Bangladesh highly vulnerable to tremor due to its geological structures
Dhaka, Apr 8 (UNB) - Although Bangladesh bears no relation with the recent devastating earthquake and tsunami in Japan, the country is highly vulnerable to tremors due to its geological structure, tectonic set-up and geographical location.
According to a geologist, Bangladesh is located in the northeastern part of the Indian sub-continent at the head of the Bay of Bengal. Tectonically, it lies on the northeastern Indian plate near the edge of the Indian carton and at the junction of three tectonic plates - the Indian Plate, the Eurasian Plate and the Burmese microplate.
Dr Syed Humayun Akhter, Professor of Geology, Dhaka University, said these form two long active tectonic structures where plates converge - the India-Eurasia plate boundary to the north forming the Himalaya Arc, and the India-Burma plate boundary to the east forming the Burma Arc.
"Bangladesh and Japan are situated in two distinct geological and tectonic locations separated by the 5000 km wide Eurasian plate. There is no link between the Japanese earthquake and the seismicity in Bangladesh."
He informed that Japan is located in the Circum Pacific Seismic belt, also known as the Pacific Ring of Fire, where 90 percent of the world's tremors occur. Only seven percent occurs in Alpide-Himalayas-Java-Sumatra seismic belt, and two percent in the Mid-Atlantic Ridge seismic belt. Bangladesh belongs to the Alpide-Himalayas seismic belt with low earthquake frequency.
The geologist, however, said the Indian plate is moving 6 cm each year towards the northeast, and sub-ducting under the Eurasian plate at 45 mm and the Burmese plate at 35 mm in the north and east respectively each year. This continuous motion is taken up by active faults.
He said the probability of an earthquake from a given fault depends on the rate of motion and on the time since the last rupture. Active faults of regional scale capable of generating moderate to great earthquakes are present in and around Bangladesh.
Prof Humayun said these include the Dauki fault, about 300 km long trending east-west and located along the southern edge of the Shillong Plateau (Meghalaya- Bangladesh border), the 150 km long Madhupur fault trending north-south situated between Madhupur Tract and Jamuna flood plain, the Assam-Sylhet fault, about 300 km long trending northeast to southwest located in the southern Surma basin, and the Chittagong-Myanmar plate boundary fault, about 800 km long running parallel to the Chittagong-Myanmar coast.
He said: "As Bangladesh is located at the junction of the India Plate, Burma microplate and Tibet sub-plate, the country is very much vulnerable to earthquakes. A number of major and devastating earthquakes jolted the region in the past 500 years, as well as in geological history."
Prof Humayun, also Director of the Seismology and Geodesy Division, Dhaka University Earth Observatory, said another tectonic element, the 'Himalayan Arc' is characterized by three well-defined fault systems that are 2500 km long stretching from northwest syntaxial bend in Pakistan in the west to the northeast syntaxial bend in Assam in the east.
"The Himalayan Arc is only 400 kilometres north of Dhaka. It poses a great threat to Bangladesh as significantly damaging historical earthquakes have occurred in this seismic belt. A huge amount of energy is stored in the seismic gap of this belt and it may be released at any moment."
The tectonic set-up and the plate motions together make Bangladesh potentially vulnerable to earthquakes, he said.
Prof Humayun also said that the Dauki fault is very dangerous and a great threat for densely populated Bangladesh. The Shillong Plateau is a micro-plate within the Indian plate and Surma basin is subducting under this plate at a higher rate. This high rate of convergence generated an earthquake of magnitude 8.7 in 1897 and another in 1787 that shifted the course of the Brahmaputra River.
About Chittagong-Myanmar-Sumatra plate boundary, he said no big earthquake has been recorded in the 600 kilometre segment between north Andaman to Myanmar of this 3500 km long belt in the last few hundred years. A huge amount of seismic energy is stored in this segment, but obviously it will be released at some point and that will trigger tsunamis in the Bay of Bengal.
"If any tsunami occurs in this segment, it will hit the coastal belt of the country within 30 minutes."
The geologist said Bangladesh, a major part of the Bengal Basin, is home to one of the largest and most active deltas of the world formed by the Ganges-Brahmaputra-Meghna river system. It is almost entirely formed of recent alluvial and deltaic deposits.
"Topographically, the country is mainly composed of low-lying flat country with the exception of the slightly elevated Pleistocene uplands in the central and northwestern parts of the country and Tertiary hilly terrain in the northeast and southeast parts."
He said these hills are part of the folding associated with the Burma Arc plate boundary. Dhaka is situated in the central part of the bank of the Buriganga River and at the southern tip of the Madhupur Tract dating back to the Pleistocene age.
Prof Humayun said the Madhupur Tract is an area of recent uplift within the delta and the surface of the tract is in general higher on the west, sloping very gently eastward to disappear beneath younger sediments.
He said Dhaka is surrounded by the old Brahmaputra floodplain in the north and the east, by the Ganges-Meghna flood plain in the south and by the Jamuna flood plain in the west. Dhaka is slightly elevated above the surrounding floodplains and represents mostly flat land with minor undulations.
The geologist said topographically Dhaka is of low relief with many low depressions. The Madhupur Tract is structurally controlled. The Pleistocene sediments of Madhupur Tract have been affected by numerous episodes of faulting. These faults are probably the branching out surface faults from the low-dipping western extension of the Burma Arc detachment fault.
Dhaka lies within a 50 to 500 km distance from the seismogenic faults and sits on the Burma Arc detachment fault.
He said the country's position adjacent to the very active Himalayan front in the north and Burma deformation front in the east exposes it to strong shaking from a variety of earthquake sources that can produce tremors of magnitude 8 or greater.
Prof. Humayun said the potential for a magnitude 8 or greater earthquake on the nearby Himalayan and Burmese fronts, which depends on the rate at which elastic strain accumulates, is very high.
The tectonic set-up of Bangladesh, especially its position close to three converging lithospheric plates with the presence of seismogenic faults, suggests that the country, including its capital Dhaka, must have suffered from severe earthquakes in history, he added.
The continuing plate motion is being absorbed within the earth's crust as strain energy that will inevitably be released by earthquake ruptures of active faults. The likelihood of a damaging earthquake from a given fault increases as the time from the previous large earthquake increases. It is possible for this energy to be released at any moment in a devastating earthquake.
If the 1885 Bengal Earthquake or the 1897 Great Indian Earthquake were to be replicated, or if an earthquake of magnitude 6 were to occur beneath Dhaka, the situation of Dhaka City with its population of nearly 13 million, widespread brick masonry, and non-engineered and poorly constructed buildings would be worse than ever before. The aftermath of such a destructive earthquake could even lead Dhaka residents to abandon the city.
http://www.unbconnect.com/component/news/task-show/id-45171
According to a geologist, Bangladesh is located in the northeastern part of the Indian sub-continent at the head of the Bay of Bengal. Tectonically, it lies on the northeastern Indian plate near the edge of the Indian carton and at the junction of three tectonic plates - the Indian Plate, the Eurasian Plate and the Burmese microplate.
Dr Syed Humayun Akhter, Professor of Geology, Dhaka University, said these form two long active tectonic structures where plates converge - the India-Eurasia plate boundary to the north forming the Himalaya Arc, and the India-Burma plate boundary to the east forming the Burma Arc.
"Bangladesh and Japan are situated in two distinct geological and tectonic locations separated by the 5000 km wide Eurasian plate. There is no link between the Japanese earthquake and the seismicity in Bangladesh."
He informed that Japan is located in the Circum Pacific Seismic belt, also known as the Pacific Ring of Fire, where 90 percent of the world's tremors occur. Only seven percent occurs in Alpide-Himalayas-Java-Sumatra seismic belt, and two percent in the Mid-Atlantic Ridge seismic belt. Bangladesh belongs to the Alpide-Himalayas seismic belt with low earthquake frequency.
The geologist, however, said the Indian plate is moving 6 cm each year towards the northeast, and sub-ducting under the Eurasian plate at 45 mm and the Burmese plate at 35 mm in the north and east respectively each year. This continuous motion is taken up by active faults.
He said the probability of an earthquake from a given fault depends on the rate of motion and on the time since the last rupture. Active faults of regional scale capable of generating moderate to great earthquakes are present in and around Bangladesh.
Prof Humayun said these include the Dauki fault, about 300 km long trending east-west and located along the southern edge of the Shillong Plateau (Meghalaya- Bangladesh border), the 150 km long Madhupur fault trending north-south situated between Madhupur Tract and Jamuna flood plain, the Assam-Sylhet fault, about 300 km long trending northeast to southwest located in the southern Surma basin, and the Chittagong-Myanmar plate boundary fault, about 800 km long running parallel to the Chittagong-Myanmar coast.
He said: "As Bangladesh is located at the junction of the India Plate, Burma microplate and Tibet sub-plate, the country is very much vulnerable to earthquakes. A number of major and devastating earthquakes jolted the region in the past 500 years, as well as in geological history."
Prof Humayun, also Director of the Seismology and Geodesy Division, Dhaka University Earth Observatory, said another tectonic element, the 'Himalayan Arc' is characterized by three well-defined fault systems that are 2500 km long stretching from northwest syntaxial bend in Pakistan in the west to the northeast syntaxial bend in Assam in the east.
"The Himalayan Arc is only 400 kilometres north of Dhaka. It poses a great threat to Bangladesh as significantly damaging historical earthquakes have occurred in this seismic belt. A huge amount of energy is stored in the seismic gap of this belt and it may be released at any moment."
The tectonic set-up and the plate motions together make Bangladesh potentially vulnerable to earthquakes, he said.
Prof Humayun also said that the Dauki fault is very dangerous and a great threat for densely populated Bangladesh. The Shillong Plateau is a micro-plate within the Indian plate and Surma basin is subducting under this plate at a higher rate. This high rate of convergence generated an earthquake of magnitude 8.7 in 1897 and another in 1787 that shifted the course of the Brahmaputra River.
About Chittagong-Myanmar-Sumatra plate boundary, he said no big earthquake has been recorded in the 600 kilometre segment between north Andaman to Myanmar of this 3500 km long belt in the last few hundred years. A huge amount of seismic energy is stored in this segment, but obviously it will be released at some point and that will trigger tsunamis in the Bay of Bengal.
"If any tsunami occurs in this segment, it will hit the coastal belt of the country within 30 minutes."
The geologist said Bangladesh, a major part of the Bengal Basin, is home to one of the largest and most active deltas of the world formed by the Ganges-Brahmaputra-Meghna river system. It is almost entirely formed of recent alluvial and deltaic deposits.
"Topographically, the country is mainly composed of low-lying flat country with the exception of the slightly elevated Pleistocene uplands in the central and northwestern parts of the country and Tertiary hilly terrain in the northeast and southeast parts."
He said these hills are part of the folding associated with the Burma Arc plate boundary. Dhaka is situated in the central part of the bank of the Buriganga River and at the southern tip of the Madhupur Tract dating back to the Pleistocene age.
Prof Humayun said the Madhupur Tract is an area of recent uplift within the delta and the surface of the tract is in general higher on the west, sloping very gently eastward to disappear beneath younger sediments.
He said Dhaka is surrounded by the old Brahmaputra floodplain in the north and the east, by the Ganges-Meghna flood plain in the south and by the Jamuna flood plain in the west. Dhaka is slightly elevated above the surrounding floodplains and represents mostly flat land with minor undulations.
The geologist said topographically Dhaka is of low relief with many low depressions. The Madhupur Tract is structurally controlled. The Pleistocene sediments of Madhupur Tract have been affected by numerous episodes of faulting. These faults are probably the branching out surface faults from the low-dipping western extension of the Burma Arc detachment fault.
Dhaka lies within a 50 to 500 km distance from the seismogenic faults and sits on the Burma Arc detachment fault.
He said the country's position adjacent to the very active Himalayan front in the north and Burma deformation front in the east exposes it to strong shaking from a variety of earthquake sources that can produce tremors of magnitude 8 or greater.
Prof. Humayun said the potential for a magnitude 8 or greater earthquake on the nearby Himalayan and Burmese fronts, which depends on the rate at which elastic strain accumulates, is very high.
The tectonic set-up of Bangladesh, especially its position close to three converging lithospheric plates with the presence of seismogenic faults, suggests that the country, including its capital Dhaka, must have suffered from severe earthquakes in history, he added.
The continuing plate motion is being absorbed within the earth's crust as strain energy that will inevitably be released by earthquake ruptures of active faults. The likelihood of a damaging earthquake from a given fault increases as the time from the previous large earthquake increases. It is possible for this energy to be released at any moment in a devastating earthquake.
If the 1885 Bengal Earthquake or the 1897 Great Indian Earthquake were to be replicated, or if an earthquake of magnitude 6 were to occur beneath Dhaka, the situation of Dhaka City with its population of nearly 13 million, widespread brick masonry, and non-engineered and poorly constructed buildings would be worse than ever before. The aftermath of such a destructive earthquake could even lead Dhaka residents to abandon the city.
http://www.unbconnect.com/component/news/task-show/id-45171
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