*The sudden tremors or shaking of the earth’s crust is called an earthquake.
*The earth’s crust is made up of different parts of various sizes. They are called plates.
*Most of the earthquakes in the world are caused by the movements of the plates.
*‘Seismology’ deals with the study of earthquake.
*‘Richter scale’ and ‘Mercalli scale’ are the instruments to measure and record the magnitude and the intensity of an earthquake respectively.
*The place where the seismic waves originate beneath the earth’s surface is called the focus of the earthquake.
*The epicenter is that point on the ground surface which is closest to the focus.
*The waves generated by earthquake are called seismic waves and they are classified into 3 types such as:
i. Primary Waves (P Waves): These are the waves of short wavelength and high frequency. They are longitudinal waves and can travel through solid, liquid and gases.
ii. Secondary Waves(S Waves): These are the waves of short wave length and high frequency. They are transverse waves, which travel through all solid particles only.
iii. Surface Waves or Long Waves (L Waves): They are the waves of long wavelength, confined to the skin of the earth’s crust. It causes most of the earthquake’s structural damage.
*There are some specific areas where earthquake waves do not occur or occur rarely, such areas are termed as shadow zones.
*They are located between 105° and 140° from epicentre.
The Earthquake Zones in India
*The Indian plate is moving from south to north. That is why there are earthquakes in the Himalayan regions.
There are three types of Volcanoes:
i. Active Volcanoes ii. Dormant Volcanoes iii. Extinct Volcanoes
Distribution of Earthquakes
Most of the world earthquakes occur in:
*The zones of young fold mountain.
*The zones of lodging and faulting.
*The zone of junction of continental and oceanic margin.
*The zone of active volcanoes.
*Along different plate boundaries.
The Traditional Zones of Earthquakes
i. Circum-Pacific belt
ii. Mid-Continental belt
iii. Mid-Atlantic belt