How Earthquakes occur? Can its time be predicted?

How Earthquakes occur?

The earth crust stores energy due to the movement of rocks or plates. The stored energy build up with time. Take an example of two blocks sliding against each other in the picture below. The blocks can stuck against each other, as the movement is not smooth. Energy will be stored due to the continuous forces and a point will reach where an abrupt breakup will occur. Similar is the case with the earth crust. Tectonic plates move towards, against or along each other. This movement is on a large scale, hence storing a lot of energy. This energy is released in the form of Seismic waves when the strength of a rock cannot bear more stress. These waves cause the shaking of the ground in the form of earthquake.

Can the time of an earthquake be predicted?

The simple answer is 'NO'. However, there are several methods used in past as well as in present to predict the earthquake. Some methods were used in past but were of not that much help. 

1) The emission of radon gas from water wells may occur before the earthquake. They are released due to the change in water level in well, which is caused by the change in the porosity of the subsurface rocks. 

2) Changes in the properties of subsurface rocks i.e. electric resistivity, magnetism and seismic velocity. 

3) Change in elevation of earth surface due to tilting of land may happen before an earthquake. But this is not the only case. Tilting may not only occur due to the earthquake but can be caused by magmatism, drying or wetting of a land, mass wasting and orogenic process. 

4) Foreshocks may occur before a major earthquake. But this doesn't happens always. A major earthquake can also occur without foreshocks. 

5) Some animals behave differently before an earthquake. For example, snakes come out of their holes immediately before an earthquake. Horses become nervous and frustrated before earthquake.

In addition to GPS, new technique like Interferometry Synthetic Aperture Radar (INSAR) is used now a days to find out the movement along the fault zones, which may be helpful to find out the stress accumulated in the rocks and how long would it take to release the energy. This method comes with trials and errors and may be sometimes helpful to forecast earthquake along a fault. There are many fault zones in the world which make it difficult and costly to monitor all.