Natural Phenomenon- Earthquake (NCERT / CBSE Revision Notes )
A natural phenomenon that cannot be predicted is an earthquake. An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves.
The earth consists of three major layers, called the crust, the mantle and the core. The core is further divided into the inner core and the outer core. The mantle consists of semi-solid material above which the crust floats. The crust consists of oceans and continents. The crust is divided into several parts, called tectonic plates.
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The regions where one tectonic plate slides against another are referred to as fault zones, and these are the regions where an earthquake is likely to occur. Hence, these zones are referred to as seismic zones.
The place in the interior of the earth where an earthquake occurs is the focus, and the region on the surface of the earth that is the closest to focus is likely to experience the largest damage. This region is called the epicentre of the earthquake.
The instrument that measures the severity of an earthquake is a seismograph. It basically consists of a drum that rolls and a pendulum with a stylus that traces the waves of an earthquake on a sheet like a graph paper. The severity of an earthquake is measured on the Richter scale. A major earthquake measures 7 or more on the Richter scale.