The earthquake of 19 December 2001 with its location 23.6o N 90.4o E, magnitude 4.5 - 4.8, and focal depth 10 km and couple of other low magnitude earthquakes in the mid seventies located near Dhaka possibly are the knocking alarm for Dhaka city. Roger Bilham and Philip England, in their paper published in the Nature science magazine, opined that 1897 Great Assam Earthquake constitutes a significant seismic threat to nearby densely populated regions of Bangladesh and to the very large city of Dhaka. My investigation and assessment suggests that 1885 Bengal Earthquake projects much greater threat to metropolis Dhaka. The return period calculation suggests that 1885 Bengal earthquake is likely to recur by 2015 having return-period character of 130 years. John W. Whitney of USGS opined that 7 large or 1 great earthquake is likely to occur in every 135 years in and around "eastern seismic belt" of the Indian plate.

Characterising 1885 Bengal Earthquake as a major one there stands a high probability for its recurrence between 2015 and 2020. The field investigations exhibit an average fault displacement of 7 m occurred along several neo-faults aligned with Dhaleshwari and Buriganga rivers. These active faults have potentials for generating seismic energy of magnitude as high as 7.5. In addition, several local shallow faults and high amplified liquefiable zones within Dhaka city where large number of buildings and structures are constructed without following standard building codes are certainly in a very high stake of earthquake disaster risk. The building collapse at Sakhari Bazar and Savar and building tilt at couple of localities in Dhaka city in recent times indicate its vulnerability to earthquake damage.

The classic example of ignorance and negligence has been set due to Savar event where collapse of 9-storey large garments factory cost hundreds of lives and caused considerable property damage without an earthquake. It is simply unimaginable what a magnitude of loss of lives and property would occur in the sites characterising similar subsurface conditions during an earthquake. Although, the present article is not written for describing the possible reasons of Savar event, but temptations could not be resisted as to the inquisitive minds tried to explore the possible reasons. It is learnt from various news paper reports and seminars what some survived persons expressed pertaining to the building collapse. Among the information, the notable are, a) there was no boiler explosion, b) the heavy machinery used to shake entire building when started, and c) more importantly the entire building used to tremble when high horse power pumps were used to abstract groundwater from subsurface.

When these information are critically analysed there emerged certain conclusions: The structure was not built on the geologic basement which is commonly known as engineering bed rock. However, the engineering bed rock might be confused with the geologic bed rock if the same is situated over a buried channel characterised by channel fill materials. The shear strength of these channel fills is much less than the shear strength of the geologic bed rock. In addition, the buried channels are highly potential from groundwater resource point of view that continuously keeps the channel fill materials in saturated and wet condition. As a result, the ambient shear strength remains quite low. The second factor is the withdrawal of groundwater with large pumping capacity that had developed interim cone of depressions with downward pull of hydrostatic pressure towards the cone of depressions. As a result, the structure used to tremble during the pumping of groundwater.

Hence, the shaking of the structure due to long time operation of heavy machineries and the trembling due to groundwater pumping had continuously weakened the base of the foundation and eventually the structure collapsed. A major buried river has been identified between Tongi fault in the south and the Nayarhat fault in the north. An appreciable horizontal off set has also been identified along Nayarhat fault. The area between such buried river and fault is an ideal site for structural collapse. Unless, such sites are definitely identified, the risk of structural failure would remain high.

Coming back to the discussion on the recurrence of the 1885 Bengal Earthquake and the possible consequence in Dhaka city, it is high time to have subsurface imaging of the entire Dhaka city and identify the zones characterised by faults, gully-fills and buried channels. Unless this is done, the micro-zoning earthquake hazard map of this megacity is unlikely to be prepared. Until such hazard map for Dhaka city is prepared, the pre-disaster physical planning and earthquake damage risk reduction will remain at a tell-tale stage. Recurrence of an earthquake is not a fairy-tale rather it is an inherent character of each earthquake event of all magnitudes globally.

Dr. Aftab Alam Khan is Professor, Department of Geology, University of Dhaka.