Why in news?

• Recently a powerful earthquake of magnitude 5.9 on the Richter scale struck a remote town in Afghanistan.

How do earthquakes happen?

• According to the theory of plate tectonics, the Earth’s crust and upper mantle are made of large rigid plates that can move relative to one another.
• Slip on faults near the plate boundaries can result in earthquakes.
• The point inside the Earth where the earthquake rupture starts is called the focus or hypocentre.
• The point directly above it on the surface of the Earth is the epicentre.

Seismic waves:

• Earth has an elastic limit and when the stress is higher than this limit, it breaks.
• Then there is a generation of heat, and energy is released.
• Since the material is elastic, the energy is released in the form of elastic waves.
• These propagate to a distance determined by the extent of the impact & are known as seismic waves.
• An earthquake generates seismic waves that penetrate the Earth as body waves (P & S) or travel as surface waves (Love and Rayleigh).

Measurement of Earthquakes:

• Earthquakes are measured by seismographic networks, which are made of seismic stations, each of which measures the shaking of the ground beneath it.
• In India, the National Seismological Network measures the earthquakes.
• The wave parameters are measured & not the total energy released.
• There is a relationship between the quantum of energy released and the wave amplitude.
• The amplitude of the wave is a function of the time period of the wave.
• It is possible to convert the measured wave amplitude into the energy released for that earthquake which is defined as the magnitude of the earthquake.

Scales used to measure earthquake:

1. Richter scale:

• The magnitude of an earthquake is the logarithm of the amplitude of the waves measured by the seismographs.
• Richter scale magnitudes are expressed as a whole number and a decimal part, for example 4.6 or 8.2.
• The magnitude is expressed in absolute numbers, 0-10.
• An increase of the whole number by one unit signifies a 10x increase in the amplitude of the wave and a 31x increase of the energy released.

2. Mercalli scale:

• It takes into account the visible damage caused by the event.
• The range of intensity scale is from 1-12.

Designating the zones :

• India has four seismic zones (II, III, IV, and V) based on scientific inputs relating to seismicity.
• Previously India was divided into five zones with respect to the severity of the earthquakes.
• Zone-V is the most hazardous and Zone- I is the least hazardous.
• BIS is the official agency for publishing the seismic hazard maps and codes.

Earthquake map of the world :

• Principally three large areas of the earth are:

• The world’s greatest earthquake belt, the circum-Pacific seismic belt (“Ring of Fire”), is found along the rim of the Pacific Ocean, where about 81% of our planet’s largest earthquakes occur.
• Alpide Belt, passes through the Mediterranean region eastward through Asia and joins the Circum-Pacific Belt in the East Indies. About 17% of the world’s largest earthquakes occur here.
• The third prominent belt is the submerged mid-Atlantic Ridge.