Powerful tremors, the most palpable in recent memory, jolted Calcutta and swathes of south Bengal on Friday afternoon as a magnitude-5.3 earthquake struck neighbouring Bangladesh.
The tremors, which lasted for several seconds, triggered widespread panic, forced people to run down the stairs of offices and homes, caused articles to fall off shelves, rattled cutlery and utensils, swayed fans, shook beds and left many with a head rush. Social media was flooded with “stay safe” posts. But no damage had been reported till late evening, police said.
In seismic parlance, a magnitude-5 quake just about qualifies as strong. Experts said the tremors felt so powerful because the epicentre was the closest to Calcutta in recent memory. According to earthquake tracking websites, the epicentre was less than 100km from Calcutta.
The jolt
The earthquake struck at 1.22pm in Bangladesh at a depth of 9.8km, the US Geological Survey (USGS) said. The epicentre was barely 26km southeast of Taki in North 24-Parganas. Taki, a popular tourist destination, is separated from Bangladesh by the Ichamati and is just 70km from Calcutta.
The National Centre for Seismology, under the Union government’s ministry of earth sciences, reported that the quake measured 5.5 on the moment magnitude (Mw) scale, with its epicentre in the Satkhira district of Bangladesh.
“Modern-day earthquakes are not measured on the Richter scale as this scale is now obsolete. Earthquakes are measured on the moment magnitude scale, which is connected to the physics of temblors. Mw is a logarithmic scale, calculated using the seismic moment (M0), which is a measure of the energy released in the earthquake,” said Supriyo Mitra, a seismologist and professor in the department of earth sciences at the Indian Institute of Science Education and Research (IISER), Kolkata.
Safety first
The ground below the city began shaking minutes after 1.20pm. People realised it was an earthquake and scurried down the stairs to safety.
From swanky offices to gated communities and standalone homes, the panic was tangible.
The 40-ft long stretch of a road in Behala's Parnasree that cracked open following the earthquake
Arunava Majumdar, 41, who works with a pre-school chain whose office is on the 11th floor of Technopolis in Sector V, said: “I was headed to the coffee machine when my head spun sharply. Seconds later, people were shouting and the emergency buzzer on the floor went off. Announcements were made, asking everyone to take the stairs to exit the building.”
Chatterjee International in Chowringhee witnessed similar scenes. “Minutes after 1.20pm, I saw the water bottle on my desk shaking. A second later, the entire building shook. All of us sensed danger and left immediately,” said Ruchira Jaiswal, who works on the 10th floor.
State secretariat Nabanna, the Bengal Assembly, Kolkata Municipal Corporation and the office of the chief electoral officer of Bengal were among the government buildings evacuated following the tremors, said the police.
At the Baker Building of Presidency University, classes of the mathematics department being held on the fourth floor had to be suspended briefly, a teacher said. “Students panicked and scurried down the stairways,” the teacher said.
Buildings & roads
A KMC official said the civic body received a complaint that a building on Metcalfe Street had leaned to a side after the earthquake. KMC engineers, however, noticed no cracks on the building when they visited.
“A team led by an executive engineer visited the nine-storey building. It is true that the gap between the leaning building and the adjacent one narrows as one goes up,” said an engineer. “We inspected the building but found no cracks on the floor, wall or roof. Local people also told us that the building has been in this leaning position for a long time,” he added.
A road in Behala’s Parnasree developed a crack. “A crack was detected at Sagar Manna Road after the earthquake. The crack possibly developed because the guard wall of an adjacent waterbody collapsed after the earthquake,” said a KMC engineer.
Why so scary?
The geographical proximity to the epicentre and the shallow depth made the tremors feel so tangible in Calcutta, experts said.
Sankar Kumar Nath, vice-chancellor of Burdwan University and former professor of geophysics and seismology at IIT Kharagpur, told this newspaper: “Bengal and Bangladesh belong to the same geotectonic unit. They are interconnected through the Bay of Bengal. A distance of 200km to 500km from the source is considered near field in seismic parlance. Calcutta was barely 100km from the epicentre. So, the impact was so tangible here.”
Mitra of the IISER said: “Friday’s earthquake occurred at a shallow depth. That is why it created large amplitude surface waves, which caused the strong ground shaking in nearby regions. The focus of this earthquake is on a fault beneath the sediments of Bangladesh. This fault is among the system of sub-parallel faults associated with the Eocene Hinge Zone (EHZ). Similar earthquakes on such faults have happened in the past beneath Bangladesh and have had magnitudes ranging between 4 and 6.
“Given the system of active faults associated with the EHZ, such events will again occur in the near future,” he added.