More than a dozen Indian cities are at a high risk of experiencing major casualties in the event of an earthquake, says new report that studies the earthquake risk for 50 cities in seismically active zones around the country.

As much as 59% of the country is vulnerable to earthquakes, the report says. It adds that more than 90% of casualties in previous earthquakes in India have been because of the collapse of buildings that were not designed to withstand the shocks.

The Earthquake Disaster Risk Index takes into account not just the chance of an earthquake occurring in a particular location, but also the number of people that would be exposed to it and how vulnerable the buildings in the city are to collapse. It was developed by the National Disaster Management Authority in association with IIIT-Hyderabad

The study says that 13 of the 50 cities are at high risk, and 15 of them are at medium risk. “This scenario is alarming and needs immediate attention,” notes the report.

The study took 13 years and required field visits to 25 cities, as well as an extensive study of the built areas of the sites. Computer modelling was used to quantify the risk of built areas collapsing in earthquakes.

The 50 cities and one district – Bareilly in Uttar Pradesh – selected for the study are located in regions with a high population density situated in Seismic Zones IV or V. These are areas most prone to earthquakes, according to the Bureau of Indian Standards.

Also included were areas with a high “housing threat factor”, a parameter that combines the earthquake hazard and housing density of a city. Cities identified by the government to be developed as smart cities were also studied.

The National Disaster Management Authority’s earthquake hazard vulnerability profile estimates that 58.6% of the country lies in Zones III, IV and V, which means that it is prone to earthquakes of moderate to very high intensity.

Zones III to V are subject to moderate to high damage risk of an earthquake. Currently, there is no Zone I classification.

Why is so much of India subject to earthquakes?

Fifty million years ago, the Indian continental plate collided with Eurasia. Before that, the Indian plate had been drifting in the ocean, at a speed of 18 cm a year, which is a breakneck pace geologically speaking. The impact of India crashing into Asia threw up the Himalayan mountain range. It also slowed down the Indian plate, but didn’t stop it. The plate’s upper crust was scraped up into the Himalayas, but the rest of it was forced downward, into the earth’s mantle, where it continues to bore into Eurasia at the rate of 2 cm a year.

Earthquakes of magnitude 5.5 or more in the subcontinent since the year 1900 have been charted and the more significant ones marked.

Every once in awhile, the steady force applied by the Indian plate overcomes the friction of the plate it is moving against, and causes a block of the crust to suddenly slip. The energy released by this causes the crust to vibrate, all the way up to the surface. That is when the earth quakes.

What can be done to mitigate earthquake risk?

The report notes that more than 90% of casualties in previous earthquakes that have struck India have been because of the collapse of buildings. The rapid pace of construction in Indian cities has led to the unregulated development of buildings, especially high-rise ones, which are improperly planned and do not follow safety codes.

The researchers recommend that academics, the construction industry and the government all work together to raise awareness of earthquake risk and to develop, uphold and enforce construction standards. Existing buildings need to be evaluated for their earthquake-readiness and reinforced if necessary. All new construction should comply with the Bureau of Indian Standards’ earthquake engineering guidelines, they say.

It is impossible to prevent an earthquake. It is also impossible to predict when or where one will occur. The only certainty is that there will be more earthquakes. If they occur in one of the high-risk cities before mitigating activity can take place, the damage to life and property will be tremendous.