Sikkim earthquake

Bangladesh was jolted by a magnitude 6.9 earthquake (USGS, Earthquake Notification Service) on September 18. The earthquake occurred in Sikkim at Latitude 27.7230 N and Longitude 88.0640 E, and depth of focus at epicenter in Sikkim was recorded as 19.7 Km (12.2 miles). [Source of magnitude and location: USGS, NEIC].
The earthquake epicenter was 68 Km (42 miles) NW of Gangtok, Sikkim, India, 119 Km (73 miles) North-Northwest of Shilguri, West Bengal, India; 272 Km (169 miles) East of Kathmandu, Nepal ; 572 Km (355 miles) North of Kolkata, West Bengal, India (USGS NEIC) and 495 Km Northwest of Dhaka, Bangladesh.
This earthquake was characterised as strong to very strong on local scale, which could cause damage to buildings and severely threaten people. The Sikkim Earthquake killed 13 people in India and 5 in Nepal, and resulted extensive damage to several buildings in India, Nepal and Bangladesh.
Bangladesh lies on the Indian-Australian Convergent Plate Boundary which moves towards Eurasian Plate, resulting in earthquakes due to the forces and process of building of the Himalayas, which is still active.
The Sikkim earthquake was followed by two aftershocks of the magnitude 4.8 with a depth of focus 20 km at epicenter location at Latitude 27.446 0 N and Longitude 88.3910 E and 4.6 magnitude with a depth of focus 20.1 km at epicenter located at Latitude 27.377 N and Longitude 88.266 E [source of magnitude and location: USGS, NEIC].
There are about 12 large-to-great earthquakes which were recorded in or around Bangladesh, of which Chittagong Earthquake of April 2, 1762 with an intensity VIII of Modified Mercalli Scale, Central Bangladesh Earthquake of July 18, 1885 and Srimangal Earthquake of July 8, 1918 with 7.6 magnitude on Richter Scale are notable.
The Calcutta Earthquake of October 1, 1737 was the third most catastrophic earthquake, with a loss of about 300,000 lives. The Assam Earthquake of June 12, 1897, with a magnitude of 8.7 on Richter Scale is one of the 10 super-earthquakes on earth, when an area of more than 300,000 sq. km covering Bangladesh, India, Bhutan and Western Myanmar was severely shaken, with damage of more than $25.00 million.
Other recorded earthquakes are magnitude 8 magnitude Nepal Earthquake on August 26; 1833, 7.5 magnitude Assam earthquake on January,1869; 7.1 magnitude Dhubri, Assam Earthquake on July 3, 1930; magnitude 7.6 Assam, India Earthquake on March 6, 1933; magnitude 8.1 Bihar-Nepal Earthquake ofnJanuary 15, 1934;magnitude 7.2 Assam, India Earthquake on October 23, 1943 and magnitude 8.5 Assam-China Earthquake.
Records of worldwide frequency suggest that the more severe an earthquake, the less it occurs. Frequency, magnitude and ground motion of earthquake are related to the depth of focus and epicenter.
The process and the forces that caused earthquakes of the past in or around Bangladesh are still active; as such occurrence of large-to-great earthquake can be anticipated in or around Bangladesh. In recent years, Bangladesh experienced several earthquakes ranging from magnitude 4.2 to 5.2, which could be a warning sign for generation and release of energy to occur, causing an earthquake in the region.
The recurrence of a large-to-great earthquake, like that of the Calcutta Earthquake of 1737 or the Assam Earthquake of 1897, may result in one of the worst catastrophes that Bangladesh and its adjoining areas have so far experienced. This calls for further intensification of the Bangladesh Hazard Reduction Programme to face such a natural calamity, as and when it happens.

The writer is a Retired Director, Geological Survey of Bangladesh.