Earthquake is one of the most destructive natural hazard. They may occur at any time of the year, day or night, with sudden impact and little warning. They can destroy buildings and infrastructure in seconds, killing or injuring the inhabitants. Earthquakes not only destroy the entire habitation but may de-stabilize the government, economy and social structure 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.
Seismology (derived from Greek word Seismos meaning Earthquake and Logos meaning science) is the science of Earthquakes and related phenomena.
Seismograph is an instrument that records the ground motions. Seismogram is a continuous written record of an earthquake recorded by a seismograph.
Seismic Zonation map of a country is a guide to the seismic status of a region and its susceptibility to earthquakes. India has been divided into five zones with respect to severity of earthquakes. Of these, Zone V is seismically the most active where earthquakes of magnitude 8 or more could occur recent strong motion observations around the world have revolutionized thinking on the design of engineering structures, placing emphasis also on the characteristics of the structures themselves it should be realized that in the case of shield type earthquakes, historic data are insufficient to define zones because recurrence intervals are much longer than the recorded human history this may often give a false sense of security. Occurrence of the damaging earthquake at Latur, falling in zone I is a typical example of this situation.
Cause of Earthquake :
The earth’s crust is a rocky layer of varying thickness ranging from a depth of about 10kilometers under the sea to 65 kilometers under the continents. The crust is not one piece but consists of portions called ‘plates’ which vary in size from a few hundred to thousands of kilometers. The ‘theory of plate tectonics’ holds that theplates ride up on the more mobile mantle,and are driven by some yet unconfirmed mechanisms, perhaps thermal convection currents. When these plates contact each other, stress arises in the crust. These stresses can be classified according to the type of movement along the plate’s boundaries:
a) pulling away from each other,
b) pushing against one another and
c) sliding sideways relative to each other.
All these movements are associated with earthquakes.The areas of stress at plate boundaries which release accumulated energy by slipping or rupturing are known as 'faults'. The theory of 'elasticity' says that the crustis continuously stressed by the movement of the tectonic plates; it eventually reaches a point of maximum supportable strain. A rupture then occurs along the fault and the rock rebounds under its own elastic stresses until the strain is relieved. The fault rupture generates vibration called seismic (from the Greek 'seismos' meaning shock or earthquake) waves, which radiates from the focus in all directions. The point of rupture is called the 'focus' and may be located near the surface or deep below it. The point on the surface directly above the focus is termed as the epicenter' of the earthquake
It is a quantity to measure the size of an earthquake and is independent of the place of the observation.
The local magnitude is defined as the logarithm of the maximum amplitude measured in microns on a seismogram written by Wood-Anderson seismograph with free period of 0.8 second, magnification of 2,800, damping factor of 0.8 calculated to be at a distance of 100 kms. The relative size of events is calculated by comparison to a reference event of ML=0,using the formula, ML=log A-log Ao
where A is the maximum trace amplitude in micrometer recorded on a standard seismograph and Ao is a standard value which is a function of epicentral distance (Δ) in kilometers.
|Classification of earthquakes|
|Category||Magnitude on Richter Scale|
|Moderate||5.0 to 6.9|
|Great||7.0 to 7.9|
|Very Great||8.0 and more|
India has witnessed some of the most devastating earthquakes during the last century like the one in Kangra (1905), Bihar-Nepal (1934) and in Assam (1950). In the recent past, earthquakes have caused havoc in Uttarkashi (1991), Latur (1993), Jabalpur (1997), Chamoli (1999) and in Bhuj (2001).
On 26th January 2001, India experienced one of the worst earthquakes in recent times. Measuring 6.9 on the Richter scale, the earthquake caused incalculable damage not just to its epicenter, Bhuj but also to other towns of the district of Kutch and to about 500 villages out of the total of 900 villages. The reported damage to property in Gujarat was about Rs.21, 000crore and the number of human lives lost were about 14,000. Of these, more than 500 deaths were reported from Ahmedabad, situated at a distance of about 350 kms from Bhuj. In the same city, close to 150 multi-storied buildings crumbled down. Cities far away from the epicenter, like Surat, too reported damage to property.
SOME DAMAGING EARTHQUAKES IN INDIA AND APPROXIMATE NUMBER OF LIVES LOST
| Year of occurrence||Place of occurrence||Intensity||Others|
|1618||Bombay||-||-||2000 lives lost|
|1720||Delhi||6.5||-||Some lives lost|
|1737||Bengal||-||-||300,000 lives lost|
|1803||Mathura||6.5||-||The shock felt up to Calcutta.|
|1803||Kumaon||6.5||-||Killed 200-300 people.|
|1819||Kutchch||8.0||XI||Chief towns of Tera, Kathara and Mothala razed to the ground.|
|1828||Srinagar||6.0||-||1000 people killed.|
|1833||Bihar||7.7||X||Hundreds of people killed|
|1848||Mt.Abu, Rajasthan||6.0||-||Few people killed|
|1869||Assam||7.5||-||Affected an area of 2,50,000 Sq. miles.|
|1885||Srinagar||7.0||-||Kamiarary area destroyed.|
|1897||Shillong||8.7||XII||Wide spread destruction in Shillong.|
|1905||Himachal Pradesh||8.0||XI||Thousands of people killed.|
|1906||Himachal Pradesh||7.0||-||Heavy damage.|
|1916||Nepal||7.5||-||All houses collapsed at Dharchulla.|
|1930||Dhubri, Meghalaya||7.1||IX||Heavy damage in Dhubri.|
|1934||Bihar, Nepal||8.3||XI||Large number of border area people killed.|
|1935||Quetta (in Pakistan)||7.5||IX||25,000 people killed|
|1941||Andaman||8.1||X||Very heavy damage.|
|1950||Assam||8.6||XII||Heavy damage to life and property.|
|1952||NE India||7.5||-||Heavy damage.|
|1956||Bulandshahar, U.P.||6.7||VIII||Many people killed|
|1956||Anjar, Gujarat||7.0||VIII||Hundreds of people killed|
|1958||Kapkote, U.P.||6.3||VIII||Many people killed|
|1967||Koyna,||6.1||VIII||Koyna Nagar razed.|
|1986||Dharamshala (H.P)||5.7||VIII||Lots of damage.|
|1988||Assam||7.2||IX||Few people killed|
|1988||Bihar- Nepal||6.5||VIII||Large number of people killed.|
|1991||Uttarkashi||6.6||VIII||Lots of damage to life and property.|
|1993||Latur||6.4||VIII||Heavy damage to life and property about, 000 people killed.|
|1997||Jabalpur||6.0||VIII||Lots of damage to property, about 39 lives lost.|
|1999||Chamoli||6.8||VIII||Lots of damage to property about 100 people lost lives.|
|2001||Bhuj||6.9||X||Huge devastation, about ~ 14000 people lost lives|
EARTHQUAKE HAZARDS IN INDIA
India has had a long history of earthquake occurrences. About 65% of the total area of the country is vulnerable to seismic damage of buildings in varying degrees. The most vulnerable areas, according to the present seismic zone map of India, are located in the Himalayan and sub-Himalayan regions, Kutch and the Andaman and Nicobar Islands. Depending on varying degrees of seism city, the entire country can be divided into the following seismic regions:
- Kashmir and Western Himalayas - Covers the states of Jammu and Kashmir, Himachal Pradesh and sub-mountainous areas of Punjab
- Central Himalayas - Includes the mountain and sub-mountain regions of Uttar Pradesh and the sub-mountainous parts of Punjab
- North-east India - Comprises the whole of Indian territory to the east of north Bengal
- Indo-Gangetic basin and Rajasthan - This region comprises of Rajasthan, plains of Punjab, Haryana, Uttar Pradesh and West Bengal
- Cambay and Rann of Kutch
- Peninsular India, including the islands of Lakshwadeep
- The Andaman and Nicobar Islands
MEASURES FOR EARTHQUAKE RISK REDUCTION
For better understanding of all the possibilities of earthquake risk reduction, it is important to classify them in terms of the role that each one of them could play. Therefore, in the pre-earthquake phase, preparedness, mitigation and prevention are concepts to work on. Post-disaster, immediate rescue and relief measures including temporary sheltering soon after an earthquake until about 3 months later and re-construction and re-habilitation measures for a period of about six months to three years need to follow. To encapsulate, the most effective measures of risk reduction are pre-disaster mitigation, preparedness and preventive measures to reduce vulnerability and expeditious, effective rescue and relief actions immediately after the occurrence of the earthquake. Depending upon the calamity and its consequences, strategies can also be divided into long term (five to fifteen years), medium term (one to five years) and short term (to be taken up immediately in high risk areas). Since it has been realized that earthquakes don't kill people but faulty constructed buildings do, the task of reducing vulnerability of structures and buildings will be the key to earthquake risk reduction. Also, pre-disaster preparedness through a post-earthquake response plan, including training of the concerned personnel in various roles, is considered essential for immediate and effective response after an earthquake occurrence. The major action points are highlighted in the following paragraphs.
PRE-DISASTER PREVENTIVE MEASURES
- Re-framing buildings' codes, guidelines, manuals and byelaws and their strict implementation. Tougher legislation for highly seismic areas.
- Incorporating earthquake resistant features in all buildings at high-risk areas.
- Making all public utilities like water supply systems, communication networks, electricity lines etc. earthquake-proof. Creating alternative arrangements to reduce damages to infrastructure facilities.
- Constructing earthquake-resistant community buildings and buildings (used to gather large groups during or after an earthquake) like schools, dharamshalas, hospitals, prayer halls, etc., especially in seismic zones of moderate to higher intensities.
- Supporting R&D in various aspects of disaster mitigation, preparedness and prevention and post-disaster management.
- Evolving educational curricula in architecture and engineering institutions and technical training in polytechnics and schools to include disaster related topics.
Medium term measures
- Retrofitting of weak structures in highly seismic zones.
- Preparation of disaster related literature in local languages with dos and don'ts for construction.
- Getting communities involved in the process of disaster mitigation through education and awareness.
- Networking of local NGOs working in the area of disaster management.
Earthquake Facts & Statistics
Frequency of Occurrence of Earthquakes
|Great||8 and higher||1 ¹|
|Major||7 - 7.9||17 ²|
|Strong||6 - 6.9||134 ²|
|Moderate||5 - 5.9||1319 ²|
|Light||4 - 4.9||13,000 (estimated)|
|Minor||3 - 3.9||130,000 (estimated)|
|Very Minor||2 - 2.9||1,300,000 (estimated)|
|¹Based on observations since 1900.|
² Based on observations since 1990.
Year-wise description of Earth Quakes
|Number of Earthquakes Worldwide for 2000 - 2005. Located by the US Geological Survey National Earthquake Information Center|
|8.0 to 9.9||1||1||0||1||2||1|
|7.0 to 7.9||14||15||13||14||14||9|
|6.0 to 6.9||158||126||130||140||140||116|
|5.0 to 5.9||1345||1243||1218||1203||1509||1307|
|4.0 to 4.9||8045||8084||8584||8462||10894||10264|
|3.0 to 3.9||4784||6151||7005||7624||7937||5782|
|2.0 to 2.9||3758||4162||6419||7727||6317||3249|
|1.0 to 1.9||1026||944||1137||2506||1344||20|
|0.1 to 0.9||5||1||10||134||103||0|
|Total||22256||23534||27454||31419||* 31199||* 21390|
List of Some Significant Earthquakes in India
|1819 Jun 16||23.6||68.6||Kutch,Gujarat||8.0|
|1869 Jan 10||25||93||Near Cachar, Assam||7.5|
|1885 May 30||34.1||74.6||Sopor, J&K||7.0|
|1897 Jun 12||26||91||Shillongplateau||8.7|
|1905 Apr 04||32.3||76.3||Kangra, H.P||8.0|
|1918 Jul 08||24.5||91.0||Srimangal, Assam||7.6|
|1930 Jul 02||25.8||90.2||Dhubri, Assam||7.1|
|1934 Jan 15||26.6||86.8||Bihar-Nepalborder||8.3|
|1941 Jun 26||12.4||92.5||Andaman Islands||8.1|
|1943 Oct 23||26.8||94.0||Assam||7.2|
|1950 Aug 15||28.5||96.7||Arunachal Pradesh-China Border||8.5|
|1956 Jul 21||23.3||7.0||Anjar, Gujarat||7.0|
|1967 Dec 10||17.37||73.75||Koyna, Maharashtra||6.5|
|1975 Jan 19||32.38||78.49||Kinnaur, Hp||6.2|
|1988 Aug 06||25.13||95.15||Manipur-Myanmar Border||6.6|
|1988 Aug 21||26.72||86.63||Bihar-Nepal Border||6.4|
|1991 Oct 20||30.75||78.86||Uttarkashi, Up Hills||6.6|
|1993 Sep 30||18.07||76.62||Latur - Osmanabad, Maharashtra||6.3|
|1997 May 22||23.08||80.06||Jabalpur, MP||6.0|
|1999 Mar 29||30.41||79.42||Champoli, UP||6.8|
|2001 Jan 26||23.40||70.28||Bhuj, Gujarat||6.9|