What causes earthquakes and where do earthquakes happen

Earthquake and Earthquake Engineering

An earthquake is a sudden slipping or movement of a portion of the Earth's crust or plates, caused by a sudden release of stresses. Earthquake epicenters are usually less than 25 miles below the Earth's surface and are accompanied and followed by a series of vibrations.

What causes earthquakes and where do earthquakes happen

The earth has four major layers: The inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin layer on the surface of earth. But this layer is not a single cover, it is made up of many pieces like jigsaw covering the surface of the earth. These keep slowly moving around each other, slidepast one another and bump into each other. These puzzle pieces are called tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

Types of earthquakes

Most earthquakes in the world occur along the boundaries of the tectonic plates and are called Inter-plate Earthquakes. A number of earthquakes also occur within the plate itself away from the plate boundaries, called Intra-plate Earthquakes.

How are earthquakes recorded 

Earthquakes are recorded by instrument called seismographs. The recording they made, is called a seismogram. The seismo gram consists of two parts, a base and a weight, to held it firmly in the ground. When an earthquake causes the ground to shake, the base of the seismograph shakes too, but the hanging weight does not. Instead the spring or string that it is hanging from absorbs all the movement. Thus the difference between the moving and immovable part is recorded.

How are earthquakes measured?? 

The size of an earthquake depends on the size of the fault and the amount of slip on the fault, but this cannot be measured directly as faults are deep in the earth. The seismogram recordings made on the seismographs at the surface of the earth are used to determine the intensity of earthquake. A short line with less zigzag portions represents a small earthquake and a lengthy line with a lot of zigzag sections shows a large earthquake. The length of line on the seismograph depends on the size of the fault and the wigginess of the line depends upon the amount of slip of the fault. The size or intensity of earthquake is called Magnitude of earthquake.

Er. Roy