In April 2021 two moderate to strong earthquakes occurred in the north of Bangladesh that jolted some parts of the country. The moderate one of 5.1 magnitude occurred on April 05 at 21:20 hours which was followed by another two smaller quakes of 4.1 and 3.7 magnitudes in the neighbouring areas on the next day. The strong one of 6.0 magnitude occurred on April 28 at 08:21 hours which was followed by a series of aftershocks numbering 11 with magnitudes ranging from 4.7 to 3.0 within the next 32 hours. Both the earthquakes occurred at nearly the same latitudinal settings but about 350 km apart, and within the Himalayan Frontal Thrust domain. Distances between the epicenters of these earthquakes and the Dhaka seismic center were 403 km and 397 km respectively. It is noteworthy that two earthquakes of magnitudes 7.1 and 6.9 occurred in this region in 1930 and 2011 respectively. Although these quakes occurred beyond our territory, the country shook because of geologic and tectonic continuity. It is known that Bangladesh lies in a tectonically active region of the globe where the Indian plate is colliding with the Eurasian plate in the north and subducting under the Burmese plate in the east. Due to the movement mainly along these plate boundaries Bangladesh experiences frequent occurrences of earthquakes.
Here data and information were collected from different websites, newspapers and personal communications from different parts of the country and analysed. The April 05 earthquake was felt mainly in the northwestern-most parts of the country that includes the districts of Panchagarh, Thakurgaon, Dinajpur, Rangpur, Joypurhat, Kurigram, Lalmonirhat and Rajshahi. Newspaper reports stated that trees and buildings were shaken, and out of fear many rushed outside. But, no casualties were reported. According to a few newspapers, the quake was also felt in the capital. However, the estimated maximum intensity of the earthquake was III, indicating weak shaking.The April 28 earthquake was felt in the north-western and mid-northern parts of the country including Dhaka. The north-western part includes the districts of Panchagarh, Thakurgaon, Dinajpur, Nilphamari, Rangpur, Gaibandha, Bogura, Rajshahi; and the other part includes Sherpur, Mymensingh, Dhaka and Sylhet. People expressed that they felt it while they were sitting, lying and in standing positions. In Dhaka, the feelings were a little different because they are mostly living in high-rise buildings. However, the quake did no harm to life, no damage to the buildings and no effect on natural objects. But it created panic among some. Frightened people came out of their homes at Panchagarh and Sherpur. From personal communication, it is learned that the quake was not felt at Pabna, Khulna, Patuakhali, Chattagram and Cox’s Bazar. It can be concluded here that the maximum intensity in the country was III-IV, which indicates weak to light shaking. Geologically, these areas are covered with Pleistocene terraces of Barind and Madhupur, composed of red-brown clay residuum, and Holocene Tista alluvial fan and floodplains composed of grey unconsolidated sand-silt clay of varying amounts.
When we discuss earthquakes, two terms come in front – magnitude and intensity, and many people mingle the terms which creates confusion. Magnitude expresses the size of an earthquake which is measured with the help of a seismograph. On the other hand, intensity indicates the effect of an earthquake on nature, people and structures. The intensity of an earthquake at a particular place depends on three factors – the magnitude of the earthquake, distance from the epicenter, and local rock/soil condition i.e. geological condition. Richter scale and Modified Mercalli scale are popularly used for estimating the magnitude and the shaking intensity of earthquakes respectively.
Based on limited data and information these primary observations and conclusions have been made, although there is ample scope to do meaningful research of geological significance using these moderate-strong earthquake data. Results of such works are needed for seismic risk reduction activities in such a vast alluvial terrain towards the benefit and safety of our people, as well as achieving self-sufficiency.
A. K. M. Khorshed Alam, PhD, research Geologist