Earthquake preparedness: A comprehensive approach

In the last couple of years, earthquake, an unpredictable but impending threat for our country sparked much discussion regarding the possible disastrous consequences. The country is historically exposed to a number of natural disasters. With the advancement of science and technology, almost all the natural disasters can be predicted well before their strike, except earthquake. This unpredictable disaster poses an extraordinary risk for the whole country. Bangladesh and its neighbouring countries have experienced high-magnitude earthquakes in the past, and the consequences were devastating, but the devastation may get intensified manifold at present because of huge population density in the poorly-planned urban areas. Recent earthquake occurrences and their colossal damages in neighbouring countries, like Nepal (7.8 Mw, 2015/4), Myanmar (6.9 Mw, 2016/4) and India (6.7 Mw, 2016/1) further exacerbate this issue. Bangladesh itself experienced several small to moderate shocks during this time, in which Mw 4.9 in Madhabpur (2017/11), Mw 4.7 in Rangamati (2016/6), Mw 4.6 in Sarankhola (2015/4), and Mw 4.5 in Chhatak (2017/4) are the noticeable one. It is thought-provoking to see that except the Sarankhola earthquake, rest of them belong to the high seismic prone area, as demarcated in the earthquake zone map of Bangladesh. Geologically, these seismic events are not isolated, rather a part of events that have been occurring along the north and eastern margins of the Indian Plate boundary located at the juncture of three tectonic plates -- the Indian Plate, the Eurasian Plate and the Burmese microplate. Bangladesh is located at the north-eastern margins of the Indian Plate.

Indian Plate is continuously moving, approximately 6 cm/year, towards the northeast, being subducted under the Eurasian Plate at approximately 4.6 cm/year and the under the Burmese Plate at approximately 3.6 cm/year in the north and east, respectively. This contractional motion is taken up by stress accumulation and strain portioning mainly along some active faults, which are broadly distributed over a series of reverse and strike-slip structures. Probability of an earthquake from such an active fault depends on the rate of motion and the time since the last rupture. The active faults of regional scale capable of generating moderate to strong earthquakes are present mainly in the north and eastern part of Bangladesh and its neighbouring surroundings.

In Bangladesh, the major active faults are - about 320 km long east-west trending Dauki Fault, located along the southern edge of the Shillong Plateau i.e., Meghalaya-Bangladesh border; the 150 km long Madhupur Fault trending approximately north-south located between Madhupur Tract and Jamuna flood plain; the Assam-Sylhet Fault of about 300 km length trending northeast to southwest located in the southern edge of the Surma basin; and the Chottogram-Coastal Fault, which is a plate boundary fault of about 800 km long, running parallel to the Chottogram-Myanmar coast and reaches inland up to the Tichna anticline near Bangladesh-Tripura border. In neighbouring surroundings, major active faults are - east-west running Main Frontal Thrust to the north; WNW-ESE running Oldham Fault also to the north; the Dapsi Fault strikes WNW across the western Shillong massif and continue beyond the Brahmaputra River; the NE-SW running Hafflong-Disang Fault to the northeast; and Kaladan, Kabaw and Sagaing faults progressively towards the east of Bangladesh. All these faults had generated devastating earthquakes in the past and can potentially generate large earthquakes in near future.

Among these faults, the presence of three important seismic gaps in three major active faults - the eastern segment of the Dauki Fault, northern segment of the Chottogram-Coastal Fault and Bhutanese segment of the Main Frontal Thrust causes major apprehension to the earthquake geologist. In general, a seismic gap is a locked segment of an active fault, which is supposed to produce a significant earthquake that has not slipped in a long time, in comparison to other segments along the same fault. For example, the eastern segment of the Dauki fault has not been slipped in the recent past, but was it to slip in a single earthquake, its potential maximum magnitude would constitute a significant seismic threat to nearby densely populated areas of Bangladesh, India, Bhutan and Nepal. Alarmingly, other two seismic gaps in the two major active faults are also close to Bangladesh, compared to the other segments of these faults. Hence, a possible high intensity earthquake in any of these seismic gaps will cause catastrophic consequences for a large part of the country. Despondently, in producing the earthquake zone mapping of Bangladesh, these three seismogenic gaps have not been considered. Not only that, none of the probabilistic seismic hazard maps carry out in Bangladesh considered these seismic gaps, and therefore, we are completely unaware of the possible consequences.

Now the questions are: what should be done in preparation, and what makes us well prepared for disasters? It is very important to remember that what we cannot do normally, cannot be done well during emergency period. Good preparation will enable us to respond successfully to disasters. In this connection, we need to do nationwide collaborative research involving geologists, seismologists, urban planners and civil engineers in order to develop probabilistic seismic/earthquake hazard maps for the whole country. Beforehand, it is necessary to produce a comprehensive active tectonic map by taking into account all the relevant geological/seismological attributes. The probabilistic earthquake hazard map needs to consider the different seismic hazard scenarios and to produce the map on different scales. All engineering constructions, specifically large-scale ones, need to strictly follow the seismic hazard map guideline during the construction and subsequent operation/maintenance. Relevant academicians and researchers need to write text book contents on earthquake disaster and precisely device necessary disaster response guidelines in Bangladesh perspective. They need to advise the policy makers to introduce the contents in the text books in the primary and higher secondary levels.

Following the disaster, it is indispensable to compare the pre and post-disaster aerial photos immediately to pinpoint the relevant disastrous impacts. Rescue workers from remote areas heavily rely on maps in the planning of their operations. Pre-disaster maps need to immediately update based on the post-disaster aerial photos. If maps don't show latest features, their work might be significantly hampered. Archiving maps and air photos to make them readily available for disaster response is the strategic responsibility of the Geological Survey of Bangladesh (GSB), Bangladesh Space Research and Remote Sensing Organisation (SPARRSO) and Survey of Bangladesh (SoB). Post-disaster aerial photo survey can be done by the Bangladesh Air Force (BAF) and the photos need to be immediately transfer to the GSB, SPARRSO and SoB for processing and interpretation. Total process needs to be completed within few hours, and necessary information needs to be updated in the post-disaster maps and respective web-portal. The Department of Disaster Management, GSB, SPARRSO, SoB, Bangladesh Meteorological Department and the other relevant organisations need to work in a well-synchronised manner to make the decision makers and disaster risk reduction community understand the importance of their work in all the phases of disaster risk reduction and management.

In post-disaster scenario, bottom-up approach needs to be maintained for better management. Disaster response and management maps for all over Bangladesh starting from divisional level down to ward/village levels need to be prepared. This map needs to develop, update periodically with location-based disaster risk information, and disseminate to decision makers, the general public and communities at risk to disaster in an appropriate format. The smallest map is required to be placed at individual building/home in ward/village level where it will be clearly marked what inhabitants should do and whom they should contact in case of emergency. Similarly, this arrangement needs to be in place up to the divisional level. Regular emergency drills are necessary to train the communities/stakeholders and emergency response workers to be well prepared for disasters and also help them to identify processes/equipment that may be required for improvement/repair. More than anything, it's crucially important to make the people understand that earthquake disaster risk is real, and to convince them to act accordingly in order to form an earthquake resilient society.

Md Sakawat Hossain is Associate Professor, Department of Geological Sciences, Jahangirnagar University.

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