Earthquakes are caused by movements of the giant tectonic
plates that form Earth’s crust. SEISMOLOGY is the study of
earthquakes. Most occur at cracks called FAULTS, at the boundaries
where the plates meet. Every minute, the ground shakes somewhere in the world,
but these vibrations are usually minor tremors that are barely noticed. When a
major earthquake strikes, the ground shakes violently, and buildings and
bridges topple.
Faults are deep cracks in rocks, mostly caused by
movement at plate margins. Deep earthquakes strike in subduction zones where
two plates collide and one slides below the other. Shallow earthquakes occur
mostly where two plates grind past one another. The rocks may be shifted only a
few centimetres, but over millions of years, this can add up to hundreds of
kilometres of movement sideways, and up to 30 km (19 miles) of vertical
movement.
The rocks along a fault may move up or down, sideways or
diagonally, depending on the angle of the fault plane. The angle of the fault
plane to the horizontal is known as the dip. In a normal fault, also known as a
dip-slip fault, the rocks shift straight down or up, following the line of
dip.
The distance that the rocks slip up or down during a quake or
tremor is called the throw. In a reverse fault, pressure causes one block of
rock to overhang another. As the rocks shift, the block is forced farther up
and over the other. A reverse fault with a fault plane of 45° or less is
called a thrust fault.
In a strike-slip fault, rocks scrape sideways past one another.
The amount of sideways slip is called the heave. The San Andreas Fault, which
runs along the west coast of North America, is a famous example. The rocks in
an oblique-slip fault slide past each other, and also up and down in a diagonal
movement.
Seismologists study earthquakes. They also examine the
behaviour of seismic waves passing through the Earth to find out about its
structure. Instruments called seismographs measure the intensity of seismic
waves. The magnitude (size) of earthquakes can be rated by measuring either
these waves, on the Richter scale, or the damage caused – the Mercalli
scale. Earthquakes cannot be prevented, but they can sometimes be accurately
predicted.
