So far at least 12 large-to-great earthquakes occurred in and around Bangladesh. In this context the people, government policy-makers as well as professional community viz. engineers, architects, seismic geologists, planners etc may consider recurrence interval for hazard avoidance through structural design and by proper land-use.

The Calcutta Earthquake of October 1, 1737 recorded a death toll of 300,000. This is the third most disastrous quake in this region which occurred during the last 800 years in terms of loss of life ( the highest deaths estimated 820,000 in 1556 at Shen-shu, China and the next 700,000 in 1976 at T'ang-shan, China).

The Assam Earthquake of June 12,1897 is one of the 10-graetest quakes occurred with a magnitude of 8.7 on the Richter scale and an intensity of VII on the Modified Mercalli Scale that caused a damage to the tune of US $25 million. The Bihar-Nepal Earthquake of January 15, 1934 recorded magnitude 8.1 and an Intensity X with a damage to the tune of US $ 25 million and estimated death toll of about 10,000.

The Chittagong Earthquake of April 2, 1762 recorded an Intensity of VIII on the Modified Mercalli Scale and a damage of US $5m. The Bengal Earthquake, Manikganj of July 14, 1885 recorded a considerable damage. The Srimangal Earthquake of July 8,1918 occurred with magnitude 7.6 and recorded damage of more than US $1.0m.

Realising the earthquake hazards and its impact on national economy, Geological Survey of Bangladesh (GSB) took initiative in 1988 for a geological investigation on earthquakes and the potential hazards of their recurrence. In April 1989 Dr Darell G Herd of US Geological Survey along with representatives of GSB carried out a geological investigation to determine the tectonic origin of the Assam Earthquake of June 12, 1897 on the Dauki fault. A project titled "Detailed geological mapping for coal and other mineral exploration and Neotectonic study related to natural hazards" was initiated also for identification of major geological features for Earthquake Hazard Reduction Programme. Provision to install micro-seismic equipment was kept to collect seismic data of even smaller than magnitude 3.4 for neotectonic study.

A network of modern, wide frequency ,digital seismographic stations at Sylhet, Cox's Bazar ( alternately Chittagong), Mongla (alternately Khulna) and Rangpur would provide an accurate location (within several kilometers) for most earthquake events including even of smaller magnitude of 2 to 3 for preparation of micro-seismic map of Bangladesh.

During the 1897 quake, an area of more than 300,000 square km covering Bangladesh, India, Bhutan and western Mynmar was severely shaken. The fault origin of the 1897 Assam Earthquake may have originated on the Dauki fault running east-west along India (Assam)-Bangladesh (greater Mymensingh and Sylhet) border. Several other potential areas of geological features and elements are Dhaka-Srimangal lineament, the Tista lineament, the Atrai lineament, the Brahmaputra-Jamuna lineament, the Bogra fault(?), the Mymensingh lineament , the Tangail scarp, the Chittagong fault identified from interpretation of satellite imagery by GSB. Neotectonic study on such potential areas may be carried out to collect data on paleoseismology for Earthquake Risk Assessment Map by identifying active faults, fault scarps by detailed trench investigations, searching river banks, stream channels, irrigation ditches and excavations in flood plain materials for geological evidence of multiple liquefaction events and sand blows.

With such realisation on one hand and observation of the rapid growth of high-rise building and population, industrial establishments and commercial activities on the other, in the capital city Dhaka and the port city Chittagong, a report entitled, "Natural Hazards in Bangladesh: Earthquakes" was prepared following the Chittagong earthquake of November 21, 1997 for awareness of the policymakers as well as the professional community. Considering the nature and extent of the earthquakes' threat or their recurrence, a strict compliance of the existing Building Code (which is not in force in Bangladesh) was suggested to build suitable engineered structures to minimise hazards.

The Chittagong Earthquake of July 27, 2003 occurred with a magnitude of 5.6 on Richter scale. From press repots, we observed that a crack was developed in Borkal area that called for detailed geological study, the nature and extent, slip rate and identification of earthquake features like sand boils, liquefaction, landslides etc. The 10-km crack is the rupture length of the fault segment produced by the Borkal Earthquake. This is a closely mimicked surface feature of the 1983 Borah Peak, Idaho, Earthquake (OJT 1989 under the supervision of Anthony J. Crone, USGS, Denver). Tremors in Chittagong and a small-scale tsunami (a long ocean wave produced by movement of sea floor following an earthquake) in Andaman sea on August 11, 2003 reveal the evidences of sea floor spreading and plate tectonic activated in the region.

Frequency, the nature and extent of the past earthquakes and recurrence of its behaviour suggest that a big earthquake event in Bangladesh may cause worst catastrophy, potentially more severe than that of the Calcutta Earthquake of October 1, 1737 when about 300,000 lost their life.

Earthquake is inevitable in the regions of seismic belt, but modern experiences with earthquakes in populated areas like the cities reveal the fact that properly designed engineered structures and constructed facilities can withstand even large earthquakes. So ensuring that appropriate engineering design and material standards keeping pace with sensible use of land and event prediction can thus serve effectively in reducing the loss of life and property in the event of a big earthquake.

M. Muminullah is retired Director of Geological Survey of Bangladesh.