Earthquakes And The Science Behind Its Mechanism...

Sign of Focus or Epicenter..

Hello Friends, welcome to my blog once again. Today I'll be talking about one of the most catastrophic calamities which world faces now-and-then i.e. Earthquakes. Friends many times when a news of an earthquake flashes on the TVs you must have wondered..
- How do these earthquakes occur and how can they be detected?
- Why do they occur?
- What are the possibilities that we can prevent it or mitigate its effects?
- What is the science behind it? ETC..
So people, I will clear all these doubts in my todays article…

Introduction..

Major earthquake zones..

An Earthquake is a trembling, shaking and vibrations of ground caused by passage of energy in the form of waves via breaking of rocks. The most immediate cause of most of the earthquakes is 'Faulting'(breaking and movement of rocks). The actual location of or zone below the earth surface where faulting occurs is known as 'Focus of an earthquake'. This focus lies below the surface but the point just above it is known as 'Epicenter of the earthquake'.

Detection and study of Earthquakes…(Foreshocks, Aftershocks and Swarms)

A large earthquake is preceded by a series of small but measureable tremors, called 'Foreshocks'. These foreshocks are the focus of scientific studies as they contain key to understand the larger earthquakes. Similarly, the tremors that follow the major earthquakes are known as 'Aftershocks'. These are mainly caused because of the periodic release of trapped energy within the rocks after the major wave(earthquake). In some cases the earthquake is followed by 1000s of aftershocks in a day. These aftershocks deceases as the time passes by.
So till now we have learned about two patterns of Earthquakes, the third one is a bit rare kind of pattern found i.e. 'large number of smaller earthquakes occur in a region for months or even years in a series without any major tremor'. For example, In Matsushiro, Japan more than 100,000 smaller earthquakes occurred b/w 1965-67 with zero causalities but huge damage to property in the long run. Such type of earthquake pattern are known as 'Swarms'.

Measurements of Earthquake…

The earthquake is measured in 2 ways i.e. "Intensity and Magnitude". The 'Mercalli Scale'(1-12) is used to determine the "intensity" of damage to life and property where the earthquake strikes. The 'Richter Scale' which is more common and people know about it because the media uses its value to show the havoc caused by the earthquake in an area. It calculates the amount of energy released during the earthquake. Therefore, the former has more of the societal significance and the latter is more for the geographers and scientists. An earthquake with Richter "Magnitude" value of 7.0 or greater is considered the most devastating or catastrophic one.

Earthquakes and the Plate Tectonics…

• 1st of all what is Plate Tectonics?
  In the terms of a layman, Our lithosphere is broken up into several plates. Here is the list of different major and minor plates of the earth:-
  

Rank- Tectonic Plate - Type - Size (Square Km)
1- Pacific Plate- Major- 103,300,000
2- North American Plate- Major- 75,900,000
3- Eurasian Plate- Major - 67,800,000
4- African Plate- Major -61,300,000
5- Antarctic Plate- Major -60,900,000
6-Indo-Australian Plate- Major- 58,900,000
7-South American Plate -Major- 43,600,000
8- Somali Plate- Minor- 16,700,000
9- Nazca Plate- Minor- 15,600,000
10-Philippine Sea Plate- Minor- 5,500,000
11- Arabian Plate- Minor- 5,000,000
12- Caribbean Plate - Minor- 3,300,000
13- Cocos Plate- Minor- 2,900,000
14- Caroline Plate- Minor- 1,700,000
15- Scotia Plate- Minor- 1,600,000
16- Burma Plate - Minor- 1,100,000
17-New Hebrides Plate - Minor- 1,100,000
Source- https://www.worldatlas.com/articles/major-tectonic-plates-on-earth.html

Now, The study of general movement of lithospheric plates with their relative motions and boundary interactions is known as "Plate Tectonics".
Now, the seismic events(earthquakes) are deeply connected with plate tectonics. It fully explains why these catastrophic tremors occur. The plate boundaries are divided into:-
1- Constructive Plate Boundaries(represents diverging plates in which the plate boundaries move away from each other).
2- Destructive Plate Boundaries(where two plates converge and collide against each other and heavier place subdues below the lighter one).
3- Conservative Plate Boundaries(here two plates slide against each other without any collision).

All these are associated with earthquakes the constructive boundary has low earthquake activity(for example the "Mid-oceanic ridge" in Atlantic ocean) while the other two are associated with huge earthquakes(For example, the earthquakes of "mid-continental belt" along the Alpine-Himalayan chains are caused due to collision of Eurasian plate with African and Indian plate). This example also explains the recent earthquake activities in Nepal region, as the Indian plate is sinking under the Eurasian plate the earthquake occurs due to collision of rocks beneath the ground. The other examples are the "Pacific Ring of Fire" region which is most prone to volcanic and seismic activities in the world also follows the same mechanism. A little different pattern is followed by the "Transform fault or the Conservative fault" of the California, USA; another zone prone to heavy earthquakes.
Subduction Zone is the area where most intense and widespread earthquakes occur. These are found at the meeting point of the convergent boundaries. Luckily for us this region is much below the earth surface and the seismic waves becomes almost negligible till they reach the earth surface, these earthquakes are also known as "Plutonic Earthquakes". Therefore, we can conclude that the most intense earthquakes which bring havoc on the earth surface are found near the lithospheric and upper mantle zones and not deep into the lower mantle or core.

The concept behind the Earthquakes and its havoc…

Shadow Zones of Seismic Waves..

The discovery of seismic waves(19th century) transformed our knowledge of Geomorphology and took it to a next level. It not only provided us important data for earthquake studies but also helped us in properly figuring out the internal structure of our Earth. For example, the discovery of Asthenosphere b/w lithosphere and upper mantle was made possible only due to seismic experiments as the velocity of seismic waves decrease when it passes through it. The seismic waves are mainly of 3 types i.e. P-waves(Primary), S-waves(Secondary) and L-waves( Love and Rayleigh). The P-waves are the fastest among all. They are longitudinal waves that can traverse both solid and liquid mediums in the earth's interior. The S-waves are comparatively slower and can only pass through solid medium(therefore, helped in the discovery of Asthenosphere mentioned above). All the waves start from the similar point known as focus or epicenter but P-waves are the 1st one to reach the earth surface. The L-waves starts traveling once both the waves have passes through the planet. Now, both the L&S waves are destructive in nature and are main reasons why the earthquakes bring calamities with them. When an earthquake occurs the P-waves travel much faster and to every corner of planet but the S-waves which are destructive in nature cannot travel through liquid medium and due to outer-core they do not reach the opposite side of the world from where epicenter lies as shown in the diagram. Here we have some "seismic shadow zones of P&S waves" where the earthquake has no effect.

Tsunamis…

The Mechanism of Tsunami..

Many of my friends here have a perception that Tsunamis are a disaster which occurs because of some kind of sea or oceanic disturbances. But in reality these are strongly related to earthquakes. The Tsunamis(means Harbor Waves in Japanese) are triggered by high magnitude Earthquakes in the ocean floors or by violent volcanic eruptions. The origin of tsunami starts with an earthquake which has its focus under the sea floor and when the energy is released the waves start piling up one over the other because the depth of ocean starts decreasing and the Tsunami wave as it moves towards the shore line the wave momentum gets shattered by the shallow shores and beaches. Therefore, it strikes the land with great destruction and havoc to life and property. The Tsunamis are nothing but the "Deep-sea Earthquakes". A typical Tsunami can be 100km high or even more which travels at a speed of more than hundreds of Kilometers per hour. Normally, when we travel over the ocean via ships, there are many tsunamis that form and die due to the small intensity earthquakes going on in the divergent plate boundary area of the deep sea ridges but we ignore them like some normal high tides. The point to be noted here is that the tsunamis if remains within the ocean and stays away from shore line it will cause negligible damage where as if it moves towards the shore and strikes the landmass it is the most destructive calamity found on earth because where ever it strikes everything turns into ocean. It barely leaves any survivors or any property.

Some of the prominent examples of furious Tsunami waves in 20th century are:-

1- Aleutian Tsunami.
2- Kamchatka Tsunami.
3- Chilean Tsunami.
4- Alaskan Tsunami.
5- Japanese Tsunami.(2011- latest)

The article may sound a bit technical to the non-geography or non-science background people but still i have tried my best to explain each and everything very leniently. If u have got some good information from my blog then please let me know in the comments section and if u have any correction, suggestions or additions please feel free to leave your ideas in comments.