Earthtquakes
An Earthtquake is the shaking of the earth’s surface caused
by rapid movement of the earth’s rocky outer layer.Earthtquakes occur when
energy stored within the earth, usually in the form of a strain in rocks,
suddenly releases. This energy is transmited to the surface of the earth by
earthquake waves.
The destruction an earthquake causes depends on its
magnitude and duration, or the amount of shaking that occurs. A structure’s
design and the materials used in its construction also affect the amount of
damage the structure incurs. Earthquakes vary from small, unnoticeable shaking
to large shocks felt over thousands kolimeters. Earthquakes can deform the
ground, make buildings and other structures collapse, and create tsunamis
(large sea waves). Live may be lost in the resulting destruction.
Earthquakes occur at a rate of several hundred per day
around the worl. A worldwide networks of seismographs (machines that record
movements of the earth) detects about 1 million small eratquakes per year. Very
large earthquakes, such as the 1964 alaskan earthquake and the 2004 tsunami in
aceh, Indonesia, which caused millions of dollars in damage and thousand of
victims, occur worldwide once every few yaers. Moderate earthquakes, such as
the 1989 tremor in California, USA, the 1995 tremor in kobe, japan, occur about
20 time year. Moderate earthquakes also cause millions of dollars in damage and
can harm many people.
In the last 500 years, several million people have been
killed by earthquakes around the world. Worldwide, earthquakes have also caused
severe property and structural damage. Adequate precautions, such as education,
emergency planning, constracting stronger, more flexible, safely designed
structures, can limit the loss of life and decrease the damage caused by
earthquakes.
Seismologists have been monitoring the frequaency and
locations of earthquakes for most of the 20th century. They
generally classify naturally occurring earthquakes into one of two categories:
interplateearthequakes occur where the crust is fracturing within a plate. Both
interplate and interplate earthquakes may be caused by tectonic of volcanic
forces.
Tectonic earthquakes are caused by the sudden release of
energy stored within the rocks along a fault. The release enrgy is produced by
the strain on the rocks due the movement within the earth, called tectonic
deformation. The effects is like the sudden breaking and snapping banck of a
stretched elastic band.
Volcanic earthquakes occur near active volcanoes but have
the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are
caused by the upward movement of magma under the volcano, which strain the
rocks locally and leads to an earthquake. As the fluid magma rises to the
surface of the volcano, it moves and fractures rocks masses and causes
continuous tremors that can last up to
several hours or days.
