Earthquake is defined as shaking or vibrating of earth surface.
Any three causes of earthquake are:
i) Tectonic activities: The surface of earth is affected by the tectonic forces. Tectonic forces include compression and tension in the plates. Due to these forces, faults occur on the earth crust, which make certain portion of the plate to rise up and some other portion to fall down. Such phenomenon produce vibration on the earth surface and earthquake occurs.
ii) Volcanic eruption: During volcanic eruption, lava comes out with a force resulting disturbances on the adjoining area of the earth crust causing an earthquake. Generally, volcanic eruption is followed by earthquake.
iii) Other causes: Other causes of earthquake are: Expansion and contraction of rocks, structures of the land surface, flood, landslide and soil erosion, artificial explosion during the construction of tunnels, roads, mine etc. it disturbs the earth surface and cause earthquake.

The point on the surface of the earth vertically above the hypocenter is called an epicenter of the earthquake.
The science that deals with the study of an earthquake is called seismology.

The scale that measures the magnitude of earthquake is called Richter scale.

On the basis of nature of shaking earthquakes are classified into two types. They are:
i) Vertical earthquake
ii) Horizontal earthquake
In vertical earthquake, the vibration is up and down and in horizontal earthquake, the vibration is lateral. Horizontal earthquake is more devasting than vertical earthquake.

The effects of earthquake on environment are listed below:
i) It may form cracks on the earth's surface and weaken the rocks for landslide.
ii) It destroys life and property.
iii) It destroys development structures like roads, canals, communication networks, electrical supply and water supply.
iv) It collapses monuments, buildings, temple, towers and industries.
v) Water resources get contaminated during earthquake which leads many types of diseases.

Safety measures of earthquake are listed below:
i) Earthquake resistant buildings should be built.
ii) Education concerning the safety measures of earthquake should be introduced in the school curriculum.
iii) The public should be made aware of way of self-protection from earthquake.
iv) Skillful volunteer manpower should be developed and kept alert for emergency rescue.

Precautions to be adopted during earthquake are as follows:
i) As far as possible crouch under the table and protect head from injury. Stand on the frame of door until the shaking stops and stand from the sides of window and outer wall ofhouse.
ii) Help to evaluate disabled people like children, old aged people.
iii) Stay far from big rocks, glasses and weak walls.
iv) Turn off gases, electric current and water supply as soon as possible.
v) Stay in a safety place with patience and follow the instruction broad cast from radio by the government.