EARTHQUAKE IN BANGLADESH: A NATURAL DISASTER AND PUBLIC AWARENESS

Transcription

1 EARTHQUAKE IN BANGLADESH: A NATURAL DISASTER AND PUBLIC AWARENESS A. A. Khan 1, M. Hoque 1, S. H. Akhter 1 & M. A. Hoque 1 ABSTRACT: The nature and the distribution of the earthquake events in different seismic zones of the country are intrinsically related to various tectonic elements. The increased frequency of earthquake events in Bangladesh in the last 30 years suggests reviving tectonic activity. In case of severe earthquake and increased probability of earthquakes the risk on the loss of life and damage to the property in Bangladesh will be quite high. Four severest risk zones in the country are inferred those include northern part of Dinajpur, Rangpur, Mymensingh, Sylhet, Tangail, northern part of Dhaka, Khulna, Jessor, Kushtia, and Chittagong. Considering the devastating impact of such impending earthquake on land and society and the lack of adequate infrastructures for earthquake studies, the installation of network of high-sensitivity modern seismographs with all components is immediately needed. Valid predictions of earthquakes can thus be made and warnings are issued in order to minimize loss of lives and damage to properties. Emphasis is given to the needs for undertaking public awareness programmes through physical planning of human settlements, following the building codes, and implementing the management techniques of settlements for both pre-disaster preparedness and post-disaster management. INTRODUCTION Earthquakes, Volcanism and related catastrophes are the terminal imps of the earth s dynamic processes. The earthquakes are caused by the explosion or the release of accumulated strain due to various stress-fields in the earth's materials. Earthquakes have been a source of terror and wonder for as long as people have inhabited the earth. Terror stands for unexpected, sudden-onset earthquake events those are capable of producing many casualties among the local population and massive destruction of property, while it is wonder because people are awe-struck by the forces of nature that suddenly disrupt the environment and alter the surface of the planet (IASPEI brochure). Earthquakes are one of the major natural hazards threatening life, property, and economic well being in many nations. Death tolls from major events can be sighted as 255,000 in Tangshan, China in 1976 and 10,000 in Mexico City in The economic loss in the 1995 Kobe, Japan, earthquake was more than U.S.$100 billion. Nations striving for full economic development may find the investments and progress of decades wiped out in a few 1 Department of Geology, University of Dhaka, Dhaka 1000, Bangladesh THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

2 minutes. The devastation on the loss of lives and economy due to an earthquake in developed countries like China and Japan where the advancement of earthquake research is quite appreciable, it is needless to say that a catastrophic condition will emerge if a large magnitude earthquake occur 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. Here we have given to give an overall scenario of the earthquakes in and around Bangladesh and recommended some relevant steps to ameliorate the public awareness. EARTHQUAKE VULNERABILITY Earthquakes are related to faulting and tectonic instability of an area. The overall tectonics of the Bangladesh and adjoining region is conducive for the frequent and recurring earthquakes. The geo-tectonic setting (Figure 1) of the country is seismically very active. These are Himalayan arc, Shillong plateau & Dauki fault system in the north, Burmese arc and accretionary wedges in the east and Naga-Disang-Haflong thrust zone in the Northeast. Threatened earthquake disaster in side Bangladesh may be expected from these active seismic zones outside the national boundary. Tsunami is the terrific tidal wave caused by the underwater earthquakes, 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 as because more than Twenty earthquake events have so far been recorded in the Bay of Bengal in recent years (Figure 2a). Hence, the possibilities of tsunami in the Bay of Bengal can not be ruled out. The dimensions of tsunami waves dwarf all our usual standards of measurement. A tsunami often extends more than 150 km and sometimes as much as 900 km from crest to crest and velocity of that in the open ocean must be reckoned in hundreds of miles per hour (Bernstein, 1954). If any tsunami is produced in the northern Indian Ocean it may create heavy loss of lives and properties because Bangladesh is in the tip of the V-shape bay and the water depth gradually decreases towards land. Tsunamis are hardly detectable in the open deep sea but when they approach shallow water or the shore they build upto their terrifying height 4-30 m high upon beaches and tens of time higher into V-shape bay. A tsunami is not a single wave but a series usually separated by intervals of 15 minutes to an hour or more. Usually the 3 rd and 8 th are the highest and sweep away lives and properties. THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

3 Figure 1. Geotectonic setting of Bangladesh and adjoining region and active faults and lineaments bearing seismogenic potentiality. (Data source: Khan and Chouhan, 1996 and Hoque, 2001). A good background of historical information of earthquake is very important to evaluate the seismicity of Bangladesh in close coincidences with the geotectonic elements. Information of earthquake in and around Bangladesh is available for the last 250 years. The major earthquakes those affected Bangladesh and its surroundings including the historical earthquakes are in records from 1664 till The earthquake record suggests that since 1900 more than 100 moderate to large earthquakes occurred in Bangladesh, out of which more than 65 events occurred after 1960 (Figure 2b). This clearly indicates an increased frequency of earthquakes in the last 30 years. The increase in earthquake activity in Bangladesh is an indication of fresh tectonic activity or the propagation of fractures from the adjacent seismic zones. THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

4 Figure 2a. Location of epicentre of earthquakes in the Bay of Bengal and northern Indian Ocean (after NEIC, USGS internet). THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

5 Figure 2b. Location of epicentres in Bangladesh (Period ). Source: National Earthquake Information Centre, United States Geological Survey THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

6 STATUS OF EARTHQUAKE STUDY IN BANGLADESH Although Bangladesh are 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. Earthquake monitoring facility in Bangladesh is totally absent. Bangladesh Meteorological Department established a seismic observatory in Chittagong in It remains the only observatory in the country till today and lacks proper functioning. The first seismic zoning map of the subcontinent was compiled by the Geological Survey of India in Bangladesh Meteorological Department adopted a seismic zoning map in In 1974, Chouhan and Khan proposed a seismic zoning map of Bangladesh. In 1977, the Government of Bangladesh constituted a Committee of Experts to examine the seismic problem and make appropriate recommendations. The Committee of Experts proposed a seismic zoning map of Bangladesh in 1979 giving basic seismic coefficient (Z) as 0.08, 0.05, and 0.04 respectively for the zones. The Committee of Experts has also given a formula (V = ZIKCSW) for determining the shear force at the base of a building, where V = shear force, I= Importance factor ( ), K = factor for structural type ( ), C = factor for flexibility of structure ( ), S = Soil foundation factor ( ), W = design vertical load. Khan et al., (1998) has proposed a multiple elements seismic zoning map of Bangladesh based on maximum probable ground surface acceleration (g) and the tectonic elements applied to respective zones. However, apart from some sporadic studies by some interested individuals, there exists no definite institution for earthquake study in Bangladesh. Being a thickly populated developing country, the policy makers, planners, scientific personnel and general people are totally unaware of the status of earthquakes in Bangladesh. LEVEL OF SEISMICITY IN BANGLADESH The present level of seismicity in Bangladesh is not precisely defined. References may be sighted to some quantitative assessments of seismicity in Bangladesh made by some research personnel/organisations. The earthquake zoning map compiled by the Meteorological Department of Pakistan and subsequently adopted by Bangladesh Meteorological Department suggests that Zone-I, seismic factor g/5 to g/10, Zone-II, seismic factor g/10 to g/15, Zone-III, seismic factor g/15 to g/20, and Zone-IV, seismic factor g/20 or less. The Committee of Experts suggested three seismic zones assigning basic seismic coefficients 0.08, 0.05, and 0.04 for Zone-I, Zone-II, and Zone-III respectively. THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

7 Chouhan and Khan (1974) gave a quantitative assessment and opined that the annual rate of strain (4.5 X ergs) accumulation corresponds to M = 6.5 and enough strain to the tune of 2.5 X ergs was available for an earthquake of magnitude M > 7.5. The study further suggests that the present accumulated strain upto 1990 to the tune of 3.4 X ergs is likely to precipitate an earthquake greater than magnitude 7.5 should a single event occurs with plausible time forecast of between years 1993 and Choudhury and Ali (1994) identified the significant seismic sources capable of producing earthquakes of magnitude greater than 7 around Assam fault zone, Tripura fault zone, Sub-Dauki fault zone, and Bogra fault zone. Khan et al., (1998) has calculated the probability of occurrence and the return period of the major earthquakes in Bangladesh and found that the return periods of the earthquakes having magnitudes 6.8 and 7.4 are 50 and 100 years respectively. The probability of occurrence of 6.8 and 7.4 magnitude earthquakes are 98 and 99 percent respectively. Khan and Chouhan (1996) inferred various active tectonic trends for future earthquakes occurrence in Bangladesh. The inferred tectonic trends are likely to be severely activated as mostly strike-slip motion by the recurrence of the major earthquakes located in the respective tectonic trends. Khan et al., (1998) has proposed maximum ground surface acceleration (g) in the respective seismic zones by Abridged Mercalli Intensity Scale likely to be demonstrated during recurred earthquake located in that zone. However, the proposed maximum ground surface acceleration values should be corrected pertaining to seismic absorption coefficient/attenuation factor of soft sediments and liquefaction character of those sediments. All the above studies lend support to conclude that all the seismic source parameters are vulnerable to severe earthquake in Bangladesh. MITIGATIONAL APPROACH AND PUBLIC AWARENESS The occurrence of earthquakes in an earthquake prone region can not be prevented. Rather, what could be done is only to make a prediction and issue warning for minimising loss of lives and properties. Although precise prediction is not always possible, an acceptable valid prediction of an earthquake will certainly minimise the loss of lives and properties. A valid prediction depends on four essential elements (Figure 3). There are good number of evidences that Quaternary sediments are affected by various earthquake events in Bangladesh pertaining to uplift, subsidence, ground deformation and massive liquefaction. Since water plays an important role in fault creep and fault slip, a small amount of water can produce an effect on lubricated surface for fault displacement with a stress drop of only 10 to 100 bars. In order to meet the requirement for short term and imminent earthquake prediction, the on-line real time seismology has been introduced in Japan. In addition to the above THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

8 earthquake disaster mitigation approach (Figure 4), the following public awareness should be followed: - pre-disaster physical planning of human settlements, - building measures for minimising the impact of disaster, and - management of settlements. Seismic Regionalization Recurrence period EARTHQUAKE PREDICTION Statement of odds by various observations for an earthquake of the predicted kind Magnitude range and tectonic flux A. Detailed geological mapping B. Crustal deformation tide-gauge stations geodetic measurements continuous recording of ground deformation C. Seismicity observatory studies micro-earthquakes changes in seismic velocities active faulting and folding D. Others heat flow measurements tectonic mechanisms after shock studies micro-earthquake studies real-time seismology seismic propagation studies seismic zoning structural dynamic studies Figure 3. Four essential elements for the valid prediction. THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

9 EARTHQUAKE DISASTER MITIGATION APPROACH Pre-disaster planning - Measures at physical planning level - Assessment of potential risk zones - Assessment of potential man-made risk zones - Land-use pattern - Infra-structural network - Safety standards and norms - Building shape, height, and group -Evacuation and emergency preparedness Building measures - Damage rating - Building code - aseismic design and practice Management - Both pre-disaster and post-disaster administrative principles - Implementation of building code - Relocation - Mass awareness both for pre-disaster preparedness and postdisaster management Figure 4. Earthquake disaster mitigation approach. SUMMARY AND RECOMMENDATION Bangladesh is extremely vulnerable to earthquake activities. 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, Jessor, Kushtia, and Chittagong 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. In order to determine the exact level of seismicity in Bangladesh, an extensive programme to study of earthquake activities should be undertaken and be institutionalised. In order to achieve the desired goal, need for a national level research institute is strongly recommended. An extensive seismological observatory network must be set-up in Bangladesh equipped with the modern and sensitive earthquake monitoring facilities. This includes THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER

10 strainmeters, 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. Acknowledgement:This paper was presented in a Seminar organized by National Oceanographic and Maritime Institute (NOAMI) at Bangladesh University of Engineering and Technology (BUET), Bangladesh, 10 th November REFERENCES Anonymous (1979), Seismic zoning map of Bangladesh and outline of a code for earthquake resistant design of structures. Final report by the Committee of Experts on Earthquake Minimization. Geological Survey of Bangladesh. 42p. Bernstein, J. (1954), Tsunamis, Scientific American, vol. 191, no.2, p Choudhury, J.R. and Ali, M.H. (1994), Seismic zoning of Bangladesh. Sem. Recent Dev. Eathq. Disas. Mitig., Organized by IEB and IAEE, Dhaka. Chouhan, R.K.S. and Khan, A. A. (1974), Seismic zoning of Bangladesh. 5th Sym. Earthq. Engg. Roorkey, India. p Hoque, M.A. (2001), Geophysical and Geochemical Signatures and Its Geotectonic significance of the Eastern Folded Belt (EFB) of the Bengal Basin, Unpublished M. Sc. thesis, Department of Geology, University of Dhaka. Khan, A. A., Hoque, M., Akhter, S.H. and Chouhan, R.K.S. (1998), Multiple element seismic zoning vis-a-vis state of seismic hazard in Bangladesh. Proceedings of International Conference on Disaster Management, Guahati, India. p Khan, A. A. and Chouhan, R. K. S. (1996), The crustal dynamics and the tectonic trends in the Bengal basin, J. Geodynamics. v.22(3/4), U.K. p THE JOURNAL OF NOAMI VOLUME 18 NUMBER 2 DECEMBER