Dhaka and some other districts were once again jolted by a mild tremor last week. I fact during the last few years Bangladesh on regular intervals has been experiencing some mild to medium tremors. Although the earthquakes in Bangladesh so far did not cause major devastation, there is apprehension that the country may face disaster at any time.
Earthquakes are one of the major natural hazards threatening life, property, and economic well being in many nations. Nations striving for full economic development may find the investments and progress of decades wiped out in a few minutes.
Experts apprehend that Bangladesh is extremely vulnerable to earthquake activities. A catastrophic condition will emerge if a large magnitude earthquake occurs in an earthquake prone country like Bangladesh. Inquiring minds have long sought to understand the processes responsible for this violent activity. The reasons for the unawareness about potential earthquakes are the lack of systematic monitoring and follow up of trends of seismicity and micro-seismic events.
There is perception among seismologists that an overall scenario of earthquake events in and around Bangladesh lends support to a vulnerable situation and it is recommended that some relevant steps must be taken to ameliorate the public awareness.
There are detailed guidelines for earthquake resistant design of concrete and steel structures in the Bangladesh National Building Code -BNBC 93. Unfortunately, the Code is yet to be officially enforced. The Ministry of Works formed a committee in 1997 to develop mechanism for its enforcement; however, it has not made any progress.
It is feared that in the absence of enforcement mechanism, many of the new buildings, which have been built or being designed and constructed in different parts, do not have adequate provisions for seismic resistance. With increasing urbanization leading to construction of a large number of engineered and non-engineered buildings in the cities, the number of people living in unsafe houses would obviously increase. To avoid catastrophes in case of future earthquakes it is imperative that measures are initiated to strictly enforce the provisions of BNBC 93.
The meteorological Department of the Government established an observatory at Chittagong in 1954 with facilities for seismic and geomagnetic observations. This remains the only observatory in the country till today. The seismic equipment consisted of three-component short and long period, low magnification, Sprengnether seismographs. The N-S component has been out of order for a long time. The observatory also suffers from serious noise/vibration effects from nearby major highway and main railway lines, as well as from the sea, which is only a few hundred meters away.
There is lack of awareness not only among the public, but also among the decision-makers about the earthquake hazard in the country. The main reason for this is that excepting the 1918 Srimongol Earthquake, no major earthquake has affected Bangladesh in this century.
Earthquakes are related to faulting and tectonic instability of an area. The overall tectonics and the nature of fault movement within Bangladesh and the adjoining region are conducive for the frequent and recurring earthquakes. Threatened earthquake disaster inside Bangladesh may also be expected from active fault zones outside the national boundary. Tsunami is the terrific tidal wave caused by the underwater earthquakes, which usually strikes the Pacific and Atlantic coasts. Bay of Bengal including Java trench in the southeast of Bay of Bengal have also such seismogenic potentiality because more than twenty earthquake events have so far been recorded in the Bay of Bengal in recent years.
A good background of historical earthquake information is essential to evaluate the seismicity. Information on earthquake events in and around Bangladesh is available for the last 250 years. The earthquakes those affected Bangladesh and its surroundings including the historical earthquakes are in records from 1664 till today. The earthquake record suggests that more than 100 moderate to large earthquakes occurred inside Bangladesh since 1900, out of which more than 65 events occurred after 1960. More than 125 earthquake events have occurred in and around Bangladesh since the beginning of the new millennium. Of which about 27 events of magnitude ranging 4 to 5 have occurred inside Bangladesh. Fifteen new epicentres have been identified inside Bangladesh since January 2001.
This clearly indicates an increased frequency of earthquakes in Bangladesh. The increase in earthquake activity in Bangladesh is an indication of fresh tectonic activity or the propagation of fractures from the adjacent seismic zones. Although Bangladesh is extremely vulnerable to seismic activity, the nature and the level of this activity is very poorly defined. The main constraint is the earthquake observational and monitoring facilities, which is totally absent in Bangladesh.
The recurrence of earthquakes in an earthquake prone region cannot be prevented. Rather, what could be done is only to make a prediction and issue warning to minimise loss of lives and property. Earthquake disaster mitigation approach involves a) pre-disaster planning, b) building measures, and c) management. Pre-disaster planning involves measures at physical planning level, assessment of potential risk zones, assessment of potential man-made risk zones, land-use pattern, infrastructural network, safety standards and norms, building shape, height, and group, and evacuation and emergency preparedness.
Building measures involve damage rating, building code, and a seismic design and practice. The management involves both pre-disaster and post-disaster administrative principles, implementation of building code, relocation, and mass awareness both for pre-disaster preparedness and post-disaster management.
Four zones have been identified as the severest zones in Bangladesh in terms of maximum ground surface acceleration and the probable movements of the deep-seated crustal faults and lineaments. The severest zones include northern part of Dinajpur, Rangpur, Mymensingh, Sylhet, Tangail, northern part of Dhaka, Khulna, Jessore, Kushtia, and Chittagong. 1885 earthquake of Manikganj, 1897 earthquake of Great Assam, 1918 earthquake of Srimangal, 1930 earthquake of Dhubri, and 1950 earthquake of Assam all are quite matured to recur any time and may create devastation in Bangladesh.
It is suggested that an extensive seismological observatory network must be set-up in Bangladesh equipped with the modern and sensitive earthquake monitoring facilities. This includes strain meters, accelerometers, velocity and displacement seismographs. Monitoring facilities of micro-seismicity and on-line real-time seismological facilities must be introduced. Highly sensitive seismographs with all components of recording system must be installed for continuous monitoring and for valid prediction.