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| What
is an Earthquake? |
An earthquake is caused by a sudden slip on a fault, much like
what happens when you snap your fingers. Before the snap, you
push your fingers together and sideways. Because you are pushing
them together, friction keeps them from moving to the side.
When you push sideways hard enough to overcome this friction,
your fingers move suddenly, releasing energy in the form of
sound waves that set the air vibrating and travel from your
hand to your ear, where you hear the snap.
The same process goes on in an earthquake. Stresses in the
earth's outer layer push the sides of the fault together. The
friction across the surface of the fault holds the rocks together
so they do not slip immediately when pushed sideways. Eventually
enough stress builds up and the rocks slip suddenly, releasing
energy in waves that travel through the rock to cause the shaking
that we feel during an earthquake.
Just as you snap your fingers with the whole area of your
finger tip and thumb, earthquakes happen over an area of
a fault,
called the rupture surface. However, unlike your fingers,
the whole fault plane does not slip at once. The rupture
begins
at a point on the fault plane called the hypocentre, a point
usually deep down on the fault. The epicentre is the point
on the surface directly above the hypocentre. The rupture
keeps spreading until something stops it (exactly how this
happens
is one of the hot research topics in seismology).
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| What
is a Fault? |
Earthquakes occur on faults. A fault is a thin zone of crushed
rock between two blocks of rock, and can be any length, from
centimetres to thousands of kilometres. When an earthquake
occurs on one of these faults, the rock on one side of the
fault slips with respect to the other. The fault surface can
be vertical, horizontal, or at some angle to the surface of
the earth. The slip direction can also be at any angle. We
classify these into two basic cases: strike-slip and dip-slip
motion.
A strike-slip earthquake occurs on an approximately vertical
fault plane as the rock on one side of the fault slides horizontally
past the other. With dip-slip earthquakes, the fault is at
an angle to the surface of the earth and the movement of
the rock is up or down. The terminology for the type of dip-slip
movement depends on what the top side of the fault--the hanging
wall--does relative to the bottom side--the foot wall. The
walls are so named by their appearance in mines in England:
miners hung their lanterns on the hanging wall and walked
on the foot wall.
On a normal fault, the hanging wall moves down and the foot
wall up; on a reverse fault, the reverse happens - the hanging
wall moves up and the foot wall moves down. A thrust fault
is a special category of reverse faults, in which
the fault
plane lies at a low angle to the earth's surface. Finally,
there are blind thrusts, thrust faults that do not extend
all the way to the earth's surface. The ground above the
blind thrusts bends instead of breaking, so that the surface
has only rolling hills.
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