Do we know what's inside the Earth?

We do have some sources that give us some information about the centre of Earth. But we haven't gone too deep and nor we can possibly go inside the Earth's interior that 6371 kms deep.

So, the question arises, in such conditions how do geologists give us information with respect to Earth's interior?

So, today, as a sub-section, we shall discuss as to how the 'Earthquakes' we have all heard and experienced at least once tell us about the Interior of the Earth.

How do Earthquakes originate?

Origination of Earthquakes is covered under the plate movements over the Asthenosphere (a semi-solid molten layer of magma). Sliding of lithosphere over asthenosphere causes plate boundary movement. There are three types of plate boundary movement. An outcome of Earth's endogenic forces (that originate inside the Interior of Earth) - PLATE TECTONICS.

1. One where two plates come close to each other (Convergent)
2. Where two plates run away from each other in opposite directions (Divergent)
3. When two plates run parallel to each other in opposite directions, Sliding and Striking each other (Transform Boundary)

Anyway, whatever has been the plate movement, there develops a crack (fault) along the Earth's surface and friction between two or more plates causes energy to be released in all directions inside the interior of the Earth. Remember, earthquake always occurs along a fault plates and the movement of tectonic plates is located in the lithosphere only (solid part of the Earth).

Earthquake can also be understood as disturbance in the 'Iso-Static Equilibrium'. The pressure on Earth's crustal surface is balanced by the highly volatile and explosive magma. On the disturbance of this balance, nuclear tests, energy is released causing shaking and we call that tremor from inside the Earth's surface as 'Earthquake'.

Why is the Energy Released?

The energy released inside the Earth's interior causes shaking (quake) and waves are generated due to the movement of particles. The waves that are generated beneath the Earth surface are called Body Waves (as being located inside the Earthquake the denser the medium is inside the Earth, the speeder is the movement of earthquake waves.

It should be noted that the higher the wavelength of waves (distance between two crests) and high frequency cause heavy destruction due to maximum release of energy. Hence, the higher the energy released, greater the chances of destruction caused by the Earthquake.

The waves generated due to shaking of the Earth's interior are known as 'Seismic Waves'.

It's a natural event but at the same time has factors as both man-made and natural. Volcanic eruption, subduction of plates and transform plate boundary movement types and hitting of landslides on the Earth's surface cause a tremor inside the Earth's surface. Construction of heavy dams and the pressure of stored water in the reservoir is said to have propelled earthquake waves. Mining and drilling activities of human beings have time and again caused Earthquakes to emerge, as seen in the collapse of the surface of the mines.

Centres in an Earthquake:

The location where plate movement has occurred and fault plane has been formed and from where shaking has been caused and energy has been released in all directions is called the FOCUS/ HYPOCENTRE of Earthquake. Waves travel in all directions from this point. Focus is the point below the Earth's surface from where the Earthquake has originated.

Seismology: The Study of Seismic Waves

The point where Body Waves after originating from focus hit the Earth's surface at the most closest distance, generally perpendicular to vertically above the focus. It is a point just above the Earth's surface known as 'Epicentre'. At this point, Body Waves hit the rocky Earth surface and a new set of waves are generated called the surface waves. The surface waves are the most destructive. Surface waves are also transverse waves like Secondary waves of Body Waves. A crest-trough-like appearance. The greater the closeness to the Epicentre of Earthquake, maximum is the destruction faced by that area.

Body Waves are further classified into Primary and Secondary Waves (P-waves) and Secondary waves (S-waves).

P-waves:

These waves travel in all mediums. Though, they travel fastest in the solid medium. They are the fastest waves and the first to be recorded on a Seismograph. They move on note of compression and expansion. They are longitudinal waves like sound waves, which we all study in Physics. The movement of particles in P-waves is parallel to the direction of the propagation of P-waves.

Secondary (S-waves):

They are the second fastest waves. Not as fast as P-waves. S-waves travel only in solid medium as shear stress is not supported in liquid/gas.

Surface Waves are classified into Love Waves (L-Waves) and Rayleigh Waves (R-waves)

L-waves:

These waves cause horizontal shifting of Earth and their movement trims with the fashion in which a snake moves.

R-waves are the most destructive ones of all. They have an elliptical motion where they cause upward-downward and sideways shifting of Earth's surface. They create a rolling motion on the surface they move thus leading to the collapse of even the most sturdy buildings.

It should be noted that waves rebound on reflection and refract when they enter into materials of different densities.

Seismic Waves are recorded on an instrument called 'Seismograph'. The instrument infers about the Earthquake with the help of two scales.

Emergence of Shadow Zones:

We did learn about the nature of the Body Waves (P-waves) and (S-waves). P-waves are capable of traveling in all states of matter, still between 105° and 145° in both the halves of Earth, there is found to be the 'Shadow Zone of P-waves' (the part where the presence of those particular waves is absent.

Shadow Zone of P-waves:- 145° minus 105° equals to 40° in one-half. Both the halves of Earth constitute 80° of Shadow Zone. It's Shadow Zone appear only as a patch or band inside the surface of the Earth.

The mysterious question that lays behind this logic is that when P-wave can travel in all mediums...what is that in that Shadow Zones (in some patches inside the Earth's interior) that is impenetrable for P-waves. A question is still pertinent to be solved regarding Earth's interior.

S-waves travel only in solid medium and they have the Shadow Zone extending from 105° on the sphere to the complete straight line angle across the diameter of our Earth. Hence, 180° - 105° = 75° which later constitutes to 150° Shadow Zone constituting 41.67% of Earth's Interior.

Hence, the nature of Seismic activity ( Trend of P-waves and S-waves) allow us to develop a predictive inference inside the Earth's surface.

Scales in Measurement of Earthquake:

Richter Scale:- This scale measures the magnitude (amount of energy released) during the Earthquake. The most prominently used scale in an Earthquake scaled 0-10. The scale increases it's position by a logarithm of a decimal. Means, every 1 unit increment on the scale equates to 10 times multiplication of the magnitude of Earthquake so on and so forth. It quickly helps in early warning signs whenever Earthquake waves are active and thereby defining the Zone of Earthquake.

Mercalli Scale:

This scale measures the intensity (amount of destruction caused by an Earthquake) on a scale of 1-12. Mercalli scale is more concerned with the post-mortem conditions of earthquakes. It has it's own relevance but measuring destruction comes much later as what comes as the first priority as what and how much energy has been released due to the Earthquake. To explain, suppose, a high magnitude Earthquake is dangerous but if it happened in a rapidly populated region, destruction would be less and hence lesser values on the Mercalli scale. Proving here despite the Earthquake being dangerous still Mercalli scales shows a smaller value as destruction was less.

Earthquakes occur inside the Earth at different levels:

Some are Shallow foci (0-70km), Intermediate Foci ( 70-300 km) and Deep Foci. (300-700 kms)

Generally, the focus of Earthquake is located close to the Earth's surface. Greater the closeness of focus to the Earth's surface greater is the destruction of Earthquake as energy quickly reaches the Earth's surface causing collapse. However, if the focus is deeply situated, the energy lapse takes time and the magnitude and intensity of the Earth is dwindled over time. 70-80% of Earthquake occurrences are Shallow foci.

Occurrences of Earthquakes:

Earthquake occurrences of 7 or 8+ on the Richter Scale are rare to occur. The deadliest Earthquake on the planet happened in the San Andreas Fault in North America, an outcome of a strike-slip fault in a transform boundary movement.

The Pacific Ring of Fire constitutes 68% of Earthquake as there are many minor plates in this region which on colliding against each other release tremendous energy leading to shaking of the Earth's surface.

In 2015, India faced shaking tremors when the Epicentre of Earthquake was in Nepal, the place where there is the convergence of two continental plates, the Eurasian Plate and the Indo-Australian Plate. Himalayas have been tectonically active due to continental-continental convergence.

The Anatolian region of Turkey has been a massive sufferer of Earthquake as Turkey has been located against a junction of places where there has been transform boundary movement between the Eurasian, Anatolia and Arabian Plates.

Earthquake - Tsunami (Effects)

Earthquakes immediately is observed to be as the destructive forces of Nature (Catastrophism). Disaster management tips should be adopted so that the humankind becomes resilient to earthquakes.

However, if the shaking/tremor inside the Earth's interior find it's epicenter in oceanic waters causing the development of waves in the ocean as an outcome of an Earthquake which results into a Tsunami.

The Earthquake lasts only a few seconds or the most destructive ones (for a minute as well) but the Nature tries to balance the Iso-Static Equilibrium as soon as it is disturbed. Obviously, the devastation caused by it is observed when this catastrophic natural event is more.

Author's Note:

We get some indirect inferences about Earth's interior however there is no end to an understanding of our Nature. There is always something more to it. By explaining the Earthquake, I tried to make this concept as lucid as possible for a greater audience. Usage of geographical terms and technical words were used only where it was essentially required. Feel free to drop any questions if you have and give suggestions if you want me to make another explanation series on Earthquake itself further or any other topic you people want me to cover, would love to that. The Power of Writing lies in how better we can explain something and how it is received by the audience. Drop your comments for me to know your opinions better. Your suggestions are always welcome.

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Credit: 

• Class XI, CBSE, NCERT, Fundamentals of Physical Geography.