- Earthquakes are caused due to release of energy. 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. As the overlying rock strata press them, the friction locks them together.
- However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly. This causes a release of energy, and the energy waves travel in all directions.
This release of energy, along a fault line, may be due to several factors. They may be categorised as:
Natural causes
Tectonic earthquakes
- The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet.
- The Earth’s crust consists of seven large lithospheric plates and numerous smaller plates and the edges of the plates are called the plate boundaries. These plates move towards each other (a convergent boundary), apart (a divergent boundary) or past each other (a transform boundary).
- The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Earthquakes are caused by a sudden release of stress along these faults in the earth’s crust.
- As seen in below figure, most of the earthquakes take place along the plate boundaries. A rather more susceptible region around the pacific plate is called ‘ring of fire’ due to very high frequency of earthquakes in the region.
- The continuous motion of tectonic plates causes a steady build-up of pressure in the rock strata on both sides of a fault. It continues until the stress is sufficiently great that it is released in a sudden, jerky movement. The resulting waves of seismic energy propagate through the ground and over its surface, causing the shaking we perceive as earthquakes.
- There are mainly 3 types of faults along the plate boundaries as shown in figure
Volcanic earthquakes
- Volcanic earthquakes are caused by slip on a fault near a volcano. Volcanoes are often found in areas of crustal weakness and the mass of the volcano its self adds to the regional strain.
- They occur as a result of regional strain exerted in an area of weak faults. They can also be generated from changes of pressure under the volcano caused by the injection or removal of magma (molten rock) from the volcanic system.
- After the withdrawal of magma from a system, an empty space is left to be filled. The result is a collapse of surrounding rock to fill the void, also creating earthquakes.
- They are generally not as powerful as tectonic quakes and often occur relatively near the surface. Consequently, they are usually only felt in the vicinity of the hypocentre.
Anthropogenic causes or induced seismicity
- Induced seismicity refers to typically minor earthquakes and tremors that are caused by human activity that alters the stresses and strains on the Earth’s crust. Most induced seismicity is of a low magnitude.
- In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors. These are called collapse earthquakes.
- Ground shaking may also occur due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes.
- The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.
