The shaking of the earth’s ground, typically causing great destruction due to the movements within the earth’s crust is called an earthquake. We have discussed broadly the earthquake in another post (click here). Here we will discuss deep into this phenomenon that can be asked into UPSC.
Why does the earth shake?
It is just a quick recap of the post (where we have discussed, why the earth shakes). The release of energy occurs along a fault. A fault is a sharp break in the crustal rocks. Rocks along a fault tend to move in opposite directions. As the overlying rock strata press them, the friction locks them together. However, their tendency to move apart at some point of time overcomes the friction. As a result, the blocks get deformed and eventually, they slide past one another abruptly.
This causes a release of energy, and the energy waves travel in all direction. The point where the energy is released is called the focus/ hypocenter of an earthquake. The energy waves travelling in different directions reach the surface. The point on the surface nearest to the focus is called the epicentre.
When two tectonic plates collide with each other or slide past each other, energy is released causing an earthquake. This energy travels in all direction in the form of waves. These waves are of two types viz. Body waves and surface waves. Body waves are generated due to the release of energy at the focus and move in all directions travelling through the body of the earth. Hence the name body waves.
The body waves interact with the surface rocks and generate a new set of waves called surface waves. These surface waves are the most destructive waves. The velocity of waves changes as they travel through material with different densities. The denser the material, the higher is the velocity.
There are two types of body waves. They are called P-waves or Primary waves and other is called the S-waves of Secondary waves. P-waves are faster and they are first to arrive at the surface. They travel through all the three materials viz. gaseous, liquid and solid. S-waves arrive at the surface with some time lag. S-waves can travel only through solid materials.
Propagation of Earthquake Waves
So, there are four types of earthquake waves. They all travel in different manners. As they move or propagate, they cause vibration in the body of the rocks through which they pass. P-waves vibrate parallel to the direction of the wave. This exerts pressure on the material in the direction of the propagation (like a spring). This creates density differences in the material leading to stretching and squeezing of the material.
Other three waves vibrate perpendicular to the difference of propagation. Hence they create trough and crests in the material through which they pass. Therefore, surface waves are considered to be the most damaging waves.
Shadow Zone/ Body Waves
Scientists have placed the seismographs all over the earth to understand the nature of the earthquake. When an earthquake occurs, there exist some areas where waves are not reported. These areas are called ‘Shadow Zone’.
It was observed that seismographs located at any distance within 105° from the epicentre, recorded the arrival of both P and S-waves. Thereafter, both the waves disappear. However, beyond 145° from the epicentre, P-waves appears but not the S-waves. Thus a zone between 105° to 145° is the shadow zone of both the waves. And the shadow zone of S-waves is the entire area beyond 105° from the epicentre.
Yes, the earthquake is not experienced in shadow zone.
As we have discussed, surface waves are of two types viz. Rayleigh waves and Love waves.
They are elliptical movement in an anti-clockwise manner.
They move in a zig-zag manner. When these waves slow down, their wavelength increases, hence to travel same distance they exert more push resultantly more damage.
Types of Earthquake
These are the most common type of earthquake which occurs because of either collision of plates or when plates slide past one another.
These earthquake are only confined to areas of active volcanoes
In the areas of intense mining activity sometimes the roofs of underground mines collapse causing minor tremors.
Ground shaking may also occur due to the explosion of chemical or nuclear devices.
Reservoir Induced Earthquake
The earthquakes that occur in the areas of large reservoirs are reffered to as reservoir induced earthquakes.
The earthquake events are scaled either according to the magnitude or intensity or the shock. The magnitude scale is known as the Richter Scale. The magnitude relates to the energy released during the earthquake. The magnitude is expressed in absolute numbers, 0-10.
The intensity scale is named after Mercalli, an Italian seismologist. The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.