5.7-magnitude quake in Narsingdi was main shock, aftershocks likely for weeks: DU expert
The 5.7-magnitude earthquake that struck Narsingdi was the main shock, and the subsequent tremors are its aftershocks, according to Professor Dr Md Zillur Rahman of the University of Dhaka's Department of Disaster Science and Climate Resilience.
He told The Business Standard today (4 December) that these later quakes are direct reactions to the initial event.
The expert's remarks came following a mild earthquake struck Bangladesh early today (4 December), according to preliminary seismic data, reported by the European-Mediterranean Seismological Centre. The 4.1-magnitude quake occurred at 06:14am.
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Earlier on 21 November, at least 10 people, including a newborn, were killed and hundreds others injured after a powerful 5.7-magnitude earthquake jolted Bangladesh in the morning, damaging multiple buildings in Dhaka and other districts.
Why the 5.7 quake is considered the 'main shock'
Professor Rahman explained that the primary 5.7-magnitude earthquake occurred independently – it was not triggered by any previous tremor. Underground stress had been building up along a fault line, and when movement occurred, that stored energy was released as the earthquake.
The smaller tremors happening now are "deep" earthquakes, which he said were not expected under normal circumstances. They are taking place because the larger quake disturbed the balance of the earth's surface. One section of the surface rose while another sank, and the region is now trying to regain equilibrium.
How long might the aftershocks continue?
The duration of this readjustment process depends largely on the strength of the main quake. Stronger quakes can produce aftershocks for more than a year.
The 5.7-magnitude quake is considered moderate. However, Narsingdi sits on thick sedimentary deposits, very soft soil, which means the adjustment period may last more than a month. Still, Professor Rahman noted that there remains a possibility that no further aftershocks will occur.
What happened beneath the surface?
According to the expert, the main quake generated such a strong jolt that it created major fractures across a 7-8 kilometre stretch. The ground split and shifted several centimetres up and down along this area, triggering the need for widespread readjustment underground.
This is why all recent tremors have remained concentrated within Narsingdi and around the same fault line. These are still classified as aftershocks because none has been stronger than the main 5.7-magnitude quake. Only a tremor larger than the main shock, he warned, would be cause for serious concern.
