Various types of disaster


What caused the massive 2004 Indian Ocean Tsunami?

  • On December 26, 2004 incoming gigantic tsunami waves generated due to a M 9.3 Great undersea earthquake off the coast of Banda Aceh, northern Sumatra.
  • This earthquake occurred along a thrust fault in the subduction zone where the Indian tectonic plate is going below the overriding Burmese plate. As a result, the ocean floor broke and there was a vertical displacement of about 15 to 20 meters along the fault causing large scale displacement of water and thus, generating tsunami waves.
  • Since 1900, only five earthquakes, worldwide have exceeded magnitude 9.0 and all of them occurred in subduction zones at shallow depths and broke the ocean floor with displacement of the order of greater than 10 metres generating gigantic tsunami waves. These earthquakes which are referred to as “Megathrust” earthquakes were
    1. 1952 Kamchatka, Russia,
    2. 1960 Chile (the world’s greatest so far with M 9.5),
    3. 1964 Prince William Sound, Alaska,
    4. 2004 Sumatra
    5. 2011 Tohoku, Japan.
  • Tsunami waves are also known to have been generated by earthquakes of greater than 8.5 magnitudes but with lesser intensities. Since the tsunami waves are long period waves with wave lengths of 200-250 kilometres, their height in the open sea ranges between few centimetres to a metre and cannot be distinguished by people travelling on a ship in an open sea. The speed of a tsunami wave is related to the depth of the ocean, greater the water depth higher the speed. Typically, for an average ocean depth of 4 km, like in the Indian Ocean/Bay of Bengal region, the speed of the tsunami waves can go up to 720 km/h or about the speed of a jet airliner.
  • More than 16,000 people perished in the Andaman and Nicobar islands and along the eastern coast of India. Although the occurrence of tsunami across the world is well known, it was the first experience for the people of India.
  • Earlier, there are records of tsunami waves touching the Indian shores
    1. in the West in 1945 and
    2. in the East in 1941.
  • Since, then several successful warnings were given by the Centre whenever there was an undersea earthquake of any significance in the Indian Ocean. In terms of tsunami research several models were developed in which scenarios were created which will help in predicting the time of tsunami wave arrivals, their heights and inundation along the east and west coast of India in case of earthquakes occurrence in the two subduction zones
    1. in Markran, south of Pakistan in the west and 
    2. Andaman and Nicobar and Sumatra in the east, identified to be sources of tsunami generation.

India set up a warning system in 2005 and upgraded it to a state-of-the-art Indian Tsunami Early Warning System two years later at the Indian National Centre for Ocean Information Services in Hyderabad. It has the capability to issue tsunami bulletins within 10 minutes of a major earthquake in the Indian Ocean.


Earthquake Prone Zone

Bureau of Indian Standards, based on the past seismic history, has grouped the country into four seismic zones, viz. Zone-II, -III, -IV and –V.

  • Of these, Zone V is the most seismically active region, while zone II is the least.

Seismic Zone Intensity on Modified Mercalli (MM) intensity scale associated with various zone is as follows:

  • II (Low intensity zone)                       VI (or less)
  • III (Moderate intensity zone)             VII
  • IV (Severe intensity zone)                  VIII
  • V (Very severe intensity zone)            IX (and above)

Different zones:

  • Zone-V comprises of entire northeastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, parts of North Bihar and Andaman & Nicobar Islands.
  • Zone-IV covers remaining parts of Jammu & Kashmir and Himachal Pradesh, Union Territory of Delhi, Sikkim, northern parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
  • Zone-III comprises of Kerala, Goa, Lakshadweep islands, and remaining parts of Uttar Pradesh, Gujarat and West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamilnadu and Karnataka.
  • Zone-II covers remaining parts of the country.



Earthquakes are the manifestations of sudden release of strain energy accumulated in the rocks over extensive periods of time in the upper part of the Earth.

Aftershock: An earthquake that follows a large magnitude earthquake called, ‘main shock’ and originates in or around the rupture zone of the mainshock. Generally, major earthquakes are followed by a number of aftershocks, which show a decreasing trend in magnitude and frequency with time.

Fault: A weak plane in the Earth’s crust and upper mantle along which two blocks of rock mass rupture or slip past each other. Faults are caused by earthquakes and earthquakes are likely to reoccur on pre-existing faults, where stresses are accumulated.

Epicentre: It is the point on the surface of the earth, vertically above the place of origin (hypocentre) of an earthquake. This point is expressed by its geographical Coordinates in terms of latitude and longitude

Dust storm:

A dust storm or sand storm is a meteorological phenomenon common in arid and semi-arid regions.

  • Dust storms arise when a gust front or other strong wind blows loose sand and dirt from a dry surface. Particles are transported by saltation and suspension, a process that moves soil from one place and deposits it in another.


  • As the force of wind passing over loosely held particles increases, particles of sand first start to vibrate, then to saltate (leaping movement of sand or soil particles as they are transported in a fluid medium over an uneven surface). As they repeatedly strike the ground, they loosen and break off smaller particles of dust which then begin to travel in suspension. At wind speeds above that which causes the smallest to suspend, there will be a population of dust grains moving by a range of mechanisms: suspension, saltation and creep.
  • Particles become loosely held mainly due to drought or arid conditions, and varied wind causes.
  • In desert areas, dust storms are most commonly caused by either thunderstorm outflows, or by strong pressure gradients which cause an increase in wind velocity over a wide area. The vertical extent of the dust or sand that is raised is largely determined by the stability of the atmosphere above the ground as well as by the weight of the particulates.
  • Drought and wind contribute to the emergence of dust storms, as do poor farming and grazing practices by exposing the dust and sand to the wind.

Why now in Rajasthan?

  • According to Skymet Meteorology Division in India, a low level cyclonic circulation over Pakistan and adjoining Rajasthan region along with high day temperatures had triggered the dust storm.
  • West Rajasthan becomes prone to such dust storms as it enters into the pre-monsoon season. This was the first widespread storm of the season covering a large area. The winds are usually westerlies due to which the dust storms travel a long way.

Heat Wave:

A heatwave is a combination of temperature and humidity for a prolonged period. It is an extended period of very high summer temperature with the potential to adversely affect communities.

  • High temperature are often accompanied by high humidity, that the body cannot tolerate are defined as extreme heat. A heatwave is a very dangerous situation and major threat to lives.
  • According to IMD, Heat Wave occurs when temperatures are greater than 4.5 degree Celsius above what’s usual for the region.
  • When temperatures soar above 47 degrees Celsius, it is known as a severe heatwave.

  • Heat Wave is said to occur when the maximum temperature exceeds 40⁰C in the plains and 30⁰C in the hills. Further, the departure from the seasonal average should be 5-6⁰C, while a severe heat wave is 7⁰C or more above average. In temperate countries, the ceiling is much lower.
  • India’s biggest “hot-spot”, as measured by IMD stations, is Nellore in Andhra Pradesh. It suffered from as many as 30 heat wave days in 1964 and 35 in 1996, the biggest numbers recorded by any station. It figures most frequently among locations in India – as many as 18 years out of 39 – which have registered more than 15 heat wave days in a year over the half-century
  • Occurance: Heat waves form when high pressure aloft strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream ‘follows the sun’. High pressure up traps heat near the ground, forming a heat wave.

Who will be affected?

  • Heat Waves affect human beings, animals and even damage crops.
The people most affected by heatwaves are the homeless, farmers, policemen, construction workers, postmen and vegetable vendors—i.e, those who live or work outdoors. Severe heatwave conditions in the past have resulted in school and college vacations being extended.


The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke. The signs and symptoms are as follows:

  • Heat Cramps: Ederna (swelling) and Syncope (Fainting) generally accompanied by fever below 39*C i.e.102*F.
  • Heat Exhaustion: Fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating.
  • Heat Stoke: Body temperatures of 40*C i.e. 104*F or more along with delirium, seizures or coma. This is a potential fatal condition.

Forest Fire

Fire SpreadThere are three general patterns of fire spread that are recognized.

Types of Fire Spread

Ground fires—which burn organic matter in the soil beneath surface litter and are sustained by glowing combustion.

Creeping ground fire at Grand Canyon National Park, Arizona

Surface fires—which spread with a flaming front and burn leaf litter, fallen branches and other fuels located at ground level.

Surface fire at Grand Canyon National Park, Arizona


Crown fires—which burn through the top layer of foliage on a tree, known as the canopy or crown fires. Crown fires, the most intense type of fire and often the most difficult to contain, need strong winds, steep slopes and a heavy fuel load to continue burning.

Crown fire at Yellowstone National Park, Wyoming


The term ‘drought’ is applied to an extended period when there is a shortage of water availability due to inadequate precipitation, excessive rate of evaporation and over-utilisation of water from the reservoirs and other storages, including the groundwater.

Drought is a complex phenomenon as it involves elements of meteorology like precipitation, evaporation, evapo-transpiration, groundwater, soil moisture, storage and surface run-off, agricultural practices, particularly the types of crops grown, socio-economic practices and ecological conditions.


Types of Droughts

Meteorological Drought: It is a situation when there is a prolonged period of inadequate rainfall marked with mal-distribution of the same over time and space.

Agricultural Drought: It is also known as soil moisture drought, characterised by low soil moisture that is necessary to support the crops, thereby resulting in crop failures. Moreover, if an area has more than 30 per cent of its gross cropped area under irrigation, the area is excluded from the drought-prone category.

Hydrological Drought: It results when the availability of water in different storages and reservoirs like aquifers, lakes, reservoirs, etc. falls below what the precipitation can replenish.

Ecological Drought: When the productivity of a natural ecosystem fails due to shortage of water and as a consequence of ecological distress, damages are induced in the ecosystem.


A cloudburst is an extreme amount of precipitation, sometimes accompanied by hail and thunder that normally lasts no longer than a few minutes but is capable of creating flood conditions.

A cloudburst can suddenly dump 72,300 tons of water over one acre. However, cloudbursts are infrequent as they occur only via orographic lift or occasionally when a warm air parcel mixes with cooler air, resulting in sudden condensation.

It’s occurrence:

  • Cloud burst is a situation when the inter-molecular forces between the H2O molecules get very high due to the rapid decrease in the temperature or excess of electrostatic induction in the clouds causing the lighting to remain inside the cloud only, which causes hyperactive energy inside the cloud. The water molecules get denser and denser and get condensed but do not leave the cloud due to excess of electroforces.
  • As the water concentration get higher and higher and so the weigh gets heavier the water no longer is able to maintain force with the clouds and so they fall and it precipitates.

In the Indian subcontinent, a cloudburst usually occurs when a monsoon cloud drifts northwards, from the Bay of Bengal or Arabian Sea across the plains, then onto the Himalaya and bursts, bringing rainfall as high as 75 millimeters per hour.

Hilly terrains aid in heated air currents rising vertically upwards, thereby, increasing the probability of a cloudburst situation. It is not necessary that cloudbursts happen only in the mountains or high altitudes. Cloudbursts do happen in plains as well. Cloudbursts are common in the tropics, although they can occur anywhere. They occur most often in desert, mountainous regions and interiors of continents.



Tropical Cyclone

Tropical cyclones are intense low-pressure areas confined to the area lying between 30° N and 30° S latitudes, in the atmosphere around which high velocity winds blow. Horizontally, it extends up to 500-1,000 km and vertically from surface to 12-14 km. A tropical cyclone or hurricane is like a heat engine that is energised by the release of latent heat on account of the condensation of moisture that the wind gathers after moving over the oceans and seas.

There are some initial conditions for the emergence of a tropical cyclone are:

(i) Large and continuous supply of warm and moist air that can release enormous latent heat.

(ii) Strong Coriolis force that can prevent filling of low pressure at the centre (absence of Coriolis force near the equator prohibits the formation of tropical cyclone between 0°-5° latitude).

(iii) Unstable condition through the troposphere that creates local disturbances around which a cyclone develops.

(iv) Finally, absence of strong vertical wind wedge, which disturbs the vertical transport of latent heat.


Structure of Tropical Cyclone

Tropical cyclones are characterised by large pressure gradients. The centre of the cyclone is mostly a warm and low-pressure, cloudless core known as eye of the storm.

Generally, the isobars are closely placed to each other showing high-pressure gradients. Normally, it varies between 14-17mb/100 km, but sometimes it can be as high as 60mb/100km. Expansion of the wind belt is about 10-150 km from the centre.


Consequences of Tropical Cyclones

It was mentioned that the energy to the tropical cyclone comes from the latent heat released by the warm moist air. Hence, with the increase in distance from the sea, the force of the cyclone decreases. In India, the force of the cyclone decreases with increase in distance from the Bay of Bengal and the Arabian Sea. So, the coastal areas are often struck by severe cyclonic storms with an average velocity of 180 km/h. often, this results in abnormal rise in the sea level known as Storm Surge.

Naming of Cyclones:

The North Indian Ocean region tropical cyclones are being named since October 2004. The region, comprising Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand, each of them suggest seven names. The names given by the countries are used alphabetically one after another.

Last modified: Saturday, 12 October 2019, 12:01 PM