Earthquake: What tectonic map reveals for Bangladesh

Although no large-scale earthquake has occurred in Bangladesh, the recent quakes in Nepal warn us that such a major disaster may hit the country any moment. The earthquakes in Nepal and their impacts on Bangladesh indicate that entire Bangladesh will suffer heavy loss of lives and properties, if an earthquake of the similar magnitude strikes the country. Due to the proximity to Nepal, the earthquakes were felt in almost every district in Bangladesh and these caused small-scale damage such as cracks in high-rise buildings and schools.
REGIONAL TECTONICS: Bangladesh is located in a potential earthquake zone where the Indian plate is pushing it northwards to the Eurasian plate. The convergence of these two continental plates has caused severe earthquakes in Pakistan, India, Nepal and other adjacent regions.
In 1982 Matin and Husain prepared a tectonic scheme for Bangladesh and adjacent areas based on geological, drilling, logging, seismic, aeromagnetic and gravity data. All previous tectonic maps of Bangladesh and adjacent areas were compiled. These maps were prepared based on the geomorphological concept.  Matin and Husain presented a new tectonic map based on historical-genetic and plate tectonic principles. The following information, which these authors published in 1986, outlines various zones of the tectonic framework of Bangladesh:
"The Bengal foredeep is a submontane foredeep of the Arakan Yoma geosynclines that entered the organic stage of its development in the Paleocene and Eocene period. The NE-SW trending Eocene Hinge line divided the basin into two distinct zones. The western part of the foredeep between the Hinge Zone and Indian shield outcrops, where the molasses sediments were deposited on a platform (shelf) basin, are considered the "External Zone". Areas east of the Hinge Zone up to Arakan-Yoma Mesozoic sedimentary outcrops, where molasses accumulation happens in the geosynclinal basin are considered by the authors as the "Internal Zone." Further the Internal Zone is divided into (I) The Eastern Chittagong Subzone and (II) the Western Dhaka Subzone, whereas the external zone is divided into (I) the Rajshahi Subzone, (II) the Hinze Subzone and (III) the Upper Assam Subzone.
About three-fourths of the country are covered by quaternary-recent alluvial sediments deposited by the Ganges-Brahmaputra-Meghna river system. The Paleocene to Pleistocene sediments are exposed in the tertiary hill ranges of Chittagong, Chittagong Hill Tracts, Sylhet, the Pleistocene terraces of the Madhupur tract of Mymensingh, Barind Tract and Lalmai hills in approximately one-fourth of the country.  

The sediments in the external zone have been deposited on the "Basement Complex" and the age of the rocks ranges from the Permian to the Pleistocene periods. The internal zone consists of Jenam formation of the Oligocene age, Bhuban and Bokabil formations of the Early Miocene age, Tipam Sandstone and Girujan Clay formations of the Middle Miocene age, the Lower and Upper Dupi Tila formations of the Late Miocene to Middle Pliocene age, the Modhupur Clay formation of the Late Pliocene to Pleistocene age and Alluvium formations of the Pre-Holocene to Holocene age" (1986, New Concepts on the Tectonic Zonation of Bengal Foredeep, SEAPEX, Singapore).
The people of Bangladesh lack knowledge about this map and many of the key signs of impending earthquake risks it reveals. This map was compiled by a group of scientists who have had proper educational training and extensive professional experience in dealing with the scientific data mentioned above. This map should be used as a base tectonic map and can be enhanced by incorporating and obtaining new local and regional data. The government of Bangladesh, policymakers, architects, engineers, city planners, geologists and others should have clear understanding of each zone of the map in order to proceed with development and policy-making in a risk reduction manner. Knowledge about these concepts and of tectonic elements will help them understand urban planning and development of sensitive projects such as high-rise buildings, industries, roads and highways, water treatments systems, gas and water supply systems, recreational facilities and other sensitive structures.
This map will help the people of Bangladesh in understanding the environmental hazards for investing, designing and constructing expensive and sensitive projects in Dhaka and other parts of the country.
TECTONICS OF DHAKA CITY: There is an intimate relationship between the regional and local tectonics of an area. The risk of environmental hazard of an area depends on both local and regional tectonics and their relationship with one another. Therefore, it is imperative to thoroughly analyse the local tectonics based on the most advanced and up-to-date scientific equipment and experts' opinions. The tectonic map of Dhaka city [which will feature in the second part of the article tomorrow] is prepared based on the geomorphological principle.
This map gives us an idea about the presence and characteristics of faults that surround Dhaka city. According to the new tectonic map, Dhaka city is located in the "Dhaka (semi-folded) Sub-zone of the Internal Zone of Bengal Foredeep. On the north it is bordered by Tangail Tripura High, on the east Chittagong (folded) sub-zone, on the West  by Hinze Subzone and on the south by Patuakhali trough" (1986, New Concepts on the Tectonic Zonation of Bengal Foredeep).  
There are two well-known distinct geologic features in Dhaka city: the uplifted block of the Modhupur Tract and the floodplain deposits that surround the Modhupur Tract. Floodplain deposits consist of alluvial sand, silt and clay and the thickness of the deposits ranges from 6 to 15 metres. The floodplain deposits are underlain by alluvial sand of the Dhamrai formation and the average thickness of sand ranges from 100 to 200 metres (Morris, 2000).
The stratigraphic sequence in the Modhupur Tract area off the city from Middle Miocene to recent geologic time ranges from Girujan clay (50 to 100 metres) to Dupi Tila formation (100-180 metres), Modhupur Clay (6-25 metres, Bashabo formation (3-25 metres) and Lowland Alluvium (0-5 metres). In terms of sedimentary stratigraphic sequences and soil mechanics, it appears that the Modhupur Tract area of the city is suitable for construction of high-rise buildings and other structures. On the other hand, the subsiding floodplain area is not a sustainable area for high-rise building construction.
According to the Geological Survey of Bangladesh, the most stable areas for constructing buildings and houses in the city include Kotwali, Motijheel, Syedabad, Kamalapur, Shegunbagicha, Kakrail, Dhanmondi, Gulshan, Banani, Uttarkhan, part of Uttara, Mirpur-1, Mirpur-2, Mirpur-3, Mirpur-6 and Mirpur-12. On the other hand, the most unstable areas with poor soil properties for constructing buildings and houses include areas of Meradia, Shatarkul, part of Badda, Bhatara, Mirpur-14, the lowland area adjacent to the Botanical Garden, part of Kalyanpur and Mohammadpur, the lowland area of Pallabi and many other areas including Kalapani.
From a geotectonic perspective, however, both categories of areas are at a greater risk for constructing high-rise buildings and other structures. Dhaka city is highly vulnerable to earthquakes because it sees numerous active and dormant fault lines, which are the weak points for potential earthquake damage. These fault lines include those at Tongi, Pagla, Balu, Baunid, Dhamrai, Kaliakair, Turag, Shitalakhya lineament, Arial Khan lineament, Banar, Old Brahmaputra, Buriganga, Bansi, Modhupur, Dhaleswari and the Padma.
Meer Husain is a professional environmental geologist, founder of Humane Water (formerly Academy of Healthy Water, Ecosystem and Environment, a non-profit organisation), Kansas in the USA, and Md Jasim Uddin is exploration geologist, Bangladesh.  
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