Earthquake. (A comprehensive Note)

Earthquake can be defined as:

”Earthquakes are those movements of Earth’s crust that make the ground vibrate and shake.”  OR

”An earthquake is the release of energy that has been slowly built up, during the stress of increasing deformation of rocks.”

Earthquakes occur when energy stored in elastically-strained rocks is suddenly released. This release of energy caused intense ground shaking in the area near the source of Earthquakes and send waves of elastic energy, called Seismic waves, throughout the earth. Earthquakes can also be generated by bomb blasts, volcanic eruptions and sudden slippage along faults. Earthquakes are definitely a geologic hazard for those living in earthquake-prone areas, but the seismic waves generated by Earthquakes are invaluable for studying the interior of the Earth. The study of how seismic waves behave in the Earth is called Seismology.

Seismic Waves: 

When a sudden break or shift occurs in the earth’s crust, energy radiates out as seismic waves, Just as the energy from the disturbance in the body of water radiates out in the wave form. In every earthquake, there are several different types of seismic waves.

Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth.  Surface waves–some times called long waves or simply L-waves– are responsible for most of the damage associated with earthquakes, because they cause the most intense vibration. Surface waves stem from body waves that reach the surface.

There are basically two types of Seismic Waves:

Body Waves and Surface Waves.

01. Body Waves:

Body waves move through the inner part of the earth, while surface waves travel over the surface of the earth.  Surface waves–some times called long waves or simply L-waves– are responsible for most of the damage associated with earthquakes, because they cause the most intense vibration. Surface waves stem from body waves that reach the surface.

Types of Body waves:

There are two main types of the body waves.

  • Primary waves:

Also called P-waves or compressional waves, travel about 1-5 miles/ second, depending on the material they are moving through. This speed is greater than the speed of other waves, so P-waves arrives first at any surface location. They can travel through solid, liquid and gas, and so will pass completely through the body of earth. As they travel through rock, the waves move tiny rock particles back and forth–pushing them apart and then back together–in line with the direction the wave is travelling. These waves typically arrive at the surface as an abrupt thud.

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  • Secondary Waves:

They are also called S-waves or Shear waves like a little behind the P-waves. As these waves move they displace rock particles out ward, pushing them perpendicular to the path of waves. This results in the first period of rolling associated with earthquakes. Unlike P-waves, S-waves do not move separate through the earth. They only travel through solid material, and so are stopped at the liquid layer in the Earth’s core.

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02. Surface Waves:

These are also called Rayleigh waves. Surface waves (Rayleigh waves) are something like the waves in the body of water–they move the surface of earth up and down. This generally causes the worst damage because the wave motion rocks the foundation of man-made structures. L-waves are the slowest moving of the all waves, so the most intense shaking usually comes at the end of an earthquake.

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All these waves can easily identified, on the seismography record and by measuring the difference in the arrival times of these waves. it is possible to estimate the distance of the source from the observatory place.

Top 10  largest Earthquakes (with magnitude more than 5) in the world: 

  • 2004 Indian Ocean Earthquake – 9.1-9.3
  • 2011 Tohoku Earthquake – 9.0
  • 2005 Sumatra Earthquake – 8.7
  • 2015 Chile Earthquake – 8.3
  • 2014 Chile Earthquake – 8.2
  • 2017 Mexico Earthquake – 8.1
  • 2008 Sichuan Earthquake – 8.0
  • 2016 New Zealand Earthquake – 7.8
  • 2016 Ecuador Earthquake – 7.8
  • 2015 Nepal Earthquake – 7.8
  • 2001 Gujarat Earthquake – 7.7
  • 2005 Kashmir Earthquake – 7.6
  • 2015 Afghanistan Earthquake – 7.5
  • 2013 Bohol Earthquake – 7.2
  • 2017 Mexican Earthquake – 7.1
  • 2013 Honshu Earthquake – 7.1
  • 2010 Haiti Earthquake – 7.0
  • 2016 Honshu Earthquake – 6.9
  • 2003 Boumerdes Earthquake – 6.8
  • 2017 Aegean Sea Region Earthquake – 6.7
  • 2003 Bam Earthquake – 6.6
  • 2016 Central Italy Earthquake – 6.2
  • 2006 Java Earthquake – 6.2


Measurement of Earthquakes:
 

Earthquakes range from tremors so small that they are barely detectable to great shocks that can destroy entire cities. This reflects their Magnitude, the amount of shaking of the ground as the quake passes, as measured by Seismography. Magnitude is assessed on the Richter Scale which assigns a number to an earthquake based on the severity of that ground motion. This open ended scale, developed in 1935 by Geophysicist Charles Richter, ranges from 0 to 8+ . It is logarithmic, so that the earthquake of magnitude 4 causes 10 times as much ground motion as one of the magnitude 3 and 100 times as much as a quake of magnitude 2.
Another measure of an earthquake’s size is its intensity. This measure reflects the impact of an earthquake on the cultural landscape- on the people, their activities and structures. Intensity is reported on the Mercalli Scale, which was first developed by Italian geologist Giusepppe Mercalli in 1905 and modernized in 1931. It assigns a number ranging from I to XII to an earthquake (Roman Numeral are always used). For instance, an earthquake of intensity IV is felt outdoors, and hanging objects swing. At intensity XII damage is total, and even heavy objects are thrown into the air. Following is detailed description of Mercalli Scales:

Strength of An Earthquake/ Range of an Earthquake:

  1. Instrumental: Detected only by instruments.
  2. Very feeble: Notice by people only at rest.
  3. Slight: felt by people at rest, like passing of truck.
  4. Moderate: generally perceptible by people in motion. Loose objects are disturbed.
  5. Rather Strong: dishes broken, bells rung, pendulum clocks stopped. People awakened.
  6. Strong: felt by all, some people frightened. Damage slight, some plaster cracked.
  7. Very strong: Noticed by people in autos. Damage caused to poor constructions.
  8. Destructive: Chimneys fall, much damage in substantial buildings, heavy furniture overturned.
  9. Disastrous: Many buildings destroyed.
  10. Very disastrous: few structures left standing.
  11. Catastrophic: total destruction.

At the earth’s surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicentre is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landsliding and occasionally volcanic activity.
An earthquake’s point of initial rupture is called its focus or hypocentre. The term epicentre means the point at ground level directly above this.

Causes of Earthquakes:

There are several causes of an earthquake, but here in this post we are discussing most common causes of an Earthquake.

01. Elastic Rebound Theory:

Most naturallay occuring earthquakes are related to the tectonic nature the Earth. Such earthquakes are called tectonic earthquakes. The Earth’s lithosphere is a patchwork of plates in slow but constant motion caused by the release of space of the heat in the Earth’s Mantle and core. The heat causes the rock in the earth to flow on geological timescale, so that the plates move slowly but surely.
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Plate boundaries lock as the plates move past each other, creaing frictional stress. When the frictional stress exceeds a critical value, called local strength, a sudden failure occurs. The boundary of tectonic plates along which failure occurs is called the fault plane. When the failure at the fault plane results in a violent displacement of the Earth’s Crust,energy is released as a combination of radiated elastic stain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. This process of gradual build-up of strain and stress punctualed by occasional sudden earthquake failure is referred to as the Elastic Rebound Theory. This theory was discovered by making measurements at the number of points across a fault. Prior to an earthquake, it was noted that the rocks adjacent to the fault were bengin. These bends disppeared after an eartquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.

02. Volcanic Activity:

Most Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes serve as an early warning of vocanic eruptions.

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03. Plate Tectonics:

Seismic activity occurs primarily near lithospheric plate boundaries. Every type of plate movement contributes in origin of earthquakes. As the crust or the outermost shell of the Earth is cracked. The main cause for this crack is the movement of molten mantel which is just beneath the hard crustal shell. Each consolidated part of cracked crust is called Tectonic plate. There are many Tectonic plates namely, Indian Plate, Eurasian Plate, African plate, Antarctic plate, Eurasian plate, Indo-Australian plate, North American plate, Pacific plate and South American plate. Tectonic Earthquakes are caused when the crust becomes subjected to strain, and eventually starts moving.Image result for tectonic movement

This movement causes Earthquakes. These plates are either moving apart from one another or moving toward one another, or this movement may be transformed. In each case the boundaries posses stress. After severe stress these plates breaks the stress and starts moving. Thus causing an effect of vibration and thrilling resulting earthquakes.

Example: – Due to tectonic movements, San Francisco has noticed many small shakes.

04. Loss of Isostatic Adjustment:

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The isostatic balance between the raised and depressed blocks of land on the surface of the Earth is not always maintained. When erosion takes takes place on on the mountains, it results in the deposition on the sea floor and the isostatic balance is disturbed. But according to the theory of Isostacy, some cause or combination of causes continuously depress the region of deposition and raise the region of erosion (because the lithosphere floats on the asthenosphere as the copper blocks float on the mercury). This lowering and rising of the earth crust cause earthquakes. It is worth noting that earthquakes are not always associated with isostatic adjustment. An earthquake takes place only if the isostatic adjustment is sudden and quick.

05. Accumulation of water:

Whenever there is large scale accumulation of water, it causes strain in the underlying rocks. When the strain crosses the elastic limit, rocks are broken and sudden vibrations in the form of an earthquake. Such earthquakes are associated with dames, lakes and tanks.

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06. Land Sliding:

Landsliding is also a minor cause of earthquake. Due to the heavy rains the earthen mountainous masses slips down in the form of huge bulks. These falling bulks cause havoc and vibration in the surface of nearby area. However such types of earthquakes are slightly noticeable and does not trigger destruction.

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07. Emition of water vapors under high pressure:

The sudden release of strained water vapors under high pressure cause strong thrilling on the surface of earth. This sudden fail of stress results in disturbance in outer crust on the ephemeral surface of the earth. Anything which will tend to destroy the perfect equilibrium or the balance of the globe will likely cause an earthquake. Each side and every part must necessarily carry the same weight and the state of isostasy must be maintained. Whenever there is a imbalance between two interconnected masses, a movement in the masses may occur. This movement will create disturbance within the surface of Earth, which create vibration and thrilling in the crust, which is known as earthquake.

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08. Sudden collapse of caves under karst region:

The abrupt collapse of the roof of karst site or huge caves also cause thrilling and vibration in the surface of earth. But such types of earthquakes are of minor nature and does not caused destruction. However these vibration are, no doubt, included in earthquakes. Image result for collapse of karst

09. Meteoritic Impact:

Meteorites cannot cause strong earthquakes as such but they do induce tremors felt over a large area just like earthquakes. It need not be a meteor strike, even a meteor burning in air close to earth can cause explosion and induced tremor. By definition, earthquakes are cause by movement across some fault plane. But volcanic earthquakes do not fall under this category and so do the meteorites. However the fall of huge meteor can cause severe tremor in earth’s surface. If this tremor is intense , then it will be called an earthquake.

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10. Havoc of a Powerful Bomb Blast: 

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When ever there is a powerful bomb like thermonuclear bomb detonated, waves of high magnitude are released through the Earth mass. These waves are composed of P-waves, S-wave and L-waves. Although such types of earthquakes are not much destructive, however it can harm weak buildings.

 

 

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