Building collapse, tilt, and sink also occurred at some parts of the Dhaka metropolis in very recent past. Ironically, every time these disastrous incidents occurred, there had been some scape-goat explanations. Every time the reason for such disaster was pointed towards structural anomaly, construction defects, non-compliance of building codes, and violation of approved plans. However, according to report, we may suspect some other reasons also. "'I was plastering a room on the third floor when at first I heard the sound of an electric generator. Soon I heard the sound of crumbling and saw the walls of the northern part of the building collapsing', Shahaj Uddin, a mason, told The Daily Star at DMCH". The whole building collapsed in a few seconds, as all but the first floor was empty, said witnesses living in adjacent buildings. The 'sound of an electric generator' and the 'collapse of walls of the northern part of the building' are of significance and need critical analysis.

Talking about building code it is not only a single element dependence rather multiple elements dependence. The common practice for engineering design is to calculate only static load. No dynamic load is calculated to incorporate in engineering design. A proper building code is accomplished based on not only engineering factors but also with geological factors. Unfortunately, no geological factors are ever considered. No realistic steps have been taken to stop the "bolt from the blue" appearance once for all. We all have been very vocal in expressing the fact that Dhaka metropolis is sitting on the fireball of disaster while even without the fireball is exploded, the tragic disaster crops out repeatedly. Moreover, as usual, all the concerned agencies seem to be very reluctant to dig out the actual causes.

We all know that Dhaka megacity is overstressed due to its extremely high population, pressure on housing, transportation and other necessities for day-to-day life. As a result, infrastructural development and expansion have increased several folds over the last thirty years or so. Moreover, the development and expansion are moving in the vertical direction in a converging mode when it is extremely important to move laterally in a diverging mode. Dhaka metropolis has a very limited area where only less than half is covered by original engineering bedrock. More than half of the metropolis area is characterised by depressed/subsided land, low land, and network of buried/incised channels of varying depths. These areas had been eventually filled with channel-fill deposits and then artificial-fill deposits over the original bedrock. Almost all the low lands were connected by numerous channels those carried sands, silts, and clays during every rainy and flood season. The channel-fill deposits are rarely few hundreds of years while the artificial-fill deposits are only few tens of years. In addition, the channel-fill deposits are frequently mixed with eroded red soil from elevated land and are often misleading with original red soil.

The buried/incised channels are also the avenues for continuous sub-surface flow of water that make such areas highly susceptible to liquefaction. It is not necessary that only earthquake vibration will generate liquefaction rather continuous slow ambient vibration would produce local liquefaction pockets. The channel-filled areas are also potential for generating local underground scouring. The situation becomes more risky when artificial filling is done over the channel-fills. The artificially filled areas also develop pockets of organic soil. Hence, the combined effects on to the building structures lead to high vulnerability to collapse.

It is very unfortunate that the last collapsed building was located over the edge of a depressed land and near the incised/buried channel previously known as Mohakhali Khal. There are more channels like Mohakhali Khal within Dhaka metropolis. Begunbari Khal, Kalayanpur Khal, Dholai Khal, Diabari Khal, and Gazaria Khal are noteworthy. Begunbari Khal once (forty-fifty years back) was connected to Baridhara-Gulshan lake, Dhanmondi lake through present Pantha Path, and Gazaria Khal to the east. Incised Kalayanpur Khal occupies a large area of Kalayanpur. Dholai Khal occupies a large part of Old Dhaka. In addition, enormous abandoned channels characterise a large area between Uttar Khan and Rampura. Rampura-Badda area unfortunately happens to be the depo-centre of low-lying areas. All these abandoned channels are associated with depressed lands and have undergone deposition of channel-fills and over-bank fills. These areas eventually came under artificial fillings with the advent of massive structural development. Similar situation was prevailing at the site of collapsed garments factory at Savar. Possibly RAJUK does not have such geological map of Dhaka city. If it is so, RAJUK must have hazard vulnerability map of Dhaka Metropolis on top priority basis. It is necessary to have geologically characterised site map of every development work whether it is building, or highway or runway or bridge or dam, or railway track. More and more the geological factors would be ignored, more tragic incidents would occur. It is also utmost important to have regulations for geological clearance prior to any infrastructural site development and major structures.

Dr. Aftab Alam Khan is Professor, Dhaka University and Vice President, Bangladesh Earthquake Society.