know about EARTHQUAKE

An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves.

The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.

 Earthquakes are measured using observations from seismometers

The surface of the Earth is in continuous slow motion. This is plate tectonics--the motion of immense rigid plates at the surface of the Earth in response to flow of rock within the Earth.

There are two main causes of earthquakes.

Firstly, they can be linked to explosive volcanic eruptions; they are in fact very common in areas of volcanic activity where they either proceed or accompany eruptions.

Secondly, they can be triggered by Tectonic activity associated with plate margins and faults. The majority of earthquakes world wide are of this type. Most earthquakes are causally related to compressional or tensional stresses built up at the margins of the huge moving lithospheric plates that make up the earth's surface .

            The immediate cause of most shallow earthquakes is the sudden release of stress along a fault, or fracture in the earth's crust, resulting in movement of the opposing blocks of rock past one another.

            These movements cause vibrations to pass through and around the earth in wave form, just as ripples are generated when a pebble is dropped into water.

                Volcanic eruptions, rockfalls, landslides, and explosions can also cause a quake, but most of these are of only local extent.

            Shock waves from a powerful earthquake can trigger smaller earthquakes in a distant location hundreds of miles away if the geologic conditions are favorable. 

Earthquakes occur where the earth's plates meet along plate boundaries.

For example as two plates move towards each other, one can be pushed down under the other one into the mantle. If this plate gets stuck it causes a lot of pressure on surrounding rocks. When this pressure is released it produces shock waves. These are called seismic waves.

 The waves spread out from the point where the earthquake started is the focus.

 More damage is done near the focus. The point on the earth's surface directly above the focus is the epicentre.