# Origin of earthquakes (Elastic Rebound Theory)

Earthquakes are classified on two bases, one is surface causes, volcanic eruption, and tectonic movement. Among these types, volcanic earthquakes are the most common and disastrous, therefore discussing the tectonic type of earthquakes is very important. The reason behind tectonic earthquakes is the tectonic movement of crustal plates. There are three types of tectonic movements.

1. When two plates move towards each other, this is called the convergent movement. In convergent movement, one plate submerges the other plate. As a result of the convergent movement of the plates, the hard rocks of the submerging plate are shoved beneath the overlapping plate, thus intruding into the mantle. The internal molten magma also extrudes simultaneously. The overlapping plate and the extruded magma become the source to form mountains on the earth’s surface. During this type of movement volcanic eruptions take place.
2. At some tectonic boundaries, two plates move apart from each other, thus leaving a gap between the plates. This type of movement is known as the divergent boundary. The divergent boundary allows the surface to expand and gives passage to the internal magma to flow out through the crust. In these types of plates, movement faults are created on the continental surface and seafloor. Mid-oceanic ridge is the outcome of such a movement.
3. While the third type of plate movement is the transverse boundary, where two plates move transversely (across) to each other. This type of plate movement is causes earthquakes. To understand this phenomenon we need to understand the elastic rebound theory.

### Elastic Rebound Theory:

The elastic rebound theory was put forward by H.F. Reid. The main points of the elastic rebound theory are as under;

1. In certain zones of the earth’s crust, the stress is accumulated in the rock masses.
2. When the stress exceeds the elastic limit of the rocks, they bend and crack. the slippage along the fracture is initially prevented by friction. Under these conditions, the rocks will store up a huge amount of energy as elastic strain. Some slight movement along the fracture plane produces ”foreshocks”.
3. When the strain reaches a critical point, the friction is overcome and slippage takes place. The fractured blocks snap back to their original shape, thereby releasing the stored up energy in the form of an earthquake by vibrating back and forth. The springing back of the rocks is called the ”elastic rebound action”. Some adjustments along the fault zone after the main earthquake produces ”aftershocks”.

With the recent developments in the field of plate tectonics, we now know that crustal plates are in constant motion. While moving, the plates may either thrust against one another or may drift away. Where they move apart, oceanic ridges are formed, and where they converge and collide, mountain chains are created. Almost all the world’s earthquakes occur along the lines of joining one crustal plate with another.