a month ago

Dhaka waking up to frequent quakes

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The  earthquake that the residents of the capital city woke up to on Friday (May 5) morning was of 4.3 magnitude on the Richter scale. The United States Geological Survey (USGS) was the first to detect the seismological activity, its source as well as measured its magnitude. The epicentre of the earthquake was not far from the capital city (at Dohar, which is only 23.4 km to the southwest of Azimpur area of the city) and it originated at a shallow depth of 10 kms below the earth's surface.  Seismic events like this are not new in this country. In fact, only  10 days before May 5's tremor, on April 25, Rupganj  under Narayanganj district adjacent to Dhaka was the epicentre of another earthquake of similar strength (magnitude 4.0), whose focus was 17 kms below  the earth's surface. In recent years, between 2008 and 20012, Dhaka and its surrounding areas including Cumilla and Chandpur were shaken by some six earthquakes of moderate strength. Other quakes that followed had their epicentres in Sylhet, Chattogram and Cox's Bazar. Evidently, the frequency of the earthquakes, especially around Dhaka, is on the rise.

The strengths of those earthquakes including the last one as measured on the Richter scale have been of moderate magnitude. And so, as told by some experts on earthquake, there is no reason for alarm. Especially, a Dhaka University teacher, Masud Kamal, who teaches at the Department of  Disaster Science and Climate Resilience of the university, is reported to have told the media that the May 5's temblor was part of the process called neotectonics (movement and deformation of Earth's  crust). And the tremor might have been caused by the activity of geological fault-lines that change the flow of rivers (in this case, the river Ichamati or any of its branches or canals could be the source). And he further viewed that big earthquakes from such neotectonic activity originating from shallow depths are rare. This is no doubt reassuring for the dwellers of this megacity. In fact, there was also no report of casualty or damage to property following the tremor. However, many residents of the city reported that though the shaking lasted for 10 to 15 seconds, the perception of the jolt with which the tremor ended was anything but mild.

What is still concerning is that the epicenters of the mentioned tremors were not very far from the capital city of Dhaka, which is a jungle of concrete with hundreds of thousands of buildings, many of those are high-rise structures. And what send chills down the spine is if the National Building Code (BNBC)-2015, which provides some guidelines for building earthquake-resilient structures, was at all adhered to while constructing those structures.  The question arises seeing the poor state of monitoring by the agencies concerned. The helplessness of fire service and civil defence personnel was particularly evident during the recent devastating fires in the city. It was clear that little attention was given to the need for keeping adequate space between the buildings of the shopping complexes for the fire service vehicles and rescue workers to move. And no one noticed it, not even the monitors, to be specific, until the damage was done. One would do well to remember at this point that there is a huge risk of fire during an earthquake in modern cities. The power lines and gas pipes which often make a tangled mess all across the city will be a potential source of fire when buildings would start to collapse.Forecasts abound about how many people may die and get injured if and when such a disaster strikes. But the issue is not just about the numbers. The very prospect of a cataclysm happening in this unplanned urban maze, where around 25,000 people live within an area of one square kilometer, gives one the jitters. But is not such a fear misplaced so far as the earthquakes in question are of moderate magnitude?

So, let us see what is a magnitude on Richter scale. It is the largest amplitude of a quake (the wiggle of highest size in the graph representing the quake).But there are also other ways of measuring an earthquake. For instance, there is the so-called, Modified Mercalli intensity scale (MMI), which measures an earthquake in terms of its intensity. Intensity is a concept that is quite different from magnitude. It actually measures the amount of shaking (caused by the earthquake) and the damage caused by it at a particular location. In fact, the closer a place is from the epicentre of an earthquake, the greater its intensity (the amount of shaking) and, hence, the potential for causing damage. Earthquakes of moderate magnitude can be destructive, if they originate from shallow depths and are close to populous human settlements.

Seismologists need to do more research in light of this to moderate seismic events as in recent times their frequency around the capital city of Dhaka  is seen to be increasing.   Worse yet, earthquake experts fear of stronger temblors of magnitude 8.0 or more in Bangladesh. Predictions about the earthquakes are in fact probabilities. It is said that earthquakes may occur at intervals of 200 to 250 years, especially along the fault lines (at locations where there are fractures  in the rocks that make Earth's crust).But that does not mean that an earthquake will happen exactly after that time span. In fact, it can happen any time.

Therefore, adequate preparations should be afoot to face an earthquake of cataclysmic scale that can strike all of a sudden without prior warning.


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