What happened?
Since November, Maharashtra’s Palghar district has experienced dozens of small earthquakes. As on February 6, a seismometer located around 70 km from Palghar and operated by Gujarat’s Institute of Seismological Research (ISR), recorded 74 quakes, with 26 measuring between 1 and 1.9 on the moment magnitude scale (Mw), 39 measuring 2 and 2.9, and nine measuring over 3, according to ISR director Sumer Chopra. This pattern of several small earthquakes occurring in a brief time-window is called an earthquake swarm. “It is like a swarm of people. There is no leader, and all of them are similar [in magnitude]. So, there is nothing like a foreshock or mainshock,” says Vineet Kumar Gahalaut, director of Delhi’s National Centre for Seismology (NCS).
Why is it a worry?
The epicentres of the current swarms haven’t seen such activity in the past. Why it is occurring now is a mystery. However, swarms are common in peninsular India and mostly harmless because of their low magnitude. In fact, a city called Jawhar in Palghar district has experienced swarms several times earlier, Dr. Gahalaut said. Another vulnerable region is Amaravati in Maharashtra, which saw swarm activity last August. Subsequently, the NCS set up seismometers in the region. They found that most earthquakes there were very shallow: they originated from within a few kilometres below the surface. Further, the tremors stopped soon after the monsoon. The researchers concluded that the swarms were related to the monsoon, and were attributable to a phenomenon called “hydro-seismicity.”
Whether the Palghar quakes are a swarm, however, will only be clear after they stop. Researchers will then categorise them based on the distribution of the magnitude. Even if the magnitude distribution so far suggests a swarm, said Dr. Gahalaut, this will change if a big earthquake follows. If and when that happens, the temblors so far will be called foreshocks. Because there is no way to predict whether such a large quake will follow, Palghar residents must be careful.
Why is this happening?
In “hydro-seismicity,” which is hypothesised as the reason for swarms in peninsular India, water from heavy rainfall enters small fractures in rocks. This raises the pressure within them. A study from the ISR in 2008, published in the Journal of Geological Society of India , estimates that with every 10 metre rise in groundwater, pore pressure increases by 1 bar. This pressure is released in earthquake swarms. However, such activity typically starts in June and dies down in December, said Dr. Chopra. In Palghar’s case, it has continued into February. This raises questions about the mechanism behind it. In general, quakes are caused by geological faults, or cracks in the earth’s crust across which rocks get displaced. There are plenty of faults along the Konkan coast of India (where Palghar lies), although how many of these are active isn’t known. In 2007, a Current Science study by researchers from IIT-Bombay suggested that swarm activity along the west coast was due to a major fault parallel to it. Dr. Gahalaut said, however, that more evidence is needed to determine the extent of this fault and how active it is.
What next?
Palghar falls in Zone 3 of the seismic zoning map developed by the Bureau of Indian Standards. This means that buildings here must be able to withstand earthquakes of intensity 5.5-6.5 on the Medvedev-Sponheuer-Karnik (MSK) scale. Intensity is a qualitative measure of how people experience earthquakes, rather than the energy released, which is measured by the magnitude scale, said Deepankar Choudhury, a civil engineer at IIT-Bombay. In earthquakes measuring 5.5-6.5 on the MSK scale, people are frightened and run outdoors, and heavy furniture can move. Buildings that follow the BIS codes are likely to survive swarms and even larger quakes. So, it is crucial for the code to be implemented stringently.
