Terrestrial Disasters

Earthquakes

An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. In its most general sense, the word earthquake is used to describe any seismic event—whether natural or caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests.

The seismicity, or seismic activity, of an area is the frequency, type, and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling. At the Earth's surface, earthquakes manifest themselves by shaking and displacing or disrupting the ground. When the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides and occasionally, volcanic activity.

Earthquakes can have origin in the plate tectonics or in the volcanic eruption, rock fall, landslides, subsidence, particularly in the mining areas, impounding of dams and reservoirs, etc. Tectonic earthquakes are the most devastating ones with quite large area of influence. Others have limited area of influence and the scale of damage.

Himalayan Earthquakes

Indian plate is moving at a speed of one to five centimetre per year towards the north and northeastern direction and this movement of plates is being constantly obstructed by the Eurasian plate from the north. As a result of this, both the plates are said to be locked with each other resulting in accumulation of energy at different points of time.


Excessive accumulation of energy results in building up of stress, which ultimately leads to the breaking up of the lock and the sudden release of energy causes earthquakes along the Himalayan arch.

Some of the most vulnerable states are Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, and the Darjiling and subdivision of West Bengal and all the seven states of the northeast.

Indian Peninsular Earthquakes

Earthquakes in the Indian PeninsulaUntil recently, textbooks in geology have described peninsular India to be a stable land mass and a region of slight seismicity. Yet, during the past few decades, many earthquakes of magnitude greater than 5 occurred in this region. The central-western parts of India, particularly Gujarat (in 1819, 1956 and 2001) and Maharashtra (in 1967 and 1993) have experienced some severe earthquakes. An important observation is that some of these earthquakes were felt over a much larger area than one would expect earthquakes of equivalent magnitude to be felt in most other parts of the world. The Koyna earthquake changed the long-held image of slight seismicity of peninsular India and earth scientists became keenly interested in the investigation of seismicity and tectonics of the region. However, no clear explanation for the origin of the tectonic stress causing these earthquakes has emerged.

Unlike the earthquakes related to plate boundaries, demarcated by mid-oceanic ridges, transform faults and island arcs, these earthquakes belong to a class called "intra-plate" earthquakes, for which plate tectonics offers no explanation. Recently, some earth scientists have come up with a theory of emergence of a fault line and energy build-up along the fault line represented by the river Bhima (Krishna) near Latur and Osmanabad (Maharashtra) and the possible breaking down of the Indian plate.

Earthquake Zones of India

Geographical statistics of India show that almost 54% of the land is vulnerable to earthquakes. The varying geology at different locations in the country implies that the likelihood of damaging earthquakes taking place at different locations is different. Thus, a seismic zone map is required to identify these regions.

Based on the levels of intensities sustained during damaging past earthquakes, the 1970 version of the zone map subdivided India into five zones – I, II, III, IV and V. The maximum Modified Mercalli (MM) intensity of seismic shaking expected in these zones were V or less, VI, VII, VIII, and IX and higher, respectively.

  1. Zone V: Very high damage risk zone
  2. Zone IV: High damage risk zone
  3. Zone III: Moderate damage risk zone
  4. Zone II: Low damage risk zone
  5. Zone I: Very low damage risk zone

The seismic zone maps are revised from time to time as more understanding is gained on the geology, the seismotectonics and the seismic activity in the country. The Indian Standards provided the first seismic zone map in 1962, which was later revised in 1967 and again in 1970. The map has been revised again in 2002, and it now has only four seismic zones – II, III, IV and V. The areas falling in seismic zone I in the 1970 version of the map are merged with those of seismic zone II. Also, the seismic zone map in the peninsular region has been modified. This 2002 seismic zone map is not the final word on the seismic hazard of the country, and hence there can be no sense of complacency in this regard.

Indian Seismic Zone Map

According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity. The national Seismic Zone Map presents a large scale view of the seismic zones in the country. Local variations in soil type and geology cannot be represented at that scale. Therefore, for important projects, such as a major dam or a nuclear power plant, the seismic hazard is evaluated specifically for that site. Also, for the purposes of urban planning, metropolitan areas are microzoned. Seismic microzonation accounts for local variations in geology, local soil profile, etc.

The MSK (Medvedev-Sponheuer-Karnik) intensity broadly associated with the various seismic zones is VI (or less), VII, VIII and IX (and above) for Zones 2, 3, 4 and 5, respectively, corresponding to Maximum Considered Earthquake (MCE).

Zone 5

Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity MSK IX or greater. It is referred to as the Very High Damage Risk Zone. The region of North-east states, areas to the north of Darbhanga and Araria along the Indo-Nepal border in Bihar, Uttarakhand, Western Himachal Pradesh (around Dharamshala) and Kashmir Valley in the Himalayan region and the Kuchchh (Gujarat) and the Andaman and Nicobar group of islands fall in this zone.

Generally, the areas having trap rock or basaltic rock are prone to such earthquakes.

Zone 4

This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, the parts of Indo-Gangetic plains (North Punjab, Chandigarh, Western Uttar Pradesh, Terai, North Bengal, Sundarbans) and the capital of the country Delhi fall in Zone 4. In Maharashtra, the Patan area (Koynanagar) is also in zone no-4. In Bihar the northern part of the state like Raxaul, Near the border of India and Nepal, is also in Zone IV.

Zone 3

This zone is classified as Moderate Damage Risk Zone which is liable to MSK VII.

Zone 2

This region is liable to MSK VI or less and is classified as the Low Damage Risk Zone.

Zone 1

Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classed as Zone 1. Future changes in the classification system may or may not return this zone to use.

Consequences of Earthquakes

Earthquakes have all encompassing disastrous effects on the area of their occurrence. Some of the important ones are listed below.

Physical Consequences

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


Socio-Environmental Consequences

It becomes a calamity when earthquake strikes the areas of high density of population. It not only damages and destroys the settlements, infrastructure, transport and communication network, industries and other developmental activities but also robs the population of their material and socio-cultural gains that they have preserved over generations. It renders them homeless, which puts an extra-pressure and stress, particularly on the weak economy of the developing countries.

Earthquake Environmental Effects

Earthquake environmental effects are the effects caused by an earthquake, including surface faulting, tsunamis, soil liquefactions, ground resonance, landslides and ground failure, either directly linked to the earthquake source or provoked by the ground shaking.

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. Earthquakes are also responsible for landslides and often these 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.