An earthquake is a motion of the ground surface, ranging from a faint tremor to a wild motion capable of shaking building apart.
An earthquake is a form of energy of wave motion transmitted through the surface layer of the earth.
All the earthquakes are not of the same intensity.
Some of them are very severe, others are very mild and still, others are not even noticed.
Major or strong earthquakes are only a few.
Though our earth experiences many earthquakes every day, however, the frequency of earthquakes varies largely from place to place.
The instrument used for recording the earthquakes is known as seismograph.
The point within the earth’s crust where an earthquake originates is called the focus. It is also referred as seismic focus.
It generally lies within the depth of 60 kilometers in the earth crust.
The point vertically above the focus on the earth’s surface is known as ‘epicenter’.
Earthquake waves originating from the focus travel in all directions.
But their intensity is the highest at the epicenter that is why the maximum destruction occurs at and around the epicenter.
The intensity of vibrations decreases as one move away from the epicenter in all directions.
Causes and Effects of Earthquakes
Folding, faulting and displacement of rock strata are the main causes of earthquakes.
Some examples of this type of earthquakes are the San Francisco earthquakes of California in 1906, the Assam earthquakes of 1951, the Bihar earthquakes of 1935.
The second important cause lies in the plenomenon of volcanic eruption.
It causes vibrations in the earth’s crust.
But, these earthquakes are limited to the areas of volcanic activity.
It’s important example is the earthquake which continued for six days preceding the eruption of Mauna Loa volcano of Hawaii Island in 1868.
Minor earthquakes often accompany or are the result of landslides, seepage of water causing the collapse of the rocks of cavern or underground mines and tunnel.
These are the least damaging earthquakes.
Violent earthquakes are generally very disastrous.
An earthquake often forms cracks and fissures in the earth’s crust. It changes the drainage system of an area as was witnessed in Assam after its 1951 earthquake.
Earthquakes also cause vertical and horizontal displacement of rock strata along the fault line.
They prove the most catastrophic and devastating when they cause fires and seismic sea waves.
Such tidal waves are called Tsunamis.
Distribution of Earthquake
There are two well-defined belts where Earthquakes occur more frequently.
These belts are the Circum-Pacific belt and the Mid-world mountain belt.
The first belt i.e., the Circum Pacific comprises the western coast of North and South America; the Aleutian Islands and island groups along the eastern coasts of Asia such as Japan and Philippines.
It encircles the Pacific Ocean from end to end.
The earthquakes in this belt are associated with the ring of mountains and volcanoes.
It is estimated that about 68 percent of earthquakes of the world occur in this belt alone.
The second belt-extends from the Alps with their extension into the Mediterranean the Caucasus and the Himalayan region and continues into Indonesia.
About 21, percent of total earthquakes of the world originate in this belt.
Remaining 11 percent occur in the other parts of the world.