India being a large landmass is particularly prone to earthquakes. The Indian subcontinent is divided into five seismic zones with respect to the severity of the earthquakes.
The classification of the zones has been done by the geologist and scientist as early as 1956 when a 3-zone (Severe, Light and Minor hazard) Seismic Zoning Map of India was produced. Since then the issue of seismic hazard has been addressed by different experts and agencies.
The aforementioned map was based on a broad concept of earthquake distribution and geotectonics. The severe hazard zones are roughly confined to plate boundary regions, ie, the Himalayan frontal arc in the North, the chaman fault region in the north west and the indo burma region in the north east.
The lower hazard zone is confined to indian shield in the south and then moderate hazard zone confined to the transitional zone in between the two.
The bureau of Indian standards is the official agency for publishing the seismic hazard maps and codes. It has brought out versions of seismic zoning map: a six zone map in 1962, a seven zone map in 1966, and a five zone map 1970/1984.
The last of these maps is currently valid; this map was created based on the values of maximum MM intensities recorded in various parts of the country, in historic times.
Zone V is the most vulnerable to earthquakes, where historically some of the country’s most powerful shock have occurred. This region included the Andaman & Nicobar Islands, all of North-Eastern India, parts of north-western Bihar, eastern sections of Uttaranchal, the Kangra Valley in Himachal Pradesh, near the Srinagar area in Jammu & Kashmir and the Rann of Kutchh in Gujarat. Earthquakes with magnitudes in excess of 7.0 have occured in these areas, and have had intensities higher than IX.
Much of India lies in Zone III, where a maximum intensity of VII can be expected. Four of the major metropolitan areas lie in Zone IV, i.e. New Delhi, Mumbai and Calcutta. Only Chennai lies in Zone II. A large section of south-central India lies in Zone I along with a section stretching from eastern Rajasthan into northern Madhya Pradesh. Some areas of Orissa, Jharkhand and Chhatisgarh also lie in Zone I.
In recent years India has been a host to many earthquakes of varying magnitude and intensity. The following table gives a detailed chronology.
|Latitude (Deg N)||Longitude(Deg E)|
|1819 June 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||Shillong Plateau||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-Nepal Border||8.3|
|1941 Jun 26||12.4||92.5||Andaman Islands||8.1|
|1943 Oct 23||26.8||94.0||Assam||7.4|
|1950 Aug 15||28.5||96.7||Arunachal Pradesh-China Border||8.5|
|1956 Jul 21||23.3||70.0||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||Chamoli Dist, Up||6.8|
Seismic waves are the vibrations from earthquakes that travel through the Earth; they are recorded on instruments called seismographs. Seismographs record a zigzag trace that shows the varying amplitude of ground oscillations beneath the instrument.
Sensitive seismographs, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. The time, locations, and magnitude of an earthquake can be determined from the data recorded by seismograph stations.
The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions.
For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value Earthquakes with magnitude of about 2.0 or less are usually call micro earthquakes; they are not commonly felt by people and are generally recorded only on local seismographs.
Events with magnitudes of about 4.5 or greater – there are several thousand such shocks annually – are strong enough to be recorded by sensitive seismographs all over the world. Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher.
On the average, one earthquake of such size occurs somewhere in the world each year. Although the Richter Scale has no upper limit, the largest known shocks have had magnitudes in the 8.8 to 8.9 range. Recently, another scale called the moment magnitude scale has been devised for more precise study of great earthquakes. The Richter Scale is not used to express damage.
An earthquake in a densely populated area which results in many deaths and considerable damage may have the same magnitude as a shock in a remote area that does nothing more than frighten the wildlife. Large-magnitude earthquakes that occur beneath the oceans may not even be felt by humans.
Seismic Active Zone:
The design of a seismic resistant building involves the usage of seismic coefficients. For the purpose manipulating these coefficients the country is divided into FIVE zones ( as recommended in IS 1897 – 1984)
ZONE 1 Area without any damage
ZONE 2 Area with major damage ( i.e., causing damages to structures with fundamentally periods greater than 1.0 second ) earthquakes corresponding to intensities V to VI of MM scale ( MM – Modified Mercalli Intensity scale )
ZONE 3 Moderate damage corresponding to intensity VII of MM scale
ZONE 4 Major damage corresponding to intensity VII and higher of MM scale.
ZONE 5 Area determines by pro seismically of certain major fault systems.