Earthquakes

Earthquakes are shaking of the earth caused by to sudden release of energy. Massive Natural earthquakes are generally caused by the energy released due to Movement in tectonic plates. It’s the most unpredictable and highly destructive of all the natural disasters.

Basic Definitions:

Focus or Hypocentre: it is a point in the earth’s interior where energy is released. Epicentre: it is a point on the surface nearest to the focus. It is the point on the earth’s surface vertically above the focus of an earthquake. It is the first to experience waves.

Types of Earthquakes:

Earthquakes can be classified on the basis of the force that causes them. There are five different types of earthquakes,

1. Tectonic earthquakes: These are generated due to the sliding of rocks along a fault plane. When the movement between the plates is obstructed, accumulation of energy takes place, which releases all of a sudden all energy at the same time which creates a strong shaking. This is the most common earthquake observed all over the world;

What are faults and fractures?

Fractures and Breaks in rocks that develop in response to stress. Fractures can be of two types: Fault: Those fracture lines along which crustal plates move relative to each other are faults. This movement is not continuous. Joint: This is the point where no movement takes place.

2. Volcanic earthquakes: Rapid expulsion of volcanic material from the active volcanoes causes shaking of the earth’s surface. Large volcanos can produce massive earthquakes.
3. Collapse earthquakes: When a large amount of material collapses, it can create shaking. Such shake can be created in the following situations:
   • In areas of intense mining activity, sometimes the roofs of underground mines collapse generating minor tremors.
   • Collapse of buildings and structures such as dams. For example, the Karam Dam breach in Madhya Pradesh in August 2022. A dam breach occurred in 2023 in Sikkim following a glacial lake outburst caused massive flash floods and several deaths. Similarly a breach occurred on a dam on Subansiri River.
   • Large Landslides.
   • Collapse of sea mount (i.e. mountain under the ocean)
4. Explosion earthquakes: Ground shaking may also happen due to the explosion of chemical or nuclear devices. Such tremors are called explosion earthquakes. These Earthquakes are strong enough to be detected on the seismograms.

• For example, Tsar Bomba, the most powerful nuclear explosion created an earthquake of Magnitude 5 on the Richter scale. In fact, even the most secretive nuclear tests can be detected on the Seismograms.

5. Reservoir-induced earthquakes Hydro-seismicity: incidence of earthquake-triggered due to impoundment of water behind a dam.

Earthquake Swarms are a sequence of seismic events that occur in a local area within a relatively short period. It may continue for days, months, or years.

Earthquake Swarms:

Earthquake Swarms are a series of low-magnitude earthquakes that occur in a localized region and over a period of time ranging from days, weeks to even months. For example, Series of Unusual tremors in Palghar, in Maharashtra. National Centre for Seismology (NCS) has categorized them as ‘earthquake swarm’. Water is the most common cause of Earthquake Swarms. With every 10m rise in groundwater, pore pressure increases by 1 bar. This induces Hydro seismicity. In the Himalayan region swarms are attributed to the low strength of the earth’s crust in the area which could not hold the tectonic energy. In 2016, a series of 58 earthquakes were recorded in the Rampur area of Himachal Pradesh.

Destruction causing Earthquake waves

On the basis of their propagation Earthquake waves can be classified into two categories:

1. Body Waves: After the release of energy at the focus, waves move in all directions through the medium. The speed at which these waves travel depends on the properties of the material that they are passing through. For example, in a denser material seismic waves travel faster, covering large distances. There are two types of Body Waves:

• P-Waves or Primary waves:

1. P-waves are longitudinal waves, i.e. the movement of material is the direction of its propagation. They exert pressure on the material in the direction of movement, which creates density differences in material: Squeezing and stretching. Sound waves and Pressure waves are a type of Primary wave.
2. They travel through solid as well as fluid mediums.
3. They carry the highest amount of energy and therefore travel fastest and farthest. But they have the least amount of amplitude (amount of to-and-fro motion), and therefore cause less damage as compared to other waves.

• S-Wave or secondary waves:

1. S waves are transverse waves,e. they vibrate the rock strata perpendicular to the direction of their propagation. In other words, they create troughs and crests perpendicular to the direction of propagation.
2. Such motion can only happen in Solids, but not through fluids.
3. S-waves are slower than P-waves and arrive at the surface with some time lag. Every second of the time lag between the S and P waves means 8 km.

• Surface waves: Surface waves are generated when body waves intersect with surface rocks. Such waves move along the surface of the earth. It carries low energy but has a high amplitude. Thus, they are slowest and are last to be reported by seismograph, but are most destructive. They too are of two types:

1. Love waves: move on the surface from side to side perpendicular to the direction of propagation.
2. Rayleigh waves: move up and down on the surface.

Measuring earthquakes:

Seismic stations are established at different locations to measure and record the earthquake waves. It is measured either according to Magnitude (strength of shaking) or intensity (impact on the environment) of shock.

• Seismograph: Instrument that records waves.
• Seismogram: Chart on which Earthquake waves are recorded.

Different types of earthquake waves create different types of movement on the Seismogram. The time delay between the arrival of different waves can be used to measure the distance of the station from the location of occurrence of the Earthquake (Focus).

Locating Earthquakes:

With the help of seismograms from at least 3 seismic stations, the epicentre can be located. The method of detecting the location of the epicentre is known as Triangulation.

Magnitude scale:

It measures the strength of shaking of the ground; It does this by computing the energy released by measuring the amplitude that the earthquake carries.

1. Richter scale: The first widely used method, the Richter scale, was developed by Charles F. Richter in 1934. It used a formula based on the amplitude of the largest wave recorded on a specific type of seismometer and its distance from the epicentre. It is expressed on a 0-10 scale in absolute whole numbers.

Richter Scale	Description
< 2	Micro Earthquakes – People cannot feel it
2 – 3.9	Minor Earthquakes – Seismographs can’t detect them
4-4.9	Light Earthquake – Causes disturbances; Noise.
5 – 5.9	Moderate Earthquakes – Buildings are damaged
6 – 6.9	Strong Earthquake – About 800 each year on earth; causes huge losses in a large area.
7 – 7.9	Major Earthquake – Takes lives on a large scale.
8 – 9.9	Great Earthquake – 1 in every 20 years; Even heavier objects are thrown into the air, and buildings are toppled.
10+	Meteoric Earthquakes – Never recorded in history.

• Limitations of Richter Scale: Unfortunately, the Richter scale, does not provide accurate estimates for large magnitude earthquakes.

2. Moment magnitude scale (MW): It is based on the total moment release of the earthquake. The moment is a product of the distance a fault moved and the force required to move it. It is derived from modelling recordings of the earthquake at multiple stations.

Limitations of magnitude scales:

• Have a little relationship with the amount of destruction.
• Again, as we have seen in the case of P-waves; The waves carrying the highest amount of energy may not create the highest amount of destruction.
• The amount of destruction depends on the intensity and the direction of shaking.

Intensity scale:

Intensity is a measure of the shaking and damage caused by the earthquake; this value changes from location to location.

Mercalli scale: Developed by Italian Seismologist Giuseppe Mercalli. It empirically measures the visible damage caused by the event. Its range is 1-12.

Limitations of Intensity Scale: The computation of intensity is not immediate, and therefore it cannot help the first responders. This limitation also applies to the Moment Magnitude scale.

Thus, the Richter scale, although not perfect, is the best available tool for us to quantify destruction in the easiest way possible and simple enough to understand for both the first responders as well as the general public.

Vulnerable Areas:

There are three major regions in India susceptible to earthquakes in India.

• Western Himalayan Region: Jammu and Kashmir, Himachal Pradesh, Uttarakhand,
• Eastern India: Sikkim and Darjiling district of (West Bengal), Seven states of Northeast.
• Central-Western parts of India: Gujarat has received major earthquakes in 1819, 1956 and 2001, and in Maharashtra 1967 and 1993. The occurrence of earthquakes is hard to explain in this zone as there is no Faultline here.

The rest of India is one of the oldest and most secure land masses. Recently, a theory of the emergence of Faultline and Energy built-up along the fault line represented by river Bhima (Krishna) near Latur and Osmanabad (Maharashtra) and the Possible breakdown of the Indian plate has emerged, that can explain the occurrence of earthquakes in the Central-Western Part of India.

Earthquake Vulnerability Zones

National Geophysical Laboratory, Geological Survey of India, Department of Meteorology, GoI, along with the recently formed National Institute of Disaster Management, has made an intensive analysis of more than 1,200 earthquakes in India and they divided India into the following five earthquake zones:

1. Very high damage risk zone: Zone 5: North Eastern states, areas north of Darbhanga and Araria along the Indo-Nepal border in Bihar, Uttarakhand, West HP (around Dharamshala) and Kashmir Valley in Himalayan region and the Kutch (Gujarat).
2. High damage risk zone: Zone 4: Jammu and Kashmir (other than Valley), Himachal Pradesh, Northern parts of Punjab, Eastern parts of Haryana, Delhi, Western Uttar Pradesh, and Northern Bihar.
3. Moderate damage risk zone: 3
4. Low damage risk zone: 2
5. Very low damage risk zone: 1

BIS Zoning

The Bureau of Indian Standards (BIS) too divides India into several zones based on the hazard level. These are used by engineers to build resilient structures. While GSI focuses on seismic faults, tectonic movements, and lithospheric dynamics, BIS zones are based on macroseismic intensity and structural safety concerns.

The Bureau of Indian Standards (BIS) classifies India into four seismic zones based on earthquake intensity and frequency, as per IS 1893:2016 (Part 1). These zones indicate varying degrees of seismic hazard across the country. Although both standards are similar in terms of location and geography.

• Zone II (Low Damage Risk Zone): This is the least seismically active region. Approximately 43% of India falls under this category.
• Zone III (Moderate Damage Risk Zone): This zone experiences moderate seismic activity. It covers about 27% of India.
• Zone IV (High Damage Risk Zone): This region is prone to severe earthquakes. It covers about 18% of India.
• Zone V (Very High Damage Risk Zone): This is the most seismically active region of India. It covers about 12% of India.

Effects of Earthquake:

The effects of earthquakes can be broadly grouped into two categories – primary and secondary hazards. Primary hazards are caused by the direct interaction of seismic wave energy with the ground.

The immediate concern for life and property:

1. Ground shaking: Shaking of the ground caused by the passage of seismic waves, especially surface waves near the epicentre of the earthquake is responsible for the most damage during an earthquake.
2. Avalanches and Landslides.
3. Floods from dam and levee failures,
4. Fires, Structural Collapse, Falling objects,
5. Tsunamis: waves generated by tremors in Ocean waters.

Secondary bearing upon landforms – Chain Effects of Earthquakes:

1. Differential ground settlement: Due to earthquakes if only part of the foundation is affected by ground failure or part of the foundation is affected to a greater extent than other parts. This kind of effect is called differential settlement.
2. Mudslides: Landslides occur when masses of rock, earth, or debris move down a slope. Debris flows, also known as mudslides, are a common type of fast-moving landslide.
3. Soil liquefaction: Soil liquefaction is a phenomenon in which soil gets saturated with water due to the vibration of soil particles and their settlement, losing its strength and stiffness.
4. Ground lurching: Ground lurching is the horizontal movement of soil, sediments, or fills located on relatively steep embankments or scarps as a result of seismic activity, forming irregular ground surface cracks.
5. Environmental consequences – Sometimes Surface seismic waves produce fissures on the upper layers of the earth’s crust through which water and other volatile materials gush out, inundating the neighbouring areas.
6. On Water: often landslides cause obstructions in the flow of rivers and channels resulting in the formation of reservoirs. Sometimes, rivers also change their course causing floods and other calamities in the affected areas.

On Ground	On Manmade Structures	On Water
Fissures	Cracking	Waves
Settlements	Slidings	Hydro-Dynamic Pressure
Landslides	Overturning	Tsunami
Liquefaction	Buckling	Possible Chain-effects
Earth Pressure	Collapse
Possible Chain-effects	Possible Chain-effects

Socio-Environmental Consequences of Earthquakes:

Earthquakes are often associated with fear and horror due to their scale, magnitude and suddenness – become a calamity when they strike areas of high-density of population.

Earthquake Hazard Mitigation

It is impossible to prevent earthquakes. The best emphasis should be on preparedness and Mitigation. We can undertake several steps such as:

• Establishing earthquake monitoring centres for regular monitoring and fast dissemination of information among the people in the vulnerable areas. For example, the Use of GPS to monitor movement of plates.
• Preparing a vulnerability map and dissemination of vulnerability risk information among the people;
• Modifying the building designs in the vulnerable areas and discouraging construction of high-rise buildings, large industrial establishments and big urban centres in such areas. Making it mandatory to adopt earthquake-resistant designs and use light materials in major construction activities in vulnerable areas.
• Educating people about the ways and means of minimising the adverse impacts of disasters.
• Information dissemination: Such as Earthquake Alarms for Earthquakes.

Note: This mind map covers most steps that can be recommended in the case of Earthquakes. Such Mind-Maps would help you the most for your Mains exam.

Note 2: National Disaster Management Guidelines 2014 has been formulated for the seismic retrofitting of deficient buildings and structures.

Did you Know?

In India, the government-backed earthquake alarm on mobile phones is delivered through the “Android Earthquake Alert System” by Google, which uses the phone’s accelerometer to detect early tremors and send alerts to users with Android 5 or higher, in collaboration with the National Disaster Management Authority (NDMA) and the National Center for Seismology (NCS).

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