Earthquakes are caused by active faults, which are, caused by the sudden movement of the two sides of a fault with respect to another. The occurrence of tectonic earthquakes can be explained by the theory of elastic rebound, first advanced by H. B. REID.
Elastic Rebound Theory
The motion along the fault is accompanied by the gradual buildup of elastic strain energy within the rock along the fault. The rock stores this strain energy like a giant spring being slowly tightened.
Eventually, the strain along the fault exceeds the limit of the rocks at that point to store any additional strain. The fault then ruptures – that is, it suddenly moves a comparatively large distance comparatively short amount of time. The rocky masses which form the two sides of the fault then snap back into a new position. This snapping back into position, upon the release of strain, is the “ELASTIC REBOUND” of Reid’s theory. The rupture of fault results in sudden release of the strain energy that has been built up over the years. The most important form which this suddenly released energy takes is that of seismic waves, which cause earthquakes and destruction.
Movement of Tectonic Plates
There are 4 different types of Tectonic movements
1. Strike-slip fault
Fault sliding against one another
2. Thrust fault
Both plates push upwards. It creates shorter & wider mountain ranges.
3. Down-dropped fault
Plates pull away from each other. It creates shorter & wider mountain ranges.
4. One plate pushes below the other plate
Volcanic earthquakes occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes. Volcanic earthquakes are caused by the upward movement of magma under the volcano, which strains the rock locally and leads to an earthquake. As the fluid magma rises to the surface of the volcano, it moves and fractures rock masses and causes continuous tremors that can last up to several hours or days. Volcanic eruptions give rise to earthquakes.
Earthquakes due to Man-made Activities
Human activities can also be the direct or indirect cause of significant earthquakes. Injecting fluid into deep wells for waste disposal, filling reservoirs with water, and firing underground nuclear test blasts can, in limited circumstances, lead to earthquakes. These activities increase the strain within the rock near the location of the activity so that rock slips and slides along pre-existing faults more easily.