EARTHQUAKE

          An earthquake is a common occurrence which is sometimes felt and sometimes goes unfelt. It is an important agent that brings on changes in the earth crust.
          An earthquake is a shaking of the ground due to some reason known or unknown, external or internal. There are two kinds of quake; 

(1) vertical quake which occurs up and down and 
(2) horizontal quake which passes left to right. The former is more dangerous than the latter. The point where the shaking originates is called focus or hypocenter and the point on the earth's crust directly over the hypocenter where the quake is first felt is known as epicenter. The instrument which measures and records the magnitude of the seismic waves of the earthquake is called seismograph. It also finds out the origin, center, direction, speed and extension of the earthquake. The magnitude of the earthquake is measured in Richter scale. There are three kinds of the waves: a. Primary or Longitudinal
b. Secondary or Transverse
c. Surface or Long period waves.

4.1 Causes of the Earthquake

      a. Volcanic Activity
          The melted rocks, water vapours and gases in the interior parts of the earth move upwards due to excessive pressures and give violent pushes to come out. When they find a weak spot, they erupt violently. The tremor thus caused, is called earthquake.
  b. Tectonic
          Sometimes earthquake occurs due to the disturbance in the internal part of the earth. The inner part of the earth sometimes contracts or shrinks. Consequently, the earth begins to shake.

4.2 Areas of the Earthquake

a.     Circum Pacific Belts

This belt is associated with the Circum Pacific belt of mountains i.e., the Rockies,

b.     Mediterranean and Himalayan Zone

It follows the mountains bordering the Mediterranean Sea, Alps, Caucasus and the Himalayas. Nearly 21 percent of the earthquakes of the world originate in this belt.

4.3 Effects of the Earthquake

Disadvantages
a.     Due to the severe earthquakes, portions of the earth sink down and some portions rise up.
b.     Earthquakes cause landslides.
c.      With the occurrence of the earthquake, chasms are created from where sand and water come out on the surface of the earth.
d.     Sometimes when an earthquake takes place, the courses of the rivers and glaciers are changed.
e.      It is a great destroyer. Thousands of people are killed, wounded and buried alive in the ground. Houses, factories, bridges, etc. collapse. Roads and railways are ruined. Crops are destroyed.
Advantages
a.     Sometimes lakes are formed by earthquake, and the lakes can be used for irrigation, generating electricity and fish-rearing.
b.     New land forms are created by earthquake.
c.      Rocks break due to an earthquake. Such broken rocks change into soil because of quick weathering.
d.     Sometimes the sea coast sinks down and natural ports are created.
e.      Several kinds of minerals and mineral water come out from the inner part of the earth.