is turned, stress slowly builds up to a point, at which the stress in the spring
overcomes the locking stress caused by the friction. At that point, the brick slips and releases the
built-up stress –an earthquake!
Repeat the process, but this time, have someone listen carefully (put an ear right up to the
sandpaper). There may be a subtle crackling sound coming from between the brick and the sand
paper before the brick slips. This crackling represents foreshocks that occur before a big quake.
Modify brick apparatus for triggering. Place two bricks on sandpaper and connect the second
brick to the first with a spring. The bricks should be in a series, separated by a spring. Make sure
the spring is just taunt, but not stretched.
The stress released by one earthquake changes the stress along nearby faults. Sometimes the first
earthquake increases the stress along another fault and can cause a second earthquake; and
sometimes the first earthquake relieves stress along a different fault. This transfer of stress from
one fault to another is called stress-coupling.
Slowly turn the crank (add stress to the fault) until the first brick slips and transfers some stress
to the spring connected to the second brick.
Continue to add stress until the second brick slips in response to the first brick slipping. Notice
how the movement of the first brick changes the stress along the fault of the second brick. Notice
that the stresses in the two springs are pulling the first brick in opposite directions. If the second
brick is nudged toward the first brick, the stress along the fault of the first brick is increased and
the first brick slips. We can cause earthquakes by both adding and decreasing stress.
Accounting for delayed aftershocks –viscoelastic relaxation
Stress released by large earthquakes is absorbed by rocks of the surrounding region. The rocks of
the upper crust are relatively cool and brittle and can hold the stress. However, because of the
high temperature and stress condition at depth, rock of the lower crust and mantle cannot hold
the stress indefinitely. Instead, they slowly flow (viscous flow) to relax the stress. When this
occurs, the stress that is relaxed in the lower crust and mantle is transferred to and accumulates in
the brittle upper crust. Under the right conditions, this stress transferred from the flowing rocks
at depth can accumulate to a point that surpasses the stability of faults in the upper crust. Thus,
stresses from a big earthquake slowly readjust through the process of viscoelastic relaxation,
which can account for delayed aftershocks following a big earthquake. For example, aftershocks
are still occurring due to the 2004 M9 Great Sumatra-Andaman earthquake, due to viscoelastic
relaxation.
Place the viscous goo near the far end (away from the crank) of the sand paper. Flatten it out
until it is a little bigger than the footprint of the second brick. Place the second brick on top of
the goo and press it down to seat it in the goo. Place the first brick on the sandpaper in front of
(toward the crank) the second brick, such that the spring connecting the two bricks is just taunt.
Quickly crank the first brick so that it slides a few inches and hold the crank in one position (do
not add any more stress). Notice the stress transferred to the second spring slowly pulls the
second brick as it moves over the flowing goo. Eventually, the stress in the second spring is
relaxed and the stress in the first spring overcomes the friction beneath the first brick. You may
have to practice this to figure out how much stress you need on the second spring.
Accounting for delayed aftershocks –poroelastic relaxation
Remember, stress released by large earthquakes is absorbed by rocks of the surrounding region.
Rocks in the brittle crust have small pore spaces that are filled with water (groundwater). The
stress of an earthquake will squeeze or expand the pore spaces, and thus temporarily change the
pressure of the water in the pores (pore pressure). Following the earthquake, the water will flow
in response to the changes in pore pressure. The pore pressure affects the frictional stability of
faults. If the pore pressure in a fault rises, it acts to separate the two sides of a fault and reduces
the frictional resistance of the fault. The opposite is true is the pore pressure in a fault drops.
Because the pore pressure slowly changes in response to an earthquake, the frictional stability of
faults in the region also changes. These slow changes in pore pressure can also account for time
lags of aftershocks.
Place an ordinary brick in a pan. Place one end of a second ordinary brick on the first brick, such
that the second brick is inclined. Connect the inflow tube of the modified brick to a sink faucet.
Place the modified brick on top of the inclined brick. Increase the inclination so that the top brick
is close to sliding, but does not slide. The fault (the surface separating the two inclined bricks) is
stressed, but not enough to trigger an earthquake. Turn on the faucet. The water pressure builds-
up along the fault and triggers slip –an earthquake! You can try to simulate slow pressure
increases by slowly increasing the water flow from the faucet.