MECHANISM OF EARTHQUAKE (ELASTIC REBOUND THEORY)

image: a house destroyed completely after Gorkha earthquake in Nepal , 2015

Elastic rebound theory explains the origin and mechanism of tectonic earthquake. This theory was proposed by H. F Reid. According to this theory materials of the earth , being elastic can withstand certain amount of stress without undergoing a permanent deformation. This means it is believed that rocks are considered to be elastic. But when the stress exceeds its elastic limit a crack or fracture is developed. Frictional resistance along the fracture prevent the rocks from being slipped off from each other and thus promotes the buildup of the strain . When the stress is maximum then the fracture block are subjected to enormous strain and there comes a stage when the rock cannot bear a more straining i.e. the time when frictional resistance is overcome. At this stage , there occurs a sudden breakdown /slip off of the fractured block to the position of no strain . Thus the energy stored in the rock decades instantaneously causing underground dislocation of rocks and waves of energy are sent out through the earth . The wave of energy released are called seismic wave which is responsible for the vibration of the earth surface during earthquake. The crack or fracture formed due to stress which causes the displacement of rock is called fault. So elastic rebound theory explains the mechanism of tectonic earthquake caused due to the movement along a fault.

Important points to remember for mechanism of earthquake:

Rocks are considered to be elastic and can withstand certain amount of stress.

When the elastic limit of the rock is overcome by the stress, a fracture is developed along the rock.

At the time of maximum stress, the fractured blocks displaces from its original position.

The displaced rock is called fault and the plane through it displaces is called fault plane.

As the energy is stored on the rock but when the rock displaces from its original position the stored energy is released instantaneously in the form of waves.

The released energy in the form of waves is called seismic wave.

Seismic waves are responsible for shaking of the earth crust during earthquake.

 Hence the "Elastic Rebound Theory" explains the origin of tectonic earthquake is associates with movement along the fault.

 

 

 Intensity of earthquake:

The intensity of the earthquake is the measure of the degree of damage and destruction it can cause.

As the magnitude of the earthquake can be measured by the devices called seismograph but the intensity is not measured using any devices as it is the level of destruction cause by earthquake.

Magnitude of earthquake:

Magnitude of an earthquake refers to the amount of energy released during the earthquake.

The magnitude of the earthquake is measured by the device called seismograph.

Magnitude of the earthquake is determined on the basis of amplitude of seismic waves recorded on seismograph.

Both magnitude and the intensity of the earthquake are the basis of classification of earthquake along with :

1)     Cause of origin

a)     Tectonic earthquake

b)     Non tectonic earthquake

2)     Depth of focus

Determining the characteristics of earthquake on the basis of depth of focus.

a)     Surface earthquake

b)     Shallow-focus earthquake

c)      Intermediate-focus earthquake

d)     Deep-focus earthquake