Earthquakes is a sudden violent movement of the Earth's surface. It occurs when a lot of energy , stored within the earth , is released . This energy is transmitted to the surface of the earth by the earthquake waves . The shaking of Earth's crust determines the intensity of earthquake. Earthquakes very from a small im perceptible shaking to large shocks felt over thousands of miles . They can deform the ground , make buildings and other structures collapse , and create tsunamis ( large sea waves ) . The study of earthquakes is called seismology . The scientists who study earthquakes are called seismologists seismographs are the machines that record the earthquakes.

The point within the earth where an earthquake originates is called the focus . The point on the Earth's surface directly above the focus is called the epicenter . If the focus is near the surface . Between 0 and 70 km deep , a shallow focus earthquake is produced . If it is between 70 and 700 km deep , a deep focus earthquake is produced . As the shallow focus earthquakes occur near the surface , they tend to be larger and more damaging .

Causes :

1. Faults

The theory is that the Earth's crust is made of tectonic plates . These plates are laying side by side . The points where plates meet are called faults . Faults are basically cracks between these plates on the Earth's surface or within the earth . The surface between the two side of a fault lies in a plain . Sometimes, the fault plain dips into the ground .

The rocks hanging over the dipping fault plain slips downward into the ground . It is called a normal fault . When the hanging wall slips upward , it is called a reverse fault . Both these faults produce vertical displacement . Sometimes the tectonic plates slide horizontally in the fault plain . Such a slip in the fault plain produces horizontal displacements . The faults where such a movement takes place are called strike faults .

2. Waves

The sudden movement of rocks along a fault causes vibrations . These vibrations transmit energy through the earth in the form of waves . These waves are called seismic waves . They are classified into four types . They are p ( primary) waves , S (secondary ) waves , love waves and Rayleigh waves . P and S waves are produced within the earth . The other two waves are produced along the surface of the earth.

The P wave travels at the highest speed. It can be transmitted by both solid and liquid materials . With a P wave the transmitting rocks are alternately compressed and expanded

The S wave , travels only through solid material within the earth. The S waves are known as shearing waves . They move the ground back and forth .

Love and Rayleigh waves travel from the apicenter along the surface of the earth . These waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other stuctuers .

3. Effects

Earthquakes result in landslides and other soil movement . These events cause an enormous loss of property and life . Earthquake waves shake building's and cause weak structures totally collapse . If an earthquake occur in an ocean , it may cause tusnami waves .

Secondary damage of the earthquake includes economic loss , diseases , lack of food and dearth of clean water . Catastrophic earthquakes can spread contagious diseases . Damage to water supply lines and sewage lines may lead to the conditions that spread infectious diseases.

4. Prediction

Seismologists try to predict earthquakes . This prediction may include a specific time , place , and size of the expected earthquake. In this regard. The scientists take help from the previous record of earthquakes . This helps them to determine the expected repetition of earthquakes. Seismolgist also measure the movement along major faults. In spite of all this , the prediction just tells about the likelihood of an earthquake in a particular area and in a specific time interval . But it's occurence should always be regarded as a chance events.