All natural earthquakes take place in the lithosphere. The study of seismic waves provides a comprehensive picture of the layered interior. An earthquake is shaking of the earth’s crust. It is caused due to the release of energy, which produces waves that travel in all directions. The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions.
Types of Earthquakes
Tectonic earthquakes: These are produced due to sliding of rocks along a fault plane.
Volcanic earthquake: These are confined to areas of active volcanoes.
Collapse earthquake: The roofs of underground mines collapse causing minor tremors in the areas of intense mining activity.
Explosion earthquakes: These occur due to the explosion of chemical or nuclear devices.
Reservoir induced earthquakes: These occur in the areas of large reservoirs.
Causes of Earthquakes
It is caused due to the discharge of energy from faults and cracks in the crust of the earth. A fault in the crust of Earth is essentially a sharp break in crustal rocks. This energy release produces waves which travel in all directions. The point where energy is released is called the focus or hypocentre. It is generally located at the depth of 60 km. This causes a release of energy, and the energy waves travel in all directions. The point on the surface which is nearest to the focus is called epicentre. It is the first one to experience the waves. It is a point directly above the focus.
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 similar to sound waves. They travel through all three mediums - solid, liquid and gas. These waves have a tendency 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 these 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 epicentre was identified as the shadow zone for both the types of waves. 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.
Effects of Earthquake
The following are the immediate hazardous effects of Earthquake:
- Shaking of ground
- Disparity in ground settlement
- Natural disasters like tsunami, land slide, mud slides and avalanches
- Soil liquefaction
- Ground lurching and displacement
- Floods and fires
- Infrastructure collapse
Measurement of Earthquake
All earthquakes are different in their intensity and magnitude. The instrument for measurement of the vibrations is known as Seismograph.
Richter scale is used to measure the magnitude of earthquake. The energy released during a quake is expressed in absolute numbers of 0-10.
Mercalli scale is used to measure the intensity of earthquake. It measures the visible damage caused due of the quake. It is expressed in the range of 1-12.