Earthquake Warnings in India
The India has a history of many earthquakes. The major reason for the high frequency and intensity of the earthquakes is that India is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India shows that almost 54% of the land is vulnerable to earthquakes. The latest version of seismic zoning map of India given in the earthquake resistant design code of India assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones (Zone 2, 3, 4 and 5) unlike its previous version which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity. The latest seismic zoning map can be accessed from govt website.
The MSK ( Medvedev - Sponheuer - Karnik ) intensity broadly associated with the various seismic zones is for Zones 2, 3, 4 and 5, corresponding to Maximum Considered Earthquake. The IS code follows a dual design philosophy: (a) under low probability or extreme earthquake events (MCE) the structure damage should not result in total collapse, and (b) under more frequently occurring earthquake events, the structure should suffer only minor or moderate structural damage. The specifications given in the design code are not based on detailed assessment of maximum ground acceleration in each zone using a deterministic or probabilistic approach. Instead, each zone factor represents the effective period peak ground accelerations that may be generated during the maximum considered earthquake ground motion in that zone.
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.
Zones:
Zone-5:
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) peak horizontal ground accelerations of 0.36 g (36 % of gravity) that may be generated during MCE level earthquake in this zone. It is referred to as the Very High Damage Risk Zone. The state of Kashmir, Punjab,the western and central Himalayas, the North-East Indian region and the Rann of Kutch fall in this zone.The capital of India, Delhi,also lies in this zone.
Zone-4:
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. The Indo-Gangetic basin and the capital of the country (Delhi), Jammu and Bihar fall in Zone 4. In Maharashtra Patan area(Koyananager) also in zone 4.
Zone-3:
The Andaman and Nicobar Islands, parts of Kashmir, Western Himalayas fall under this zone. 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 0.18 for Zone 3.
Zone-2:
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.
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