EARTHQUAKE

Earthquakes are natural geological phenomena caused by the sudden release of energy in the Earth's crust, resulting in seismic waves and ground shaking. The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

SEISMIC WAVES

• Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth.
• They are the energy that travels through the earth and is recorded on seismographs.
• The two main types of waves are body waves and surface waves.

Body waves

1. Primary waves (longitudinal wave)

• The first kind of body wave is the P wave or primary wave.
• This is the fastest kind of seismic wave.
• The P wave can move through gaseous, solid rock and fluids, like water or the liquid layers of the earth.
• It pushes and pulls the rock, it moves through just like sound waves push and pull the air.

2. Secondary waves (transverse wave)

• The second type of body wave is the S wave or secondary wave.
• An S wave is slower than P wave and can only move through solid rock.
• This wave moves rock up and down, or side-to-side.
• S-waves arrive at the surface with some time Lag.

Surface waves

Love Waves

• The first kind of surface wave is called a Love wave, named after A.E.H. Love, a British
  mathematician.
• It’s the fastest surface wave and moves the ground from side-to-side.

Rayleigh Waves

• A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean.
• Because it rolls, 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, which can be much larger than the other waves.

CAUSES OF EARTHQUAKES

Tectonic Plate Movements

• Most earthquakes are caused by the movement of tectonic plates that make up the Earth's crust.
• These movements can include subduction (one plate moving beneath another), transform boundaries (plates sliding past each other), and divergent boundaries (plates moving apart).

Volcanic Activity

• Earthquakes can also be triggered by volcanic eruptions. T
• he movement of magma and the pressure build up within a volcano can induce seismic activity.

Human Activities

• Certain human activities like mining, reservoir-induced seismicity (due to the filling of large reservoirs), and hydraulic fracturing (fracking) for oil and gas can induce earthquakes, albeit usually of lower magnitude.

TYPES OF EARTHQUAKES

Tectonic Earthquakes

• These are the most common and occur due to tectonic plate movements. They can be further categorized into thrust, strike-slip, and normal earthquakes based on the direction of movement.

Volcanic Earthquakes

• These are associated with volcanic activity and occur near active volcanoes.

Collapse Earthquakes

• These occur in underground mines and caverns due to the collapse of the rock structure.

Man-made or Anthropogenic  Earthquakes

• Blasting of rock by dynamites for construction purposes.
• Deep underground tunnel
• Nuclear explosion
• Reservoir Induced Seismicity (RIS) (E.g. Koyna Reservoir witnessed Earthquake in 1967 due to RIS)
• Hydrostatic pressure of man-made water bodies like reservoirs and lakes.

WORLD DISTRIBUTION OF EARTHQUAKES

• The world’s distribution of earthquakes coincides very closely with that of volcanoes.
• Region of greatest seismicity are Circum-Pacific areas, with the epicenters and the most frequent occurrences along the Pacific Ring of Fire.
• 70% of earthquakes occur in the Circum-Pacific belt.
• Another 20% of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas, and parts of north-west China.
• The remaining occur in the interiors of plates and on spreading ridge centers.

EARTHQUAKE PRONE AREAS IN INDIA

• Earthquake of mild intensity takes place daily. Strong tremors causing large scale destructions are, however, less frequent. Earthquakes are more frequent in the areas of plate boundaries, especially along the convergent boundaries.
• In India, the region of convergence of the Indian Plate and the Eurasian Plate is more vulnerable to earthquakes. E.g. the Himalayan Region.
• The peninsular part of India is considered to be a stable block. Occasionally, however, some earthquakes are felt along the margins of minor plates. The Koyna earthquake of 1967 and the Latur earthquake of 1993 are examples of earthquakes in peninsular regions.
• The experts of Indian Seismology have divided India into Four seismic zones namely Zone-II, Zone-III, Zone-IV, and Zone-V. It may be observed that the entire Himalayan region, the states of North-East India, Western and Northern Punjab, Haryana, Uttar Pradesh, Delhi, and parts of Gujarat belong to the highest and the high-risk categories zone, named as zone V and IV.
• The remaining parts of the northern plains and western coastal areas fall in moderate risk zone and a large part of the peninsular region lies in the low-risk zone.

CONSEQUENCES OF EARTHQUAKE

Damage to human life and property

• The deformation of the ground surface because of the vertical and horizontal movement of the earth’s crust causes huge damage and destruction to human establishments and structures.
• Example: Nepal earthquake of 7.8 magnitudes caused heavy casualties because of unplanned urban construction; poor designed buildings and unscientifically designed structures.

Landslides and Avalanches

• Tremors especially in mountain areas can cause slope instability and slope failure leading to debris down the slope causing landslides.
• The huge masses of ice may fall down snow-covered peaks due to earthquakes causing Avalanches.
• Example: The Sikkim earthquake of 2011 caused landslides and serious damage to life and property, especially the Singik and Upper Teesta hydel projects.

Floods

• The earthquake can lead to devastating disturbances to dams, reservoirs and can cause flash floods. Landslide and Avalanches which may block the river course, leading to floods.
• Example: –Assam earthquake of 1950 produced a barrier in the Dihang River due to the Accumulation of huge debris causing flash floods in the upstream section.

Tsunami

• Tsunamis are the waves produced due to disruption of ocean basin and displacement of the huge volume of water. Seismic waves of an earthquake can displace sea floor and generate high sea waves as Tsunamis.
• Example: – The Tsunami of 26th December 2004 of the Indian Ocean was caused by an earthquake off coast of Sumatra. It happened because of the subduction of the Indian plate under the Burmese plate. It killed about 2.4 lakh people in the countries in and around the Indian Ocean.

MITIGATION MEASURES

1. Building Codes and Standards: Enforce strict building codes and standards for earthquake-resistant construction.
2. Early Warning Systems: Develop and implement early warning systems to provide people with some advance notice before an earthquake strikes.
3. Seismic Retrofitting: Strengthen existing buildings and infrastructure to withstand earthquakes.
4. Land-Use Planning: Avoid constructing critical infrastructure in high-risk areas and promote safer land-use planning.
5. Education and Preparedness: Educate the public about earthquake risks and encourage preparedness measures such as emergency kits and evacuation plans.
6. Monitoring and Research: Invest in seismic monitoring and research to better understand earthquake behavior and predict potential hazards.
7. Emergency Response: Develop and maintain effective emergency response plans and resources to address earthquake impacts promptly