The rate of earthquake occurrence worldwide, or India in general, has not increased in the last few years, say experts
A spate of earthquakes in north, west and east India — a total of 19 earthquakes with a magnitude of 4.5 or more on the Richter scale between June 16 and July 7 — have triggered alarm around the likelihood of increased seismicity.
The strongest of these was a 6.1-magnitude tremor, close to Hanle in the Ladakh region on June 26.
Most earthquakes have been concentrated either in the Jammu and Kashmir and Ladakh regions or in Mizoram, both of which were hit with seven earthquakes each.
Other places with an increased seismic activity have been Haryana, the Andaman and Nicobar Islands and Nepal. Some regions in Gujarat and Maharashtra also have had frequent earthquakes.
Many fear that these earthquakes are foreshocks of a larger one. But the same can be concluded only after the event has occurred and not before, according the United States Geological Survey.
The rate of earthquake occurrence worldwide, or India in general, has not increased in the last few years, according to experts. Down To Earth spoke to Ian Main, professor of seismology and director of research at the School of Geosciences, University of Edinburgh to ascertain whether there has been an increase in earthquake activity in the past few years, and if earthquakes can be predicted before they occur.
Akshit Sangomla: Is there an increase in earthquake activity around the world or in certain regions in the last few years?
Ian Main: No. The number of earthquakes to hit any part of the world has not been above what one would expect from sampling a random process where events are not causally related to one another.
Sometimes the number is more and sometimes less than the average. This is a bit like how the buses go by, you wait half-an-hour and three arrive at once, but they are not in communication with each other. So, a degree of clustering is expected in a random process and this is well understood.
The assumption of independence between events applies to large events primarily, but these are the ones used in calculations of likely ground motion for building design or insurance risk.
AS: If there has been an increase in activity in some regions then what is the reason for it?
IM: The exception is areas that are currently experiencing a sequence of causally related events, for example a set of aftershocks or a seismic swarm, which may trigger each other or be triggered by a common underground process such as subsurface fluid flow.
This leads to clustering, primarily of small events, which is more concentrated than you would expect for a random process.
AS: What do we know about earthquake activity in the past?
IM: We know earthquakes concentrate on plate boundaries such as the Himalaya, and we know why, due to the motion of the plates. We also find events in the middle of a plate sometimes because the plates themselves can deform to some extent. We know earthquakes can be triggered by human intervention, such as filling a water reservoir behind a dam.
AS: What is the status of earthquake prediction science? How close are we to predicting earthquakes and providing early warning to people before major earthquakes?
IM: We can provide a few minutes of early warning, exploiting the fact that electronic signals travel faster than the speed of transmission of ground motion (limited to Mach 1 or the speed of sound underground).
This is enough to allow people to brace for the earthquake, get under a table or a door lintel, etc.
We can also say if the probability of an earthquake is higher than usual due to a high degree of clustering and make ‘operational earthquake forecasts’. Though the probabilities remain low (less than 1 per cent a day), leading to a state of heightened alert.
People can then make their own choices as to how to deal with such a low risk, and the authorities can prepare and stand by in case needed.
Sadly, we are nowhere near providing the kind of reliable, accurate prediction of the size, time and place of a future earthquake, above chance, and within narrow limits’ that one would need to justify an evacuation before the event.
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