An Earthquake is a sudden movement of the ground that releases the elastic energy stored within the rocks, creating destructive seismic waves. The word "seismic" comes from the Greek word "seismos" meaning an earthquake. These quakes are not isolated events. They come with smaller shocks, called aftershocks, with smaller effects.
An earthquake is caused when two sides of a large fracture in the rocks within the earth slide past each other. This fracture is called a fault, which may be microscopic or thousands of kilometers in length, while their width is usually a few millimeters or a few meters in size.The size of an earthquake depends on the area of the fault that ruptured, and the distance through which the rocks on the two sides of the fault slide past one another.Small earth quakes are caused by small faults or small parts of big faults. These last only for a fraction of a second and usually unnoticed, as the rocks on either side of the faults are not displaced much. The larger ones are caused due to faults which are tens to thousands of kilometers long, taking minutes and the displacement of the rocks is around tens of meters.
Earthquakes are caused because the earth is regularly cooling since it was born, the seismic wave being one of its ways to release the elastic energy.
Earthquakes are generally categorized into three types: Tectonic, Volcanic and Artificial.
Tectonic earth quakes are the most devastating, and unfortunately the most unpredictable. The volcanic quakes are seldom important or devastative, but they help predict the eruption of volcanoes. The artificial ones are caused due to human activities, like pumping fuels deep into the earth or due to explosives, and several other reasons.
MEASUREMENT OF EARTHQUAKES:
The most popular scale used to measure earthquakes is the Richter scale, named after Charles Francis Richter (US).This scale measures the energy released at the focus of the quake, read through a seismograph which gives the direction and force of the quakes. An earthquake of size 8 is 10 times more devastative than size 7, 100 times more destructive than 6, and so on. the frequency of earthquakes decreases as the magnitude increases, as the earth experiences about 800 earth quakes of size 5-6 , but only one or two quakes of size 8-9.
Earthquakes cause great loss of life by causing damage to buildings, bridges, dams, etc. they can trigger devastating landslides. Fires can begin due to leaks in Gas plants and electrical lines. Earth quakes can also generate tidal waves, or the tsunamis, capable of destroying entire townships. These quakes also cause liquefaction of soils, which makes soil lose almost all its strength, which makes it function like quicksand. This material is capable of swallowing an entire building within itself.
Earthquakes around the world cost around 15,000 lives per year. Several countries like the United States of America, China, Japan and Russia are most actively participating in seismological research, trying to predict future earthquakes.