Dahanu town in Maharashtra’s Palghar district has been hit by some 30 low-intensity earthquakes since November last year, leading to more than 10,000 residents of 40 villages moving out of their homes into tents erected by the district administration and National Disaster Response Force. On Friday, a two-year-old girl died after falling while rushing outdoors with her family during an earthquake. Why are so many tremors hitting this area? Are they foreshocks of a larger disaster that is on the way?
WHEN DID IT START? The first quake hit on November 11, and was followed by others measuring between 2 and 4 on the Richter scale. Walls have cracked and collapsed in Dhundalwadi gram panchayat. The strongest so far has measured 3.9. The National Centre for Seismology (NCS) under the Ministry of Earth Sciences and the CSIR laboratory National Geophysical Research Institute (NGRI), Hyderabad, have set up 5 stations with seismometers in Dahanu and Talasari to identify the source of the earthquakes and understand their cause.
IS IT A SWARM? Data collected so far point to an “earthquake swarm”, a series of many (sometimes thousands) low-intensity earthquakes without a discernible main shock that can occur over weeks in active geothermal areas. A swarm lasting over 3 weeks killed 500 people on or around the Indonesian island of Lombok in July-August 2018.
In India, sequences of low-intensity quakes are common in areas that have been hit previously, like Saurashtra in Gujarat and Koyna in Maharashtra, but they are also seen in areas without a history of seismic activity. Dr Srinagesh D, who heads the Seismological Observatory at NGRI, said swarms are normal in peninsular India. Scientists have found no mining activity to explain the quakes, and have ruled out a small reservoir nearby as the cause. The measuring stations have isolated the epicenter to within 5-10 km of the quake sites, nudging scientists from the Gujarat government’s Institute for Seismological Research (ISR) towards the swarm hypothesis.
BUT IT’S PROBABLY TOO EARLY. Seismologists are wary of drawing a definite conclusion without more data. They have not ruled out the possibility of either the quakes now subsiding or of a big one coming. NCS Director Dr Vineet Kumar Gahalaut said identifying the quakes as a swarm would suggest there is little threat of a deadlier one hitting in the near future. Dr M Ravi Kumar, ISR Director General, said: “We still don’t know whether the earthquakes are a result of seismic activity, hydro-seismicity due to water percolation post-monsoon, or magmatic activity in the region.” If they continue, “a closer look” would be needed, Dr Kumar said.
MEANWHILE: A study by the Structural Engineering Division of IIT-Bombay’s Civil Engineering Department last year concluded that kachcha homes, about a third of the buildings in areas hit by the quakes, would be especially vulnerable in a large quake. Division Head Prof Ravi Sinha said district authorities must draw up a community disaster management plan, with Standard Operating Procedures so that the large number of industries in the district do not suffer debilitating damage.