Monday, September 6, 2021

How earthquake occurs?

In this article we will discuss in a simple way how earthquake occurs? What are the reasons behind it? How it is measured? What are some types of seismic waves? and many more things. Before we start let us have a look at the interior of the Earth.

Interior of Earth

We know that Earth is made up of a thin crust, a thick mantle which occupies most of the volume of Earth (about 84-85%) and a core whose outer part is molten while the inner part is solid. Earth’s outer skin is made from the crust and the outer layer of mantle, and this skin is not a continuous surface. This outer skin of Earth is composed of massive segments called tectonic plates, and the edges of these tectonic plates are called the plate boundaries.

There are some cracks within these plates which is known as “faults”. So basically these tectonic plates keep slowly moving around and sliding past one another. Most of the Earthquakes occur along the boundaries between tectonic plates or at the “faults”.

How earthquake occurs?

The tectonic plates that we discussed above, are moving constantly, and as the edges of these plates slide against each other in fault zones, then due to the friction these plates can be slowed down, which ultimately leads to very pressure generation over long periods of time. This process will continue until the force of movement overcomes the frictional force, and few parts or rocks of the crust suddenly break or become displaced which eventually release the generated pressure in the form of seismic waves that make the ground shake and hence the earthquake occurs. This type of earthquake is also known as tectonic earthquake.

Types of energy waves

P waves – P waves or primary waves are compressional waves that push and pull as they move through rock and fluids, and they are the first type of waves that was detected. S waves – S waves or secondary waves move either up and down or side to side, in the right angle to the direction in which the wave is moving. S waves move only through rock and they were detected after the P waves. S waves are slower than the P waves.

Since P waves travel faster, they shake the ground first. After that the S waves follow and shake the ground also. If we are close to the earthquake, then P and S wave will come one right after the other, but if we are far away, there will be some time gap between them.

How earthquakes are measured?
"Seismograph" by Hitchster

Scientists measure the magnitude of earthquakes by an instrument called seismographs and the recordings that are made using this instrument is called seismogram. The base of seismograph sets firmly in the ground, and there is a very heavy weight that hangs free. Whenever an earthquake occurs, it causes the ground to shake, which eventually causes the base of the seismograph to shake, but the hanging heavy weight does not shake. But why?

Because the heavy weight is attached with a spring or string, and this spring or string absorbs all of its movement during an earthquake. Scientists then record the difference in position between the shaking part of the seismograph and the motionless part i.e. hanging weight. If the seismogram shows short jiggly line that means it was a mild earthquake, and if it shows long jiggly line that means it was a large earthquake.

Other causes of earthquake

Till now we have only discussed about the natural earthquake which is also known as tectonic earthquake. But there are other ways too by which earthquake can occur. For example, before volcanic eruptions when the magma moves toward the surface, then it produces seismic waves. These seismic waves can be felt like natural earthquake.

Another example is underground nuclear explosions, which create seismic waves like any large scale earthquake. Seismic waves can also be generated during mining operations or subway construction. 

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