Earthquake and Seismic Waves
change in shape or size of an object/body when a force is applied is called deformation. there are two types of deformations
i) Elastic deformation: deformation that disappears after the removal of the force is called elastic deformation, also known as temporary deformation.
ii)Inelastic deformation: deformation that stays even after the removal of the applied force is called inelastic deformation, also known as permanent deformation.
Consider an example of breaking a wooden scale. place a wooden scale on your knees and apply some force on both ends of the scale, we will notice that the scale has bent (deformed), now remove the force we will observe that the scale gets backs to its original state. when we applied the force on both ends of the scale the applied force is stored as elastic energy in the scale and the elastic energy is released when we removed the force.
Consider the same scale again and this time we will not remove the applied force instead we keep on increase the applied force, every object has an elastic limit once the scale reaches its elastic limit and if we still apply the force then eventually it will break. when the scale breaks the stored elastic energy is released as heat energy, partly as sound waves and vibrations.
The earthquake has similar kinds of vibrations that we saw when the scale breaks.
Seismic wave
Seismic waves: waves that travel through the rock are called seismic waves, these waves are produced by earthquakes and explosions.
the device that detects seismic waves is called a seismograph. The actual recording made by a seismograph, showing the various seismic waves as they arrive at the seismic station, is called a seismogram
An earthquake produces two types of seismic waves, they are;
i)Body waves
ii)Surface weaves
Body waves are generated due to the release of energy at the focus of an earthquake and move in all directions traveling through the body of the earth, when body waves interact with the surface rocks they generate new kinds of waves known as surface waves these waves travel along the surface of the earth hence they are called as surface waves
The velocity of these waves keeps changing as they travel through materials of different densities. the denser the material is the higher the velocity. the direction also keeps changing as the waves may reflect or refract when traveling through the different materials of different densities.
Note: reflection causes waves to rebound and refraction causes waves to move in different directions.
Body waves
Body waves are further classified into two types
i) P - waves
ii) S- waves
P - waves
P-waves are the first to arrive at the surface (seismic
station). they are similar to sound waves as they travel through the three
mediums of matter (i.e; solid, liquid, and gaseous). these waves tend to
vibrate parallel to the direction of propagation of the wave, which causes
density differences. in the material
through which they travel. p-waves are responsible for the elongation and
squeezing of a material. P-waves move at a speed of 5 to 8 km/second depending
on the density and character of crustal rocks. P-waves are also referred to as
compressional waves
S - waves
S-waves arrive at the surface after the P-waves. they only travel through the solid medium which is a quite unique and important characteristic of the S-waves, it helped scientists to understand the interior of the earth. the direction of vibration of the P-wave is perpendicular to the direction of propagation of the wave, which creates crust and trough in the material through which they pass. S-waves move about 60% fast than P waves in crustal rocks. S waves are also referred to as shear waves.
Surface waves
Surface waves are further classified into two types
i)Love waves
ii)Rayleigh waves
Love waves
love waves are the fastest moving surface waves, they move the ground side by side. only confined to the surface of the crust, produces horizontal motion.
Ryleigh wave
rolls the ground just like a wave rolls across an ocean or lake, as it roles it moves the ground up and down and side to side in the same direction wave is moving. most shakings felt during an earthquake are due to this wave.
Determining Earthquake Locations
the point where the energy is released first during an earthquake and from which seismic waves start traveling outwards is called Hypocenter or focus. the focus may not be a simple point rather it can be a region up to a few Km along the fault plane. the focus is at some depth below the surface so it is not always convenient to identify an earthquake using it. A more convenient way to identify is by using Epicenter. epicenter refers to the point on the surface of the earth that lies exactly above the Focus. The usual way to describe the location of an earthquake is to state its focal depth(depth at which an earthquake ) and the location of its epicenter.
P waves & S waves travel at different velocities, we also know that P waves travel faster than S waves. the fast the P waves move S wave lag behind it, which results in the greater time lag between the arrival of both the waves at a seismograph. this time lag can be used to calculate the distance traveled by the waves, seismologists use this time lag to calculate the distance traveled by waves (say d), now using seismograph station as the center and "d" as the radius draw a circle. when the data from three or more seismographs are available we can draw three or more circles the point of intersection of these circles is the epicenter of the earthquake.
Determining the Focal depth is not as easy as the epicenter. If the focus is very deep and far from the seismic station, the calculated distances will overshoot the epicenter. then the circles we draw will overlap in a small area instead of a single point, time travel, path distance, and arrival time can reveal the depth at which an earthquake occurred.
knowing the location is one thing, we should also calculate the amount of energy released at the time of an earthquake, the energy released during earth can be Referred to as the magnitude of the earthquake.
it was found that 1 cubic meter of deformed rock can store 100J of elastic energy which is equivalent to 25 calories of heat, it's a very less amount of energy. the problem arises when billion or more than billion cubic meters of rocks are deformed then an enormous amount of energy is released which is generally called earthquakes. the largest earthquake of the highest magnitude 9.5 took place in chile in 1960, there are only other two earthquakes that are reported of magnitude greater than 9.0 one is of magnitude 9.2 took place in Alaska in 1964 and the other one is Sumatra- Andaman earthquake happened in 2004 with a magnitude of 9.1. magnitudes are of different scales, The most common are the Richter magnitude scale, the moment magnitude scale, and the modified Mercalli intensity scale.
