An earthquake (also
known as a quake, tremor or temblor) is the result of a sudden release of
energy in the Earth's crust that creates seismic waves.
The seismicity, seismism or seismic activity
of an area refers to the frequency, type and size of earthquakes experienced
over a period of time.
The
surface of the Earth is in continuous slow motion. This is plate tectonics--the
motion of immense rigid plates at the surface of the Earth in response to flow
of rock within the Earth.
There are two main causes of
earthquakes.
Firstly,
they can be linked to explosive volcanic eruptions; they are in fact very
common in areas of volcanic activity where they either proceed or accompany
eruptions.
Secondly,
they can be triggered by Tectonic activity associated with plate margins and
faults. The majority of earthquakes world wide are of this type. Most earthquakes are causally related to compressional
or tensional stresses built up at the margins of the huge moving lithospheric
plates that make up the earth's surface .
The
immediate cause of most shallow earthquakes is the sudden release of stress
along a fault, or fracture in the
earth's crust, resulting in movement of the opposing blocks of rock past one
another.
These
movements cause vibrations to pass through and around the earth in wave form,
just as ripples are generated when a pebble is dropped into water.
Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but
most of these are of only local extent.
Shock
waves from a powerful earthquake can trigger smaller earthquakes in a distant
location hundreds of miles away if the geologic conditions are favorable.
Earthquakes
occur where the earth's plates meet along plate boundaries.
For
example as two plates move towards each other, one can be pushed down under the
other one into the mantle. If this plate gets stuck it causes a lot of pressure
on surrounding rocks. When this pressure is released it produces shock waves.
These are called seismic waves.
The waves spread out from the point where the earthquake started is the focus.

