How Earthquakes are caused on the world?

Let us briefly start describing how earthquakes are cause by talking about the surface of the Earth that consists of seven major tectonic plates and many smaller ones. These plates move in different directions and at different speeds from those of the neighboring ones:

– Sometimes, the plate in the front is slower; then, the plate behind it comes and collides and mountains are formed.

– On the other hand, sometimes two plates move away from one another and rifts are created.

– In another case, two plates move side-by-side, along the same direction or in opposite directions.

These three types of inter-plate interactions are the convergent, divergent and transform boundaries (Figure 1), respectively. The relative movement of these plate boundaries varies across the Earth; on an average, it is of the order of a couple to tens of centimeters per year.

Rocks are made of elastic material, and so elastic strain energy is stored in them during the deformations that occur due to the gigantic tectonic plate actions that occur in the Earth. But, the material contained in rocks is also very brittle. Thus, when the rocks along a weak region in the Earth’s Crust reach their strength, a sudden movement takes place there (Figure 2); opposite sides of the fault (a crack in the rocks where movement has taken place) suddenly slip and release the large elastic strain energy stored in the interface rocks. For example, the energy released during the 2001 Bhuj, India earthquake is about 400 times (or more) that released by the 1945 Atom Bomb dropped on Hiroshima!

The sudden slip at the fault causes the earthquake…. a violent shaking of the Earth when large elastic strain energy released spreads out through seismic waves that travel through the body and along the surface of the Earth. And, after the earthquake is over, the process of strain build-up at this modified interface between the rocks starts all over again (Figure 3). Earth scientists know this as the Elastic Rebound Theory. The material points at the fault over which slip occurs usually constitute an oblong three-dimensional volume, with its long dimension often running into tens of kilometers.

Types of Earthquakes: Most earthquakes in the world occur along the boundaries of the tectonic plates and are called Interplate earthquakes. A number of earthquakes also occur within the plate itself away from the plate boundaries; these are called Intra-plate earthquakes. In both types of earthquakes, the slip generated at the fault during earthquakes is along both vertical and horizontal directions (called Dip Slip) and lateral directions (called Strike Slip) (Figure 4), with one of them dominating sometimes.

by : http://struczone.com