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On December 10, 1967, Koyna in Maharashtra hit the headlines globally. And it left seismologists shaken.
A devastating 6.3 magnitude earthquake in Koyna, where a grand, huge dam was built in 1962, caused much misery. But, more importantly, it shattered a long-held belief that the Deccan Traps were solid rock and not prone to earthquakes.
The needle of suspicion pointed to the reservoir. Sections of earth scientists firmly believed it had triggered the trembler. A raging debate ensued on whether reservoir-induced seismicity (RIS) is a cause for earthquakes.
Fifty years hence, scientists think they are close to digging up clinching evidence. The optimism stems from the ₹400-crore deep borehole project of the Ministry of Earth Sciences, which has completed a large part of the pilot studies and gathered data from a 3 km borehole.
Half century of research
Earth scientist and seismologist Harsh K Gupta, one of the foremost proponents of RIS, backs his nearly half a century of research to claim that Koyna is the best spot in the world to understand how earthquakes occur.
The planned 7-km-long borehole, which will go 3-4 km deep and be bent across the Donachiwada fault (mainly responsible for the earthquakes), will unearth all the answers, he told BusinessLine. The earthquake activity, which began in 1962, has so far seen more than 200 episodes of 4+ magnitude, and 22 episodes of 5+ magnitude.
The biggest, of course, was the 6.3 magnitude quake of 1967. To get an idea, the earthquake had power equivalent to the bomb that struck Hiroshima. An increase of every one point over a magnitude of 5 adds 30 times more energy release, Gupta explained.
Before the creation of the dam — 300 feet tall and 800 feet wide, with the artificial Shivsagar lake — no earthquake was reported in the region. Artificial reservoirs are created for flood control, irrigation and power generation.
Globally, there have been about 120 cases of triggered earthquakes. At four places they were above 6 magnitude. These include Kariba on the Zambia-Zimbabwe border (1963), Hsingfenking in China (1961), Kematsa in Greece (1966) and Koyna, which was the biggest, in 1967. The total area impacted was 20 km by 30 km, and there was no other force of earthquakes around 100 km.
There are very few instances of near-field study of earthquakes triggered by reservoirs or large constructions. In such regions, earthquakes usually occur at 2-9 km depths, while in known fault zones (such as San Andreas, California) or ruptures and plate collisions (such as Himalayan region), they can happen up to depths of 40 km.
Koyna is best suited for research to unravel the phenomenon, according to the International Continental Drilling Programme, a group of 30 international and 50 Indian scientists, who first met in 2011 at National Geophysical Research Institute (NGRI).
The scientists conducted the initial studies using airborne, magnetic, LIDAR and other techniques for three years. In 2014, the group met in Karad, near Koyna, reviewed the progress and decided to go for a pilot, setting up six borehole stations at 1,500 m depths. In 2016, a borehole of 3 km depth was completed and data collected till June 2017, said Gupta.
Seismologists argue that large reservoirs can hold more than 1 trillion litres of water that can crack and fissure the insides of the surrounding earth when the water levels inside rise and fall. This can induce weaknesses in the surrounding rock mass and trigger earthquakes.
By continuously monitoring the activity around the reservoir, it is possible to predict an earthquake ahead of time. Gupta and a large group of seismologists from NGRI who have been working on the area have developed a model that they claim has been tested over the past few years, and found to be reasonably accurate in terms of time and locate of quakes with 4+ magnitude.
Himalayan task
However, it has not been a smooth scientific mission. Questions have been raised about the huge costs and also the practicality in the Indian context, where the threat and chances of a massive 7+ plus quake are more likely in the Himalayan belt.
In contrast, the chances of a Koyna type big earthquake occurring in peninsular India are very rare, maybe one in hundred years.
Therefore, the need to understand and strengthen preventive and predictive capabilities in the Himalayan region are greater, contend a large section of seismologists.
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