Is Bangladesh vulnerable to earthquakes?

Is Bangladesh vulnerable to earthquakes?

                                                                   

                                                              -     Mir Fazlul Karim:

 

       There are some valid questions: Is Bangladesh vulnerable to earthquakes? Should we be concerned about an earthquake when occurrences of earthquake damages are not so significant? The country faces so many day-to-day problems related to environment, industrial pollution, traffic, water and power shortage, and annual calamities such as flood, drought, cyclone and tidal bore. Can we afford to ignore earthquake hazards?

      Earthquakes are the detectable shaking of the earth's surface resulting from seismic waves generated by a sudden release of energy from inside the earth. Any landmass which has experienced natural ground shaking in the past is vulnerable to earthquake risk and thus liable to earthquake hazard. A severe earthquake can bring devastation to the economy of the country and we cannot ignore potential danger of earthquakes.

Bangladesh: A geological location for earthquakes

     The geological structures in and around Bangladesh are capable of accumulating tectonic strain. These structures have released enough energy to produce destructive

shakes in the past.

     Fortunately, the frequency of large earthquakes in and around the country is less than in other earthquake-prone regions of the world, though sometimes the lone national seismic observatory station at Chittagong measures a relatively high frequency of low magnitude shakes.

       Bangladesh, along with its neighboring counties, shared the experience of extraordinary ground shaking due to an earthquake of magnitude 8.7 which is widely known as "The Great Indian Earthquake." The earthquake occurred due to a vertical displacement along the Dauki Fault located near the north-east international boundary between Bangladesh and India. The earthquake caused about 20m of pop-up of the Shillong Massive within a few seconds, and debris were blown even miles away from the epicenter area.

       A similar strong and extraordinary earthquake of magnitude 7.5 occurred in Bhuj on January 26, 2001, damaging many urban areas of Gujarat and killing an estimated 25,000 people. Scientists consider these as rare earthquakes, but this type of earthquake could be extremely devastating in the peripheries of the Indian peninsula.

       Bangladesh occupies a greater part of the Bengal basin. It is located in the eastern extremity of the peninsula and the Kutch basin in the western extremity is a mirror image of the Bengal basin. The regional geological structures from south to north at both the eastern and western extremities postulate a geometrical symmetry that would be receptive to similar tectonic behaviour in terms of stress distribution (except for some local differential characteristics). Considering such a geological setting, Bangladesh could be a receptive place for extraordinary earthquakes.

The rapidly growing urban centers increase the susceptibility of earthquake damage

     Generally, unplanned and populous townships are always vulnerable to earthquake hazard or damages. Bangladesh is a densely populated country. At the beginning of the twentieth century there were only 48 urban centers in the country and at present there are 491 including the densely populated cities and growth centers. A rapid change in infrastructure development has resulted in significant changes in housing pattern and transportation, sewerage, water supply, waste disposal system and communication network. All development has taken place in a very short time. The planners and city managers could not keep pace for regulating the government's planned efforts in the face of such rapid development. The lack of planned development puts the cities and growth centers in a vulnerable situation for larger earthquake damages. The experts foresee the most deadly future for Dhaka mega-city in the event of an earthquake here.

  Prediction of ground conditions

      The geology of Bangladesh is complex due to the presence of about 100m to 1000m (30,000ft) of sedimentary deposits over the basement rock of Indian plate. More than 80% of the country is covered by soft sediments (soil) or holocene deposits with unpredictable changes in the upper 100m of deposits, having considerable variations in the constituent geological materials and geotechnical properties. The geological map of the country indicates that the upper 10m of sediments in about 60% of the land area is susceptible to liquefaction during earthquake, making the ground vulnerable to immediate shear failure.

    More effort is needed for building up earthquake hazard awareness

      As the frequency of earthquakes is low in Bangladesh, the people and government are not clearly aware of earthquake devastation and we can not afford any experiment with it. Building up of public awareness could be the first and essential step towards preparedness for reduction of earthquake damages. It is necessary to remember the alarming Dhaka Earthquake 2001, when strong tremors were felt in the city and many people rushed out of their homes and offices in panic. 100 prison inmates were hurt in a stampede at the Dhaka Central Jail.

What shall we do?

     The country has had many damaging earthquakes in the past and is placed in a high seismic zone in the Global Seismic Hazard Map. We have not investigated the source structures, but due to its complex geological setting, Bangladesh is not capable of sustaining the strong shaking produced in the Himalaya and Meghalaya source area. Unfortunately, many of the infrastructures and buildings in Bangladesh may not meet BNBC standards and may be considered vulnerable from seismic safety viewpoint. Generally earthquake damages are irreparable. If we consider the potentiality of earthquake disaster, we may not be able to ignore this extraordinary geological hazard.

      We are at the early stage of possible earthquake hazard assessment and cannot expect any overnight understanding of earthquake vulnerability of the country. But steps can be taken to reduce the losses and damages by implementation of Bangladesh National Building Code (BNBC) in the construction practice, identification of appropriate subsurface geology, determining the right type of architectural setting and engineering design of both foundation and superstructures, development of fire safety options, keeping open spaces for rescue operations, and other such measures.

    At least we need an plan of action. There is an urgent need forreasonable seismic risk assessment of the country. It is a multidisciplinary task and includes technical training, institutional development, development of technical manuals, legal and enforcement aspects, and public awareness programmes.

                                     Mir Fazlul Karim is Director, Geological Survey of Bangladesh.