What causes earthquakes
and where do they happen?
has four major layers: the inner core, outer core,
mantle and crust. (figure 2) The crust and the top
of the mantle make up a thin skin on the surface of
our planet. But this skin is not all in one piece –
it is made up of many pieces like a puzzle covering
the surface of the earth. (figure 3) Not only that,
but these puzzle pieces keep slowly moving around,
sliding past one another and bumping into each
We call these puzzle pieces tectonic
plates, and the edges of the plates are called the
plate boundaries. The plate boundaries are made up
of many faults, and most of the earthquakes around
the world occur on these faults. Since the edges of
the plates are rough, they get stuck while the rest
of the plate keeps moving. Finally, when the plate
has moved far enough, the edges unstick on one of
the faults and there is an earthquake.
How are earthquakes recorded?
Why does the earth shake
when there is an earthquake?
edges of faults are stuck together, and the rest of
the block is moving, the energy that would normally
cause the blocks to slide past one another is being
When the force of the moving
blocks finally overcomes the friction of the jagged
edges of the fault and it unsticks, all that stored
up energy is released. The energy radiates outward
from the fault in all directions in the form of
seismic waves like ripples on a pond.
seismic waves shake the earth as they move through
it, and when the waves reach the earth's surface,
they shake the ground and anything on it, like our
houses and us! (see P&S Wave inset)
Earthquakes are recorded by
instruments called seismographs. The recording they make is
called a seismogram. (figure 4) The seismograph has a base
that sets firmly in the ground, and a heavy weight that
hangs free. When an earthquake causes the ground to shake,
the base of the seismograph shakes too, but the hanging
weight does not. Instead the spring or string that it is
hanging from absorbs all the movement. The difference in
position between the shaking part of the seismograph and the
motionless part is what is recorded.
How do scientists measure
the size of earthquakes?
The size of
an earthquake depends on the size of the fault and
the amount of slip on the fault, but that's not
something scientists can simply measure with a
measuring tape since faults are many kilometers deep
beneath the earth's surface. So how do they measure
an earthquake? They use the seismogram recordings
made on the seismographs at the surface of the earth
to determine how large the earthquake was (figure
A short wiggly line that doesn't wiggle
very much means a small earthquake, and a long
wiggly line that wiggles a lot means a large
earthquake. The length of the wiggle depends on the
size of the fault, and the size of the wiggle
depends on the amount of slip.
The size of
the earthquake is called its magnitude. There is one
magnitude for each earthquake. Scientists also talk
about the intensity of shaking from an earthquake,
and this varies depending on where you are during
How can scientists tell
where the earthquake happened?
Seismograms come in handy for locating earthquakes
too, and being able to see the P wave and the S wave
is important. You learned how P & S waves each shake
the ground in different ways as they travel through
it. P waves are also faster than S waves, and this
fact is what allows us to tell where an earthquake
was. To understand how this works, let's compare P
and S waves to lightning and thunder. Light travels
faster than sound, so during a thunderstorm you will
first see the lightning and then you will hear the
thunder. If you are close to the lightning, the
thunder will boom right after the lightning, but if
you are far away from the lightning, you can count
several seconds before you hear the thunder. The
further you are from the storm, the longer it will
take between the lightning and the thunder.
P waves are like the lightning, and S waves are like
the thunder. The P waves travel faster and shake the
ground where you are first. Then the S waves follow
and shake the ground also.
If you are close
to the earthquake, the P and S wave will come one
right after the other, but if you are far away,
there will be more time between the two.
looking at the amount of time between the P and S
wave on a seismogram recorded on a seismograph,
scientists can tell how far away the earthquake was
from that location. However, they can't tell in what
direction from the seismograph the earthquake was,
only how far away it was. If they draw a circle on a
map around the station where the radius of the
circle is the determined distance to the earthquake,
they know the earthquake lies somewhere on the
circle. But where?
Scientists then use a
method called triangulation to determine exactly
where the earthquake was (figure 6). It is called
triangulation because a triangle has three sides,
and it takes three seismographs to locate an
If you draw a circle on a map
around three different seismographs where the radius
of each is the distance from that station to the
earthquake, the intersection of those three circles
is the epicenter!
Can scientists predict
No, and it is unlikely
they will ever be able to predict them. Scientists
have tried many different ways of predicting
earthquakes, but none have been successful. On any
particular fault, scientists know there will be
another earthquake sometime in the future, but they
have no way of telling when it will happen.
Is there such a thing as
earthquake weather? Can some animals or people tell
when an earthquake is about to hit?
These are two questions that do not yet have
definite answers. If weather does affect earthquake
occurrence, or if some animals or people can tell
when an earthquake is coming, we do not yet
understand how it works.