Oleh: Lailatul Fadhilah
Mahasiswa STIK
Earthquakes
An earthquake is the shaking of the Earth’s surface caused by rapid
movement of the Earth’s rocky outher layer. Earthquakes occur when energy
stored within the Earth, usually in the the form of a strain in rocks, suddenly
releases. This energy is transmitted 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 contruction also effect the amount of damage the
structure incurs. Earthquakes vary from small, unnotiable shaking to large
shocks felt over thousands f kilometers. Earthquakes can deform the ground,
make buildings and other structures collapse, and create tsunamis (large sea
waves). Lives may be lost in the resulting destuction.
Earthquakes occur at a rate of several hundred per day around the
world. A worldwide network of seismographs (machines that record movements of
the Earth) detecs about 1 million small earthquakes 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 thousands of victims,
occur worldwide once every few years. Moderate earthquakes, such as the 1989
tremor in California, USA, the 1995 tremor in Kobe, Japan, occur about 20 times
a year. Moderate earthquakes also cause
millions of dollars in damage and can harm many people.
In the last 500 years, several millions 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, and contucting stronger, more flexible, safely designed
structures, can limit the loss of life and decrease the damage caused by
earthquakes.
Seismologist have been monitoring the frequency and locotions of
earthquakes for most of the 20th century. They generally classify naturally
occuring earthquakes into one of two categories: interplate and intraplate.
Interplate earthquakes are the most common; they occur primarily along plate
boundaries. Intrapale earthquakes occur where the crust is fracturing within a
plate. Both interplate and intraplate earthquakes may be caused by tectonic or
volcanic forces.
Tectonic earthquakes are caused by the sudden release of energy
stored within the rocks along a fault. The released energy is produced by the
strain on the rocks due to movement within the Earth, called tectonic
deformation. The effect is like the the sudden breakang and snapping back of a
streched elestic band.
Volcanic earthquakes occur near active volcanoes but have the same
fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused
by the upward movements of magma under the volcano, which strains the rock
locally and leads to an earthquake. As the fluid magma rises to the surface of
the volcano, it moves and fractures rock masses and causes continuos tremors that
can last up to several hours or days.
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