Syed Humayun Akhter

New structural data along the central part of the Dauki topographic front supports the hypothesis that the Shillong Plateau is a highly asymmetric south-verging Quaternary anticline driven by a north-dipping blind thrust fault that projects into Bangladesh, south of the topographic front. This thrust-fold is tectonically more important than it appears from the relatively modest accumulated deformation, and may represent a reorganization of the eastern Himalayan front. The Dauki Fault is the most likely source of the 1897 Great Indian Earthquake and poses a hazard to densely populated areas on the Ganges-Brahmaputra Delta region. The sharp linear topographic feature often mapped as the Dauki fault is instead a contact between competent Eocene limestone and much less competent younger clastic units. This contact may be depositional or locally a secondary back thrust. While the Sylhet basin has been rapidly subsiding in the Late Quaternary, the topographic front is marked by raised and...

The great 1762 Arakan earthquake caused subsidence and uplift along 700km of the Arakan coast, and is thought to derive from a huge megathrust rupture reaching northward onto the southeastern coast of Bangladesh. Paleoseismic investigations were conducted in that area to document effects of that and prior earthquakes. U/Th ages obtained from isochron analysis of uplifted dead coral heads of the Poritesspecies, collected along a south to north transect from the islands east coast reveal at least three growth interruptions caused by abrupt relative sea-level changes within the past 1300 years that we interpret to be associated with megathrust ruptures. The ages show distinct events approximately 250, 900 and 1300 years ago. The youngest of these events corresponds to the 1762 Great Arakan earthquake. The two prior events at ~1100 and 700 AD, suggest an average recurrence interval of 400-600 years. Along the coast of Teknaf, we mapped a ~2m uplifted terrace. Marine shells on top of the...

ABSTRACT A paleo-seismological study was conducted at Jaflong, Sylhet, Bangladesh, which is on the eastern part of the Dauki fault. The geomorphology around Jaflong is divided into the Shillong Plateau, the foothills, the lower terraces, and the alluvial plain from north to south. Because the foothills and lower terraces are considered to be uplifted tectonically, an active fault is inferred to the south of the lower terraces. This fault, which branches from the Dauki fault as a foreland migration, is known as the Jaflong fault in this paper. The trench investigation was conducted at the southern edge of the lower terrace. The angular unconformity accompanied by folding, which is thought to be the top of the growth strata, was identified in the trench. An asymmetric anticline with a steep southern limb and gentle northern limb is inferred from the back-tilted lower terrace and the folding of the gravel layer parallel to the lower terrace surface. The timing of the seismic event which formed the folding and unconformity is dated to between AD 840 and 920. The trench investigation at Gabrakhari, on the western part of the Dauki fault, revealed that the Dauki fault ruptured in AD 1548 (Morino et al., 2011). Because the 1897 great Indian earthquake (M ⩾ 8.0; Yeats et al., 1997) was caused by the rupture of the Dauki fault (Oldham, 1899), it is clear that the Dauki fault has ruptured three times in the past one thousand years. The timing of these seismic events coincides with that of the paleo-liquefactions confirmed on the Shillong Plateau. It is essential for the paleo-seismological study of the Dauki fault to determine the surface ruptures of the 1897 earthquake. The Dauki fault might be divided into four rupture segments, the western, central, eastern, and easternmost segments. The eastern and western segments ruptured in AD 840–920 and in 1548, respectively. The 1897 earthquake might have been caused by the rupture of the central segment.

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...

Bangladesh is a country characterized by numerous natural disasters. These natural hazards occur both on the surface (e.g., flooding and river avulsions) and within the subsurface (e.g., earthquakes) (see figure), and both types have been related to regional tectonic activity. Bangladesh is also one of the most highly populated countries in the world with a capital city, Dhaka, host to 15 million people. This urban center is located only 40 km southeast from the Madhupur Tract, a potentially tectonically hazardous region. In order to determine this region’s tectonic hazard potential, recent studies have attempted to detect and identify significant neotectonic signatures of the tract, such as faults, lineaments, and weak zones within the region. Recent earthquake evidence along the proposed Madhupur Fault suggests that the area my be tectonically unstable and vulnerable to further seismicity, placing the fast-growing and densely populated Dhaka city in potential danger (see figure).
The Madhupur Tract is in central Bangladesh, and is surrounded by the Jamuna-Brahmaputra river floodplain. The Madhupur Tract is an exposed Quaternary interfluve between two pathways for the Brahmaputra River. Possible uplift of the Madhupur Tract may have exerted a significant control on the avulsion history of the Jamuna River. The Jamuna river avulsion history is cyclic, with a periodicity of about 1800 years. Within these cycles, the Jamuna’s position has fluctuated between west and east of the Madhupur(Pickering et al., 2013). As this avulsion history is thought to be, at least partly, related to seismicity in the region, future seismicity has the potential to cause future river avulsions and related flooding. It is believed that 1885 Bengal earthquake may have been caused by the rupture of the blind Madhupur fault (see figure) on the western margin of the Madhupur. However, there is no paleoseismological evidence. The principle aim of this research is to identify the location of the Madhupur fault and the connection between fault activity and the avulsion history of the Jamuna River. To this end, we aim to determine, existence of the fault by mapping the local lithology using resistivity, and ultimately shed light of the cause of Jamuna River avulsions.

The Ganges, Brahmaputra and Meghna Rivers converge in Bangladesh with annual discharge second to the Amazon. Most of the flow occurs when the summer monsoon causes widespread flooding. The impounded water represents a large surface load whose effects can be observed in GRACE and GPS data. Bangladesh is at the center of the second largest seasonal anomaly in the GRACE gravity field, reflecting water storage in Southeast Asia. Eighteen continuous GPS stations in Bangladesh record seasonal vertical motions up to 6 cm that inversely correlate to river level. We use 304 river gages to compute water height surfaces with a DEM to separate surface water from groundwater. Porosity of 20% was used to estimate groundwater mass and calculate the water load. Results show ~100GT of water are stored in Bangladesh (7.5% of 20 annual discharge), but can reach 150GT during extreme events. The calculated water mass agrees with monthly GRACE water mass equivalents from Bangladesh within statistical limits. We compute the deformation due to this water load on an elastic half space, and vary Young’s modulus to fit GPS data from our two most continuous records. The 23 water loading can account for >50% of the variance in the GPS data. The best fitting Young’s modulus is 117-124GPa for DHAK and 133-135GPa for SUST, although the upper bound is not well constrained. These estimates lie between sediment (30-75GPa) and mantle (190GPa) values, indicating that response to loading is sensitive to structure throughout the lithosphere and is not absorbed by the weak sediments.

The Ganges, Brahmaputra, and Meghna rivers converge in Bangladesh with an annual discharge second to the Amazon. Most of the flow occurs during the summer monsoon causing widespread flooding. The impounded water represents a large surface load whose effects can be observed in Gravity Recovery and Climate Experiment (GRACE) and GPS data. Bangladesh is at the center of the second largest seasonal anomaly in the GRACE gravity field, reflecting water storage in Southeast Asia. Eighteen continuous GPS stations in Bangladesh record seasonal vertical motions up to 6 cm that inversely correlate to river level. We use 304 river gages to compute water height surfaces with a digital elevation model to separate surface water from groundwater. Porosity of 20% was used to estimate groundwater mass and calculate the water load. Results show !100 GT of water are stored in Bangladesh (7.5% of annual discharge) but can reach 150 GT during extreme events. The calculated water mass agrees with monthly GRACE water mass equivalents from Bangladesh within statistical limits. We compute the deformation due to this water load on an elastic half!space, and we vary Young’s modulus to fit GPS data from our two most continuous records. The water loading can account for >50% of the variance in the GPS data. The best fitting Young’s modulus is 117–124 GPa for DHAK and 133–135 GPa for SUST, although the upper bound is not well constrained. These estimates lie between sediment (30–75 GPa) and mantle (190 GPa) values, indicating that response to loading is sensitive to structure throughout the lithosphere and is not absorbed by the weak sediments.

A knowledge of Himalayan erosion history is critical to understanding crustal deformation processes, and the proposed link between the orogen's erosion and changes in both global climate and ocean geochemistry. The most commonly quoted age of India–Asia collision is ~50 Ma, yet the record of Paleogene Himalayan erosion is scant — either absent or of low age resolution. We apply biostratigraphic, petrographic, geochemical, isotopic and seismic techniques to Paleogene rocks of the Bengal Basin, Bangladesh, of previously disputed age and provenance. Our data show that the first major input of sands into the basin, in the N1 km thick deltaic Barail Formation, occurred at 38 Ma. Our biostratigraphic and isotopic mineral ages date the Barail Formation as spanning late Eocene to early Miocene and the provenance data are consistent with its derivation from the Himalaya, but inconsistent with Indian cratonic or Burman margin sources. Detrital mineral lag times show that exhumation of the orogen was rapid by 38 Ma. The identification of sediments shed from the rapidly exhuming southern flanks of the eastern–central Himalaya at 38 Ma, provides a well dated accessible sediment record 17 Myr older than the previously described 21 Ma sediments, in the foreland basin in Nepal. Discovery of Himalayan detritus in the Bengal Basin from 38 Ma: 1) resolves the puzzling discrepancy between the lack of erosional evidence for Paleogene crustal thickening that is recorded in the hinterland; 2) invalidates those previously proposed evidences of diachronous collision which were based on the tenet that Himalayan-derived sediments were deposited earlier in the west than the east; 3) enables models of Himalayan exhumation (e.g. by mid crustal channel flow) to be revised to reflect vigorous erosion and rapid exhumation by 38 Ma, and 4) provides evidence that rapid erosion in the Himalaya was coincident with the marked rise in marine 87Sr/86Sr values since ~40 Ma. Whether 38 Ma represents the actual initial onset of vigorous erosion from the southern flanks of the east-central Himalaya, or whether older material was deposited elsewhere, remains an open question.