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Friday, Nov 22, 2019

3,637 earthquakes in Palghar since January: NGRI data

Multiple tremors have been rocking the district since November 2018.

mumbai Updated: Jul 26, 2019 01:21 IST
Snehal Fernandes
Snehal Fernandes
Hindustan Times
Rishya Meghwali died when a portion of his house fell on him as tremors hit Dahanu on Thursday morning.
Rishya Meghwali died when a portion of his house fell on him as tremors hit Dahanu on Thursday morning.(HT Photo)
         

Palghar district has witnessed 3,637 earthquakes — an average of 17 every day — measuring between 1 and 4.1 on the Richter scale since January, data from the six seismological stations set up by the National Geophysical Research Institute (NGRI), Hyderabad has revealed.

Multiple tremors have been rocking the district since November 2018. Seismologists studying the micro earthquakes said they are tectonic in nature — they occur because rocks in the Earth’s crust break as a result of the movement of tectonic plates.

From the afternoon of July 24 to Thursday afternoon, the district recorded 55 earthquakes — four above the magnitude 3 — killing one person. Sri Nagesh D, chief scientist and head of the seismological observatory at NGRI, said the earthquake activity had been almost dying since June 27, when there were only one or two low-magnitude quakes. However, the activity picked up from July 9, eventually leading to 55 earthquakes in a day. “A region should be critically stressed for an earthquake. The Indian plate is moving towards the north-northeast. Tectonic stresses get accumulated in the weak zones owing to the motion of the plates. When these stresses exceed the strength of the rocks, they slip, resulting in earthquake,” said Sri Nagesh D.

The earthquakes’ epicentre at Dhundalwadi is on the seismologically active west coast fault, mapped by Delhi-based National Centre of Seismology. The fault line is 11-km long and 2-km wide. The district falls under the moderate seismic zone 3, which means structures should be able to withstand earthquakes measuring 5.5-6.5. Since November, a large number of earthquakes originated from a depth of more than 5kms in this region. The sequence of earthquakes on Thursday, however, was at an estimated depth of 10km — one reason for scientists to attribute it to tectonic activity and not monsoon.

“There is no connection with the monsoon as earthquakes have underlying causes that are totally different. If the depth is more than 5kms, then it is most certainly a tectonic activity,” said professor Ravi Sinha, department of civil engineering, Indian Institute of Technology-Bombay. Sinha heads a committee constituted by the state to train engineers in Palghar in assessing safety of constructions.

Previously, some experts had said when rainwater seeps into the rocks, the pore pressure inside increases and releases in the form of swarms of low-magnitude earthquakes as the region is critically stressed. However, now, experts said there is no connection to monsoon. They said tectonic plates have been shifting for millions of years and there is sudden activation and reactivation along the fault line. “We happen to be at a place where reactivation took place. As a large number of tremors have occurred at a depth of over 5km, the degree of concern becomes different. These earthquakes or tremors can go on for years,” said Sinha.

Vineet Kumar Gahalaut, director, NSC, said earthquake sequences are restricted to a small region of 4-5km. “If the occurrence has to lead to a big earthquake, I would assume the activity to be slightly more diffused. To me, it’s a swarm that will not lead to a big earthquake.”