Earthquake hazard : Dhaka city perspective |
Dr. Aftab Alam Khan
A sudden, transient motion or trembling in the earth's crust, resulting from the propagation of seismic waves caused by faulting of the rocks either at shallow and/or deeper depths is known as earthquake. The motion is caused by the quick release of slowly accumulated energy in the form of seismic waves. The release of accumulated energy may occur at any depth and time but the intensity of damage is directly proportional to the movement on a fault, which is a thin zone, both at vertical and horizontal plains, of crushed rock between two blocks of rock. A fault can range in length from a few centimeters to hundreds of kilometers. The larger the fault length, the larger the energy release by fault movements. The ground shaking and the radiated seismic energy are caused most commonly by sudden slip on a fault, or other sudden stress changes in the Earth. Sudden break within the upper layers of the earth, sometimes breaking the surface, resulting in the vibration of the ground, where strong enough will cause the collapse of buildings and destruction of life and property. Based on long term historical records, about 18 major earthquakes (7.0 - 7.9 on the Richter scale) and one great earthquake (8.0 or above) are expected in any given year globally.
Any physical phenomenon associated with an earthquake that may produce adverse effects on human activities is termed as earthquake hazard. This includes surface faulting, ground shaking, landslides, liquefaction, tectonic deformation, tsunami, and their effects on land use, man-made structures, and socio-economic systems. A commonly used restricted definition of earthquake hazard is the probability of occurrence of a specified level of ground shaking in a specified period of time. Similarly, earthquake risk is the expected (or probable) life loss, injury, or building damage that will happen, given the probability of earthquake hazard. Earthquake risk and earthquake hazard are occasionally used interchangeably.
Bangladesh, by and large, is seismically active. The occurrence of earthquakes with magnitude averaging around 5 in Richter scale is quite frequent especially in its eastern region. Although, Dhaka has not been experienced with any moderate to large earthquake in historical past, even then the earthquake of December 19, 2001 with magnitude of 4.5 and focal depth of 10 km located very close to Dhaka is certainly an indication of its earthquake source and vulnerability. In addition, micro-seismicity data also supports the existence of at least four earthquake source points in and around Dhaka. The earthquake disaster risk index has placed Dhaka among the 20 most vulnerable cities in the world. Dhaka with its population of around 13 million and enormous poorly constructed and dilapidated structures signifies extremely vulnerable conditions for massive loss of lives and property in the event of a moderately large earthquake.
The recently measured plate motions at six different sites of Bangladesh including Dhaka; (the research being jointly conducted by Lamont-Doherty Earth Observatory, Columbia University, USA and the Department of Geology, Dhaka University) clearly demonstrate that Dhaka is moving 30.6 mm/year in the direction northeast. Further, the rate of strain accumulation is relatively high in and around Dhaka. It may precipitate in an earthquake of magnitude 6.8 in the event of the release of accumulated strain. The shallow subsurface of Dhaka is also characterized by number of faults of variable dimensions. These faults are vulnerable to motion where these coincide with the zones of high particle velocity.
The coincidence of the zones of high particle velocity with the location of faults suggests that the western part of Dhaka city from Mirpur-Kalyanpur to Pagla along Buriganga river and the eastern part of Dhaka city from Uttar Khan-Badda to Demra along Balu river has emerged as high risk zone. The peak ground acceleration in these areas has been calculated ranging between 0.3 to 0.35 if an earthquake of magnitude 5.6 occurs in and around Dhaka city. The resonant length in these areas suggests an optimal height beyond five stories; additional seismic factor needs to be introduced in addition to general seismic factor which is introduced based on seismic factors of the site specifically for earthquake resistant building code. The entire Dhaka megacity has been looked upon from earthquake hazard point of view. It has been divided into four zones of earthquake hazard vulnerability ranging between very high risks and low risk.
Earthquake cannot be prevented. But certainly it is high time to be much more concerned about the probable impending earthquake in order to minimise the loss of lives and property in national interest. On the basis of the above facts, we should develop earthquake monitoring network in Bangladesh immediately. It is of prime importance to set a national institute of earthquake research to develop high skilled manpower that can perform the task for earthquake risk assessment and management. We should remember that one earthquake of moderate intensity would kill thousands of people and destroy enormous national property. Death is certain for all human beings but painful death is not desirable.
Dr. Aftab Alam Khan, Professor, Geology Department, Dhaka University is Vice President, Bangladesh Earthquake Society (BES)
Earthquake Hazard Zoning Map of Dhaka Megacity