What Is Earthquake | Seismic Waves(भूकम्पीय तरंगे) | P and S Waves|

What is an Earthquake?

                         In very simple language, the earthquake is the shaking of the earth. An earthquake can be defined as a sudden violent shaking of the ground as a result of movements in the earth’s crust. These movements result shaking of the earth in the release of energy and this releasing of energy causes this shaking and ultimately, Earthquake. 

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Types of Earthquake as per causes

1. Natural Causes

• Plate Tectonics:- Slipping of land along the faultline along convergent, divergent and transform boundaries causes earthquakes. Hence, Earthquakes can be felt closer to the Convergent, Divergent and Transform Boundaries.

• Volcanism- Volcanic activity also can cause an earthquake, but the earthquakes of volcanic origin are generally less severe and more limited in extent than those caused by fracturing of the earth’s crust.

• Fault Zones:- The immediate cause of most shallow earthquakes is the sudden release of stress along a fault rupture (crack) in the earth’s crust.

2. Man-Made

• Reservoir-induced seismicity-The pressure exerted by a column of water in a large and deep the artificial lake changes stress along an existing fault or fracture. This huge pressure may lead to localized Earthquakes.
• Excessive Exploitation of Underground water for irrigation or Industrial use can also cause small localized Earthquakes.
• Underground nuclear explosions for testing can cause Earthquakes.
• Other Causes:- mining, large scale petroleum extraction

Types of Earthquakes based on Depth

• Shallow earthquakes (0–70 km ):- they are also called “crustal earthquakes”. Most of them are originate in Lithosphere. The majority of them are of a smaller magnitude. 

•   Intermediate earthquakes:- (70–2 00 km deep) 

• Deep Earthquakes (300–700 km):- In general, the term “deep-focus earthquakes” is applied to earthquakes deeper than 70 km. These are found in Subduction zones and this is called Benioff Zones. They are also are known as “Intraplate Earthquakes” The deepest earthquake ever recorded was a 4.2 earthquake in Vanuatu at a depth of 735.8 km in 2004.

What is Wadati- Benioff Zones?

                        This is the zone where deepest, deadliest and the most number of Earthquakes are experienced. This zone is found on Subduction zones, or in Convergent Plate Boundaries.

Distribution of Earthquakes

                     Earthquakes are not evenly distributed on the planet. There are regions where maximum intensity earthquakes are experienced, whereas, few other regions experience low-intensity and very few Earthquakes.

Major Earthquake Prone Regions

1.     Circumvent Pacific Region:- It starts from the Southern tip of Andes and ends at the Northern tip of Rockies. It further leads to the Aleutian Islands, Japan, Philipines, and New Zealand. This region experienced nearly 70% of the planets’ total Earthquakes. This region is marked with many volcanic eruptions also. That is why it is also called a RING OF FIRE ( as shown in Diagram)

2.     Mediterranean-Himalayan Region:- This is the second most Earthquake-prone region that receives around 20% of Earthquakes.

3.     Other Regions:- East African Rift Valley, Mid Atlantic Ridge and Caribbean Islands along with other Earthquake Prone Regions.

Seismic Waves- What it is and their Role?

                           When energy is released from the Focus point/Hypocenter, it travels in all the directions. Till the time this energy is inside the earth, we don’t feel any shaking When this energy is received by the surface of the earth, we feel shaking of the ground.

This energy travels through waves. First, these waves travel through the interior/body of the earth and then on the surface.

These waves are called Seismic waves. These seismic waves can be categorized into two parts:-

1. Body Waves- P-waves and S-waves
2. Surface waves- Love waves and Rayleigh Waves

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BODY WAVES

P-Waves

• The first kind of body wave is the P wave or primary wave.
• This is the fastest wave, and they are the first to 'arrive' at a seismic station. 
• The P wave can travel through all mediums -solid rock, liquid, and gases
• They travel like Sound waves
• P waves are also known as compressional waves 
• P waves are also called Pressure waves.
• These are Longitudinal waves because particles move in the same direction that the wave is moving in, which is the direction of wave propagation.
• Change their direction when passes through the mediums of different density 

  

 

S-waves 

• These are the secondary wave, which is the second wave we feel in an earthquake.
• S wave is slower than a P wave
• These waves can travel only through Solid 
• S waves move rock particles up and down, or side-to-side
• These are transverse waves or shear waves or distortional waves.
• The direction of vibrations of the particles in the medium is perpendicular to the direction of propagation of the wave. 
• They create troughs and crests in the material through which they pass
• S-waves are important to know that the outer core of the earth is Liquid in nature.

Surface Waves

      surface waves are of a lower frequency than body waves. Though they arrive after body waves, it is surface waves that are almost entirely responsible for the damage and destruction associated with earthquakes.

 Love Waves 

      It's the fastest surface wave and moves the ground from side-to-side. Love waves produce entirely horizontal motion.

Rayleigh Waves

      These waves are like waves in water. A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. It moves the ground up and down and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave.

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In the next part, topics to be covered:-

1. Role of P and S waves in knowing the interior of the earth.

2. P wave and S wave shadow regions