Earthquake

The energy generated by earthquake in the form of waves is the most reliable source to know the interior of the earth.
Seismic waves passing through the earth are refracted in a way that show distinct discontinuities within earth’s interior and provide the basis for the belief that earth has:
a) A solid innercore,
b) A liquid outercore,
c) A soft asthenosphere,and
d) A rigid lithosphere.

What causes earthquakes?

Abrupt release of energy along a fault causes earthquake waves.
A fault is a sharp break in the crustal rock layer.
Rocks along a fault tend to move in opposite directions.
But the friction exerted by the overlying rock strata prevents the movement of rock layer. With timepressure builds up.
Under intense pressure, the rock layer, at certain point, overcomes the friction offered by the overlying layer and undergoes an abrupt movement generating shockwaves.
This causes a release of energy, and the energy waves travel in all directions.
The point where the energy is released is called the focus of an earthquake, alternatively, it is called the hypocentre.
The energy waves travelling in different directions reach the surface.
The point on the surface, nearest to the focus, is called epicentre.
It is the first one to experience the waves. It is a point directly above the focus.

Types of body waves

P-waves :
- P-waves move faster and are the first to arrive at the surface.
- These are similar to sound waves.
- They travel through gaseous, liquid and solid materials.
S-waves:
- Arrive at the surface with some time lag.
- These are called secondary waves.
- An important fact about S-waves is that they can travel only through solid materials.
- This characteristic of the S-waves is quite important.
- It has helped scientists to understand the structure of the interior of theearth.
- Reflection causes waves to rebound whereas refraction makes waves move in different directions.

How these properties of ‘P’ and ‘S’ waves help in determining the earth’s interior?

Reflection causes waves to rebound whereas refraction makes waves move in different directions.
The variations in the direction of waves are inferred with the help of their record on seismograph.
Change in densities greatly varies the wave velocity.
By observing the changes in velocity, the density of the earth as a whole can be estimated.
By the observing the changes in direction of the waves (emergence of shadow zones), different layers can be identified.

Surface Waves (L waves)

Also called as long period waves.
They are low frequency, long wavelength, and transverse vibration.
Generally affect the surface of the Earth only and die out at smaller depth.
Develop in the immediate neighborhood of the epicenter.
They cause displacement of rocks, and hence, the collapse of structures occurs.
These waves are responsible for most the destructive force of earthquake.
Recoded last on the seismograph.

How do P & S waves give evidence for a liquid outer core?

The seismic shadows are the effect of seismic waves striking the core-mantle boundary.
P and S waves radiate spherically away from an earthquake’s focus in all directions and return to the surface by many paths.
S waves, however, don’t reappear beyond an angular distance of ~103° (as they are stopped by the liquid) and P waves don’t arrive between~103° and 142° due to refraction at the mantle-core boundary.

Seismic shadow zone:

Area of the Earth’s surface where seismographs cannot detect an earthquake after the waves have passed through theearth.
P waves are refracted by the liquid outer core and are not detected between 103° and 142°.
S waves cannot pass through the liquid outer core and are not detected beyond 104°
This information led scientists in the early 1900’s to deduce a liquid outer core. The magnitude scale is known as the Richter scale.
The magnitude relates to the energy released during the quake.
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.

Discontinuities in earth structure:

Connard discontinuity Zone that separates the Earth’s upper crust from the lower crust.
Mohorovicic discontinuity Zone that separates the Earth’s crust from theasthenosphere.
Repetti discontinuity Zone that separates the upper mantle from the lower mantle.
Gutenberg discontinuity Zone separating the lower mantle from thecore
Lehman Discontinuity Zone that separates the Earth’s outer core from the Inner core.