SEISMICITY & CONCEPT OF SEISMIC ZONING

 

SEISMICITY & CONCEPT OF SEISMIC ZONING

 

India lies at the northwestern end of the Indo-Australian Plate, which encompasses India, Australia, a major portion of the Indian Ocean and other smaller countries. This plate is collidingagainst the huge Eurasian Plate and going under the Eurasian Plate; this process of one tectonic plate getting under another is called subduction. Himalayan-Nagalushai region, Indo-Gangetic plain, Western India and Cutch and Kathiawar regions are geologically unstable parts of the country and some devastating earthquakes of the world have occurred there. A major part of peninsular India has also been visited by strong earthquakes, but these were relatively few in number and had considerably lesser intensity.

Figure : Indian Seismic Zone Map

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. Parts of Himalayan boundary in the north and northeast, and the Kachchh area in the west were classified as zone V. 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 as shown in Figure above  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.

 

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. Seismicmicrozonation accounts for local variations in geology, local soil profile etc. Seismic zoning map of a country is a guide to seismic status of a region and its susceptibility to earthquakes. It is important to note that following are the major and important modifications made in the latest revision of IS 1893:

 

(i) The seismic zone map is revised with only four zones, instead of five. Erstwhile Zone Ihas been merged to Zone II and hence Zone 1 does not appear in the new zoning; onlyZones II, III, IV and V are there in revised code.

 

(ii) The values of seismic zone factors have been changed; these now reflect more realistic values of effective peak ground acceleration considering Maximum Considered Earthquake (MCE) and service life of structure in each seismic zone.

 

(iii) A clause has been introduced to restrict the use of foundations vulnerable to differential settlements in severe seismic zones.

 

(iv) Here it is worthwhile to mention that it is not intended in this standard to lay down regulationso that no structure shall suffer any damage during earthquake of all magnitudes.

 

(v) It has been endeavoured to ensure that as far as, possible structures are able to respond, without structural damage to shocks of moderate intensities and without total collapse to shocks of heavy intensities.