Question

What is an epicentre ?

Answer 1

Hint: In order to answer this question, we will put some light on the topic of an earthquake. An epicentre is a major part of this topic and we will try and understand it.

Complete answer:
Many of us have heard about this natural disaster or natural phenomena or even may have experienced it, which is the earthquake. Earthquake (also called a quake, tremor or tremblor) is the shaking of the surface of the earth resulting from a sudden release of energy in the earth’s lithosphere that creates seismic waves.

Earthquakes can be compared with one another by the energy they release, known as their magnitude or by their intensity of shaking, referred to as ground motion and the resulting impact on people and society. There are two basic terms or rather places, from where the earthquake occurs.
1) Focus: It is a point where rocks start to fracture. It is the origin of earthquakes and it is also called as Hypocentre.
2) Epicentre: The point directly above the focus, on the surface is called as epicentre.
When an earthquake occurs, it releases the stored up energy (Elastic Rebound Theory) in the “seismic waves” that travel away from the focus. Seismic waves travel uniformly but the effect is not uniform.

There are two types of waves, 1) Body waves (P & S waves) and 2) Surface waves (Rayleigh waves & Love waves). These (Specially P and S) waves are very much useful to find the epicentre through the “Triangulation Method”. P waves travel faster than S waves from rocks. Therefore, at points some distance away from the scene of earthquake, P waves arrive before the S waves. The difference in arrival times of the first P wave and S wave is a function of distance to the earthquake’s epicentre. The farther the receiving seismograph is from the epicentre, the greater the time lag between the first arrivals of P and S waves.

Note: It is important to note here that the epicentre lies on the surface of the earth exactly above the focus of the earthquake. The focus point is within the surface of the earth from where the earthquake emerges. The focus and epicentre are totally different points.