Seismic Zoning of India
The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of earthquakes is that India is driving into Asia at a rate of approximately 47 mm/year. Almost 54% of the land is vulnerable to earthquakes. The latest version of seismic zoning map of India assigns four levels of seismicity in terms of zone factors. Bureau of Indian Standards [IS-1893 (Part-I):2002], based on various scientific inputs from a number of agencies, has grouped the country into four seismic zones viz. Zone-II, -III, -IV and –V unlike its previous version which consisted of five zones for the country.
Of these, Zone V is the most seismically active region. It covers the area with the highest risk zone that suffers earthquake of intensity IX or greater. The IS zone code against zone factor of 0.36 Zone V. Structural designers use this factor for earthquake resistant design of structure in Zone V. The zone factor of 0.36 is indicative of effective (zero period) peak horizontal ground acceleration of 0.36 g (36% of gravity) that may be generated during MCE level earthquake in this zone. It is referred to as Very High Damage Risk Zone.
Broadly, Zone –V comprises of entire north-eastern India, parts of Jammu and Kashmir, Himachal Pradesh, Uttaranchal, Rann of Kutch in Gujarat, part of North Bihar and Andaman & Nicobar Islands. Generally, the areas having trap or basaltic rock are prone to earthquakes.
Zone IV is called the High Damage Risk Zone. The IS code against zone factor of 0.24 for zone IV. Zone –IV covers remaining parts of Jammu & Kashmir and Himachal Pradesh, Union Territory of Delhi, Sikkim, northern parts of Uttar Pradesh, Indo-Gangetic basin, Bihar and West Bengal, parts of Gujarat and small portion of Maharashtra near the west coast and Rajasthan. Delhi prone areas include the areas near Yamuna bank, east Delhi, Shahdara, Mayur vihar , Laxmi Nagar and near by areas of Gurgaon, Rewari and Noida.
Zone III is classified as Moderate damage Risk Zone which is liable to MSK VII.The IS code assigns zone factor of 0.16 for zone III. Zone –III comprises of Kerala, Goa, Lakshadweep islands, and remaining parts of Uttar Pradesh, Gujarat and West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamilnadu and Karnataka.
Zone II is the least seismically active region. 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, i. e. maximum horizontal acceleration of that can be experienced by a structure in this zone is 10% of gravitational acceleration for zone II. Zone –II covers remaining parts of the country.
The Modified Mercalli (MM) intensity, which measures the impact of the earthquakes on the surface of the earth, broadly associated with various zones is as follows:
Seiswix Zone Intensity on MM scale
II (Low intensity zone) VI(or less)
III (Moderate intensity zone VII
IV (Severe intensity zone) VIII
V (Very severe intensity zone) IX(and above)