An earthquake is a phenomenon that occurs without warning and involves violent shaking of the ground and everything over it. It results from the release of accumulated stress of the moving lithospheric or crustal plates.
The occurrence of an earthquake in a populated area may cause numerous casualties and injuries as well as extensive damage to property. Earthquakes are by far the most unpredictable and highly destructive of all the natural disasters.
Focus and Epicentre of an Earthquake
Focus – The point within the earth s crust where an earthquake originates is called the focus. It is also referred as seismic focus or hypocenter. It generally lies within the depth of 60 kilometres in the earth crust.
Epicentre – The point vertically above the focus on the earth s surface is known as epicenter . Earthquake travel in the form of the longitudinal wave from the focus to epicentre. The intensity is the highest at the epicentre. That is why the maximum destruction occurs at and around the epicentre. The intensity of vibrations decreases as one moves away from the epicentre.
Earthquakes that are of tectonic origin have proved to be the most devastating and their area of influence is also quite large. As compared to these, the earthquakes associated with a volcanic eruption, rockfall, landslides, subsidence, particularly in the mining areas, impounding of dams and reservoirs, etc. have a limited area of influence and the scale of damage. There is a geographic pattern of an earthquake around the world at the tectonic plate margins.
There are instances where earthquake occurs within the plate also. India’s increasing population and extensive unscientific constructions mushrooming all over, including multistoried luxury apartments, huge factory buildings, gigantic malls, supermarkets as well as warehouses and
masonry buildings keep – India at high risk. During the last 15 years, the country has experienced 10 major earthquakes that have resulted in over 20,000 deaths.
As per the current seismic zone map of the country, over 59 percent of India’s land area is under threat of moderate to severe seismic hazard. In fact, the entire Himalayan belt is considered prone to great earthquakes of magnitude exceeding 8.0-; and in a relatively short span of about 50 years, four such earthquakes have occurred:
- 1897 Shillong (M8.7)
- 1905 Kangra (M8.0)
- 1934 Bihar-Nepal (M8.3)
- 1950 Assam-Tibet (M8.6)
The Himalayas is the region of convergence of Indian and Eurasian plates. The Indian plate is moving at a speed of one centimeter per year towards the north and northeastern direction. In the recent past, even inter-plate
boundary areas have experienced devastating earthquakes, albeit of lower magnitude than the Himalayan earthquakes.
The Koyna earthquake in 1967 led to a revision of the seismic zoning map. The occurrence of the Killari earthquake in 1993 resulted in further revision of the seismic zoning map.
The North-Eastern part of the country continues to experience moderate to large earthquakes at frequent intervals including the two great
earthquakes. On average, the region experiences an earthquake with a magnitude greater than 6.0 every year.
The Andaman and Nicobar Islands are also situated on an inter-plate boundary and frequently experience damaging earthquakes
India is divided into four earthquake zones. There was a change in the earthquake map of India where Very low-risk zone and Low-risk zone were merged into a single ‘low-risk zone’. Zone IV and Zone V had experienced some of the most devastating earthquakes in India.
Areas vulnerable to these earthquakes are the North-east states, areas to the north of Darbhanga, and Araria along the Indo-Nepal border in Bihar, Uttaranchal, Western Himachal Pradesh (around Dharamshala) and Kashmir Valley in the Himalayan region and the Kuchchh (Gujarat). Most of the areas that can be considered safe are from the stable landmass covered under the Deccan plateau.
Socio Environmental Consequences of Earthquakes
Earthquake is often associated with fear and horror due to the scale, magnitude, and suddenness at which it spreads disasters on the surface of the earth without discrimination.
It becomes a calamity when it strikes the areas of the high density of the population. Following are the major impact of the earthquake:
Damage of property: when an earthquake occurs, all buildings from the cottage to palaces and stronger skyscrapers are greatly damaged or totally destroyed. Underground pipelines and railway lines are damaged or broken. Dams on river collapse, resultant floods cause havoc.
Human loss: The duration of tremors of an earthquake is normally of only a few seconds, but thousands of people may die in this short period. Bihar earthquake of 1934 and Kangra earthquake of 1905, 10,000 and 20,000
people died respectively. The destruction of property was tremendous and could not be estimated properly and exactly.
Changes in river courses: Sometimes river channels are blocked or their courses are changed due to the impact of the earthquake.
Tsunamis: are caused by an earthquake. It wreaks havoc on the settlement of coastal areas. It sinks large ships. The tsunami that occurred on 26-12-2004 near the coast of Sumatra (Indonesia) damaged property worth billions of rupees.
Fountains of Mud: Due to the intense impact of earthquake hot water and mud appear on the surface and take a form of fountains. In the Bihar earthquake of 1934, some cracks and fissures had developed. The fields of
the farmer was covered by knee-deep mud and the crops were destroyed.
Different Effects of Earthquake are Shown in the Table
|On Ground||On Manmade Structures||On Water|
Possible Chain effects
Possible China effects
Possible China effects
Earthquake Hazard Mitigation
Since earthquake also destroys most of the transport and communication links, providing timely relief to the victims becomes difficult.
It is not possible to prevent the occurrence of an earthquake; hence, the next best option is to emphasis on disaster preparedness and mitigation rather than curative measures such as:
Establish earthquake monitoring centers (seismological centers) for regular monitoring and fast dissemination of information among the people in the vulnerable areas.
Preparing a vulnerability map of the country and dissemination of vulnerability risk information among the people and educating them about the ways and means minimizing the adverse impacts of disasters.
Community preparedness: Community preparedness is vital for mitigating earthquake impact. The most effective way to save you even in the slightest shaking is ‘DROP, COVER, and HOLD’.
Planning: The Bureau of Indian Standards has published building codes and guidelines for safe construction of buildings against earthquakes. Before the buildings are constructed the building plans have to be
checked by the Municipality, according to the laid down bylaws.
Many existing lifeline buildings such as hospitals, schools, and fire stations may not be built with earthquake safety measures. Their earthquake
safety needs to be upgraded by retrofitting techniques.
The analysis of data from major earthquakes establishes the fact that the casualties here in India are primarily due to building collapse while the similar intensity earthquakes in the US or Japan does not lead to such enormous loss of lives as the structures in these countries are built with structural mitigation measures and earthquake-resistant features
Public education is educating the public on the causes and characteristics of an earthquake and preparedness measures. It can be created through sensitization and training programs for the community, architects, engineers, builders, masons, teachers, government functionaries teachers, and students.
By preparation of disaster management plans by schools, malls, hospitals etc. and carrying out a mock drill, by preparing documentation on lessons from previous earthquakes and widely disseminating it
Engineered structures: Buildings need to be designed and constructed as per the building by laws to withstand ground shaking. Architectural and engineering inputs need to be put together to improve building design and construction practices. The soil type needs to be analyzed before construction.
Building structures on soft soil should be avoided. Buildings on soft soil are more likely to get damaged even if the magnitude of the earthquake is not strong as shown. Similar problems persist in the building constructed on the river banks which have alluvial soil.
Risk transfer instruments – These should be developed in collaboration with insurance companies and financial institutions.
Encouraging the use of Indigenous Methods – Indigenous earthquake-resistant houses like the bhongas in the Kutch region of Gujarat, dhajji diwari buildings in Jammu & Kashmir, brick-nogged wood frame constructions in Himachal Pradesh and ekra constructions made of bamboo in Assam are increasingly being replaced with modern Reinforced Cement Concrete (RCC) buildings, often without incorporating earthquake-resistant
features and without compliance with building codes and bye-laws.
It is thus necessary to empower communities to ensure the seismic safety of the built environment by encouraging the use of simple, easy, and affordable technical solutions and institutional arrangements and making use of indigenous technical knowledge and locally available materials in the construction of earthquake-resistant buildings in suburban and rural areas.
Quick and Effective Response – Although NDRF battalions have been set up by the government, experience has shown that over 80% of search and rescue is carried out by communities itself before the intervention of
state machinery and specialized rescue and relief forces. Thus there is a need to give basic training to the community members as it is always the first responder after any disaster.
Early Earthquake Warning and Security System – Chennai-based Structural Engineering Research Centre (CSIR-SERC), a pioneer advanced seismic testing and research laboratory under the Council of Scientific &
Industrial Research (CSIR), has completed the testing of the German-developed “Early Earthquake Warning and Security System”, which was launched for the first time in India early this year. Participation of the community at the local level in planning, implementation, and monitoring process.
Cgallenges for India in Mitigation
Inadequate numbers of trained and qualified civil engineers, structural engineers, architects and masons proficient in earthquake-resistant design and construction of structures.
Need for imparting training in earthquake-resistant design and construction to faculty members in professional colleges, for revising the curriculum in professional courses
Creating public awareness of seismic risk reduction features in non-engineered construction in earthquake-prone areas.
Carrying out a structural safety audit of existing critical lifeline infrastructure and their strengthening and seismic retrofitting in earthquake-prone areas.
Mobilization of resources including funds and their management
Some Dos and Don’ts during and after the earthquake:
Have a disaster emergency kit ready which includes a torch, radio, first aid kit, emergency food, chlorine tablets for water purifier, cash and cards, etc.
Inside the House – Don’t run outside, set your family into-doorways, under the table, or if they are bedridden, move them under the beds; keep away from windows and chimneys.
Outside the House – Don’t go near the buildings, high walls, or electric wires.
While Driving – If an earthquake occurs stop driving and keep sitting in the vehicle.
To be Done Immediately
- Put off the domestic fire, and all electrical switches.
- Leave the house if possible and go to open space.
- Leave the house if a gas leak is detected after the gas is turned off.
- Save water
- Untie and free pets and domestic animals (dogs, cats, and cattle).
If Trapped Under Debris:
- Do not light a match.
- Do not move about or kick up dust.
- Cover your mouth with a handkerchief or clothing.
- Tap on a pipe or wall so rescuers can locate you.
What are the Measure Needed to Manage an Earthquake?
According to the seismologists, the earthquake itself is not the killer, but it is the unpreparedness among the society, in terms of inadequate infrastructure, which kills thousands.
- Risk recognition – The fact that there is a risk first needs to be recognised. People need to be made aware of this risk.
- Risk mapping – risk mapping of the areas in the region and divide them into zones depending upon amount of risk involved.
- Risk mitigation – This required investment in R&D to develop cost effective quake-resistant designs. As this cannot be taken up commercially, public funding is very important. Indigenous knowledge should also be used and disseminated. E.g..- Assam tupe house or ikora style– these houses are made with walls of bamboo or reed mesh (ikora) and plaster set in a wooden framework.
EARTHQUAKE ZONES IN INDIA
Each zone indicates the effects of an earthquake at a particular place based on the observations of the affected areas and can also be described using a descriptive scale like Modified Mercalli intensity scale or the Medvedev–Sponheuer–Karnik scale.
Zone 5 covers the areas with the highest risks zone that suffers earthquakes of intensity MSK IX or greater. The IS code assigns zone factor of 0.36 for Zone 5. Structural designers use this factor for earthquake resistant design of structures in Zone 5. The zone factor of 0.36 is indicative of effective (zero period) level earthquake in this zone. It is referred to as the Very High Damage Risk Zone. The region of Kashmir, the Western and Central Himalayas, North and Middle Bihar, the North-East Indian region, the Rann of Kutch and the Andaman and Nicobar group of islands fall in this zone.Generally, the areas having trap rock or basaltic rock are prone to earthquakes.
This zone is called the High Damage Risk Zone and covers areas liable to MSK VIII. The IS code assigns zone factor of 0.24 for Zone 4 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 no-4.
This zone is classified as Moderate Damage Risk Zone which is liable to MSK VII. and also 7.8 The IS code assigns zone factor of 0.16 for Zone 3.
This region is liable to MSK VI or less and is classified as the Low Damage Risk Zone. The IS code assigns zone factor of 0.10 (maximum horizontal acceleration that can be experienced by a structure in this zone is 10% of gravitational acceleration) for Zone 2.
Since the current division of India into earthquake hazard zones does not use Zone 1, no area of India is classed as Zone 1.
Earthquake Waves are of Two Types — Body Waves and Surface Waves
- P-waves are also known as the Primary waves. They are the first waves to arrive at the surface.
- The characteristics of P-waves are like sound waves. They travel through all three mediums- solid, liquid and gas.
- These waves have a tendency to vibrate parallel to the direction of wave propagation. This causes density differences in the material through which they travel.
- These waves are responsible for elongating and squeezing of material.
- S- Waves arrive after some time after the happening of Earthquake and they are called secondary waves.
- A significant characteristic of these S-waves is that they travel only through a solid medium.
- The direction of vibration of this S-wave is perpendicular to the direction of wave propagation, thereby creating crests and troughs in the material of their transmission.
Practice the following questions
Q-1 Examine why earthquakes occur? Explain the various kind of waves produced in an Earthquake?
Q-2 What are earthquakes and what causes them?