The story so far: Joshimath, a small but busy town in Uttarakhand’s Chamoli district, is in the midst of a crisis as 723 houses in all nine of the town’s wards have developed major or minor cracks on the floors, ceilings and walls. Beams have also been dislodged in many houses.
The first cracks appeared in a few houses in October 2021, but the administration failed to take adequate steps. After more cracks appeared in buildings this January, 145 families have been temporarily relocated to safer locations within the town while a debate on the future of Joshimath rages.
Joshimath is located in Zone V of India’s seismic zonation scheme, which denotes a high-risk area.
It is also worth noting that the Indian Space Research Organisation (ISRO) said in a report that the town in Uttarakhand sank 5.4 cm between December 27, 2022, and January 8, 2023. However, the report and the satellite images were later withdrawn from the ISRO website. The National Disaster Management Authority (NDMA) has directed all departments and organisations associated with survey and data collection in Joshimath not to interact with the media or share any data on social media.
What are seismic zones?
A seismic zone is an area where there is a high probability of earthquakes due to the area’s geology. Seismic zonation, which involves dividing areas based on expected ground motion, assesses the hazards related to earthquakes in such areas to provide inputs for safer constructions and other practices.
India’s seismic zone maps
A published in the Journal of theInternational Society for the Prevention and Mitigation of Natural Hazards said that almost 65% of India falls in high to very high seismic zones.
According to the 2002 version of India’s seismic zone map, earthquake-prone regions in the country are divided into four zones – zone II, III, IV, and V – based on intensity levels during past earthquakes. However, this is not the version of the map that has always been in use.
The first national seismic zoning map of India was compiled by the Geological Survey of India in 1935, after the 1934 Nepal-India earthquake that measured over 8.0 on the Richter scale and destroyed Kathmandu, Bhaktapur and Patan in Nepal. It consisted of three zones – severe, light, and minor hazard, “based on the broad concept of space-time earthquake statistics and the prevailing understanding of geotectonic” movements, as stated in a research paper by scholars A.K. Mohapatra and W.K. Mohanty.
In 1962, the Bureau of Indian Standards (BIS) published a seismic zonation map of India. This map markedearthquake epicentres in the country and built on the isoseismic map published by the GSI in 1935. It divided India into seven zones – from 0 (no damage) to VI (extensive damage). It was reviewed in 1966, using geological and tectonic features to modify the zones.
Until 1967, seismologists were under the impression that most of the Deccan plateau and peninsular India were free from seismic activity. However, in 1967, a 6.3-magnitude earthquake struck the Koyna hydroelectric project. killing hundreds of people and injuring thousands. This led to major revisions in the 1970 seismic zone map of India.
The 1970 map consisted of five zones – I, II, III, IV, and V – based on the Modified Mercalli Intensity (MMI) scale with a Comprehensive Intensity Scale (CIS-64). The MMI scale takes into account the effect of earthquakes on people, objects, and buildings, and estimates the shaking intensity from an earthquake at a specific location.
Zero was removed as a zone since it was deemed scientifically inappropriate to consider a region completely safe from earthquakes. Another major change in the 1970 version of the map was the merging of zones V and VI.
A new update to India’s seismic zone map was released in 1984. This map identified the seismic potential of regions based on past earthquakes as well as its tectonic features.
Seismic zone maps after 1984
Till 1984, IS 1893 was the main seismic code of India, and all seismic zone maps were based on it. This code was revised in 1966, 1970, 1975, and 1984. In 1991, it was decided that IS 1893 would be split into parts.
Latur in Maharashtra was struck by an earthquake of intensity IX (on MMI-CIS-64 scales) in 1993. The 6.3-magnitude earthquake killed thousands of people and occurred in a region that was placed in zone I — an area at low risk. This led to further revisions in the seismic zonation map of India.
What do India’s seismic zones signify?
The latest seismic zone map of India was released in 2002 with only four zones – II, III, IV, and V. Approximately 11% area of the country falls in zone V, 18% in zone IV, 30% in zone III and the remaining in zone II.
Seismic zones signify the intensity of earthquakes which are most likely in those classified areas. According to the revised IS 1983-2002 seismic code, zones are mapped to a modified CIS-64 scale, an alternative to the MMI scale for seismic zoning. The revised seismic code classifies areas under zone II to coincide with an intensity of VI and below on the modified CIS-64 scale. Zone III includes areas that are prone to earthquakes mapped to intensity VII on the scale, zone IV to intensity VIII, and zone V to intensity IX and above.
Zone II, which was made by combining areas under zone I and II, indicate areas of under intensity. Some examples of areas under zone II are Chitradurga, Kota, Hyderabad, Rourkela, Thanjavur, and Visakhapatnam.
Zone III is the next classification and includes areas that are prone to earthquakes of moderate intensity. Killari, believed to be the epicentre of the 1993 Latur earthquake that killed thousands, lies in zone III. The revised seismic code also includes parts of the eastern coast of India, that show similar hazard levels to that of the Killari area, in zone III. Some notable places in this zone are Chennai, Lucknow, Agra, Belgaum, Bokaro, Jabalpur, and Mumbai.
Zone IV includes areas prone to earthquakes of severe intensity, and include Patna, Pilibhit, Ludhiana, Roorkee, Gorakhpur, and Amritsar.
The entire northeast India, parts of northwestern Bihar, Kangra Valley in Himachal Pradesh, Andaman and Nicobar Islands, eastern part of Uttarakhand, Rann of Kutch in Gujarat, and Srinagar area in Jammu and Kashmir were all placed in zone V – the most seismically active zone.