NCERT Notes: Earthquake - Body Waves, Causes & Types [Geography Notes For UPSC]

• All natural earthquakes occur in the lithosphere.
• Seismic wave studies offer a full picture of the layered interior.
• An earthquake is, simply put, shaking of the earth’s crust.
• It is caused due to the energy release, which triggers waves that travel in all directions.
• The emanation of energy occurs along a fault.
• A fault is a sharp break in the crustal rocks.
• Rocks along a fault generally move in opposing directions.

• Tectonic earthquakes: The most common form of earthquakes, is caused by the movement of loose fragmented pieces of land on earth’s crust knowns as tectonic plates.
• Volcanic earthquake: The less prevalent compared to the tectonic variety, these earthquakes happen before or after the eruption of a volcano. It is caused when magma leaving the volcano is filled by rocks being pushed to the surface.
• Collapse earthquake: This earthquake occurs in underground mines. The main cause is pressure generated within the rocks.
• Explosion earthquakes: The occurrence of this type of earthquake is artificial. High-density explosion such as nuclear explosions is the primary cause.

• It is caused due to the tectonic movements of the earth.
• The energy release produces waves that travel in all directions.
• The point where energy is released is called the focus or hypocentre. It is generally located at a depth of 60 km.

• This causes a release of energy, and the energy waves travel in all directions.
• The point where the energy is released is called the focus of an earthquake or hypocentre.
• The point on the surface of the earth which is vertically above the focus is called the epicentre. It is the first place to experience the waves.

• Earthquake waves are of two types — body waves and surface waves.

P- Waves

• P-waves are also known as the Primary waves. They are the first waves to arrive at the surface.
• The characteristics of P-waves are like sound waves. They travel through all three mediums- solid, liquid, and gas.
• These waves tend to vibrate parallel to the direction of wave propagation. This causes density differences in the material through which they travel.
• These waves are responsible for elongating and squeezing of material.

S- Waves

• S- Waves arrive after some time after the happening of Earthquake and they are called secondary waves.
• A significant characteristic of these S-waves is that they travel only through a solid medium.
• The direction of vibration of this S-wave is perpendicular to the direction of wave propagation, thereby creating crests and troughs in the material of their transmission.

• The shadow zone is the zone of the earth from angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves.
• The shadow zone results from P waves being refracted by the liquid core and S waves being stopped completely by the liquid core.
• A zone between 105° and 145° from the epicentre was recognized as the shadow zone for both the wave types.
• The entire zone beyond 105° does not receive S-waves.
• The shadow zone of S-wave is larger than that of the P-waves.
• The shadow zone of P-waves appears as a band around the earth between 105° and 145° away from the epicentre.

The following are the immediate hazardous effects of Earthquake:

• Shaking of ground
• The disparity in ground settlement
• Natural disasters like Tsunami, landslide, mudslides, and avalanches
• Soil liquefaction
• Ground lurching and displacement
• Floods and fires
• Infrastructure collapse.

All earthquakes are different in their intensity and magnitude. The instrument for the measurement of the vibrations is known as Seismograph.

Magnitude scale

• Richter scale is used to measure the magnitude of the earthquake
• The energy released during a quake is expressed in absolute numbers of 0-10.

Intensity scale

• The Mercalli scale is used to measure the intensity of an earthquake
• It measures the visible damage caused due to the quake.
• It is expressed in the range of 1-12.