Earthquake epicentre in Narsingdi: No reason to be panicked over ground cracks
Geologists note that one of the most direct signs of an earthquake is a surface rupture. This occurs when a subsurface fault line extends all the way to the surface, raising or lowering one block of land relative to the other.
The cracks that have appeared in the soil across various parts of Narsingdi district following the recent earthquake are not a cause for alarm. According to the Geological Survey of Bangladesh (GSB), these fissures did not form as a result of the fault line at the earthquake’s origin or epicentre. These are shallow cracks and pose no geological risk.
A three-member expert team from the GSB inspected several locations in Narsingdi last Saturday, including the epicentre, to assess the geological impact of the earthquake. They examined ground cracks at a minimum of three sites. These fissures range from four metres to a maximum of 30 metres in length and are shallow.
During Saturday’s field visit across various areas of Narsingdi, the GSB team observed that every cracked site was adjacent to a waterbody. They concluded that the pressure exerted by water during the quake caused the surrounding soil to loosen, creating fissures.
GSB deputy director Md Mahmud Hossain Khan told Prothom Alo on Sunday that the cracks in Narsingdi were caused by “liquefaction”.
According to him, when seismic waves strike loosely compacted and dry sand layers, particularly in areas filled near ponds or canals, the intense vibration forces water from the adjacent waterbodies to surge horizontally into the sand layer. At a certain point, the ground loses its bearing capacity and begins to behave like a dense liquid. The surface layer above the liquefied soil then collapses or cracks.
The GSB team stated that it is too early to confirm whether Saturday’s earthquake was an aftershock. According to team member Anisur Rahman, it will take up to 290 days to determine this.
The deputy director added that in Narsingdi, the cracks appeared parallel to the banks of the waterbodies. However, he emphasised that there is no cause for fear. These shallow cracks can easily be filled with soil.
The capital and surrounding areas were jolted by an earthquake, whose epicentre was in Narsingdi, at 10:38 am last Friday. The following morning, another tremor occurred, with the epicentre again in Narsingdi.
Friday’s earthquake killed 10 people, including a child, and injured more than 600. Narsingdi saw the highest number of fatalities, with five deaths.
Cracks developed in at least two locations in Palash upazila as a result of the quake. In addition, long ruptures appeared at a cattle farm in Lebupara, Ghorashal, spanning multiple fractures from one end of the yard to the other.
The three-member GSB expert team inspecting the affected areas comprised deputy director Md Mahmud Hossain Khan, Anisur Rahman and Selim Reza. All three spoke to Prothom Alo.
The experts said that the site identified by the US Geological Survey (USGS) as the epicentre did not show any significant ground rupture. Rather, every site where cracks were found lay beside a waterbody. Their examination confirmed that the soil contained a high proportion of sand and that the low-lying areas had been filled with sand in the past.
He believes it cannot be said with certainty that the recent tremors signal a larger earthquake to come, as small quakes have been occurring sporadically around Dhaka over the past two to three years.
Geologists note that one of the most direct signs of an earthquake is a surface rupture. This occurs when a subsurface fault line extends all the way to the surface, raising or lowering one block of land relative to the other.
Whether it is an ‘aftershock’ will be known in 290 Days
The GSB team stated that it is too early to confirm whether Saturday’s earthquake was an aftershock. According to team member Anisur Rahman, it will take up to 290 days to determine this.
He explained that scientific criteria exist for such classification. If, within the next 290 days, smaller earthquakes occur within a 50-kilometre radius of the epicentre, they will be considered aftershocks.
Rahman added that if within the next 1,000 days a magnitude-8.5 earthquake occurs within 110 kilometres of the epicentre (Narsingdi), then the recent tremors, both Friday’s and Saturday’s, would be considered foreshocks.
GSB deputy director Selim Reza views the minor tremors following Friday’s moderate earthquake positively. He believes they indicate that the accumulated energy beneath the ground has been released gradually, reducing the risk of a larger discharge.
No reason for panic
Professor Md Zillur Rahman of Dhaka University’s Department of Disaster Science and Climate Resilience considers Friday’s tremor to be the “mainshock”.
He told Prothom Alo that aftershocks following moderate quakes typically occur within five to seven days. He believes it cannot be said with certainty that the recent tremors signal a larger earthquake to come, as small quakes have been occurring sporadically around Dhaka over the past two to three years.
Advising the people not to panic, professor Zillur Rahman said it is impossible to predict when future earthquakes might occur. However, he added that the likelihood of a major earthquake striking the capital in the near future remains low.