Dauki Fault Research Articles

An assessment of the geomorphodiversity and land use/cover change (LUCC) effects associated with landslides in Meghalaya, India

AbstractThe Shillong Plateau in the Eastern Himalayas is exposed to high seismicity, neo‐tectonic activity and orographic precipitation that aggravated landslides in the region by affecting hillslope processes. Landscape development is influenced by landslide phenomenon and depends upon geomorphic processes and geomorphodiversity (GmD). It is linked intimately with geomorphology, hydrology and soil type. This study uses a remote sensing and GIS‐based approach to assess the spatial linkage between GmD and landslide distribution in Meghalaya (India). GmD index (GmDI) is developed using geological, morphometric and geomorphological parameters and analysed with landslide distribution. The landslide distribution considering land use/cover change (LUCC) is also examined. The result shows that the region with high‐relief, valley and depression zone, mainly along the Dauki fault, have a high to a very high GmDI. About 61% of landslide localities are in zones of high GmDI, concentrated in the central and eastern parts of Meghalaya. Density analysis of the very high class (class 5) of GmD confirms the same. Most of these landslides occur in dense and light‐vegetated landmasses. However, the LUCC analysis reveals a significant increase in the built area on steeper slopes (average expansion ratio of 2.30 for slopes >15°) between 2017 and 2022, indicating a rising threat of landslides in urban areas of Meghalaya. The study presents the importance of GmD in active tectonic regions with particular reference to landslide potential and recent LUCC. It can aid decision‐makers in planning sustainable developments and risk mitigation strategies for landslide hazards and geoconservation.

Tectono-geomorphic evolution of an alluvial fan along the Dauki fault system, Bengal Basin, Bangladesh: remote sensing and pedogenic approach

ABSTRACT Late Quaternary landform evolution of the Karnajhora fan and its adjacent areas of the mid northern Bengal Basin, Bangladesh have been studied using remote sensing and pedogenic approach. Based on pedogenic characteristics, different soil-geomorphic units can be arranged according to their maturity as Hill > Distal Fan > Mid Fan > Proximal Fan. Soil-geomorphic units are further grouped into a three member soil-chronoassociation and approximated inferred ages are QGSK1 < 1 ka; QGSK2 < 3–4 ka; and QGSK3 > 27 ka. The weakly developed youngest member QGSK1 and the moderately developed member QGSK2 represents inceptisols at fersiallitisation weathering stage. While the oldest QGSK3 member represents alfisols and show relict nature and at the initial phase of ferrugination weathering stage. Neotectonics controls the configuration of the local catena. Paleoclimate and base-level fluctuations have also played significant roles in the development of soils and landforms of the study area. The parent materials of Member QGSK3 were deposited during 80–27 ka. Since the beginning of deposition of QGSK3 soils till the Last Glacial Maximum (LGM~18 ka), the climatic conditions gradually changed from warmer-moist to cooler-drier, and the area might experienced dominantly channel incision with minor aggradation. During this period, the land between the Jamalpur Terrace and Susang Hills might be eroded and washed away while the Sherpur Terrace survived as a remnant terrace. After the LGM, the climate changed from a cooler-dry condition to warmer-moist condition and the present Holocene floodplain started to develop. The parent materials of QGSK2 and QGSK1 members then started to deposit by the paleo-Karnajhora synchronously with the old Brahmaputra River. As the Karnajhora River presently shifted toward east, soils of QGSK2 member are not presently affected and are relatively better developed compared to QGSK1 member soils.

Outcrop analogy with the subsurface geology and hydrocarbon prospectivity of Jaintiapur and adjacent areas in North-East Bangladesh

The availability of hydrocarbons in the eastern part of Bangladesh indicates the presence of active petroleum systems in the Bengal basin. Although extensive research has been conducted in the Surma basin, the correlation between structural configuration, tectonostratigraphic framework, and hydrocarbon potentiality in the basin including Jaintiapur area has not yet been established. In this study, the hydrocarbon prospectivity of Jaintiapur and the adjoining Dupigaon area has been characterized through outcrop analogy with the subsurface interpretation of seismic data along L13-01, PK-SY-5, and L13-03 lines. The main three megasequences have been recognized based on sequence stratigraphy in the Sylhet region utilizing outcrop, seismic, and well-log data. The exposed Tertiary and Quaternary formations of Jaintiapur and adjoining areas can be classified into eleven litho-stratigraphic units. Detailed facies analyses indicate that these rock units were deposited in fluvial, deltaic, and shallow marine environments. The subsurface lithology of the study area has been inferred from the petrophysical data obtained from Sylhet-7, Kailashtila-4, and Jalalabad-1 wells. Jaintiapur area lies in a tectonically complex province and is controlled by the Dauki Fault System. Folding and faulting represent intense tectonic deformation in the area. Detailed analyses of surface and subsurface geology indicate that faults pass along the rivers and transverse faults segment the reservoirs. A total of seven horizons have been interpreted using Petrel software. The seismic attribute analysis identifies leads and prospects based on transverse faults and amplitude anomalies with distinct bright spots at the PK-SY-5 line. From seismic interpretation, attribute analysis, and lithofacies distribution of Sylhet-7 well and its surround area in the Dupigaon, it seems that same gas bearing sand. Gas sands of the Sylhet-7 well indicate the possibility of gas/oil in the equivalent sedimentary sequence in the Dupigaon structure as this area is located close to and more down-dip towards the kitchen area.

Seismic evidence of pop-up tectonics beneath the Shillong Plateau area of Northeast India

Our detailed 3-D seismic tomographic assimilation using high-quality phase arrival time data recorded by the local seismographic network demonstrated that heterogeneities in the crustal faults have contributed significantly to the pop-up tectonics beneath the Shillong Plateau, characterized by high-V and low-σ. The major seismogenic faults, namely, the north-dipping Dapsi thrust in association with Dauki fault in the south and south dipping Brahmaputra fault in the north, located either side of the Shillong Plateau that acted as the causative factors for the pop-up, which attributed to the lithostatic (high-V, low-σ) and sedimentary (low-V, high-σ) load, respectively. Seismicity is found confined to a depth ≤ 60 km. Uneven distribution of structural heterogeneities in the upper crust is responsible for earthquake genesis of varying strengths. It is intriguing to note that high-velocity anomalies and low-ϭ in the uppermost crust, interpreted as the Shillong Plateau that acted as a geometric asperity and the juxtaposition of high-V and low-V became the source zone of the 1897 Shillong earthquake (Ms 8.7) as a novel observation for the region. Structural heterogeneities are distinctly distributed between low-V, high-σ and high-V, low-σ in the lower crust plays a major role for future intense seismogenesis due to differential strain accumulation.

Tectono‐stratigraphic evolution of Shillong Plateau, North East India through the Permian‐Eocene window

The Shillong Plateau in the north‐eastern Indian Plate signifies a cut‐off patch of the Indian Peninsular Shield, bordered by fault and thrust sheets, with thick alluvium cover along its periphery. The basement rock in the plateau accomplished about 2 km relief difference with the adjoining Sylhet Trough of Bangladesh in the south. The plateau witnessed Permo‐Carboniferous Gondwana sedimentation at its western margin, with Early Cretaceous basaltic traps and continuous fluvio‐marine sedimentation along its southern edge since the late Cretaceous. Although it is largely believed that strike‐slip movement along the Dauki Fault shifted the plateau about 250 km eastward, however, the driving mechanism for Shillong Plateau detachment and conversion of rift‐intercratonic basin to platformal configuration has drawn more attraction. This article focuses on the tectono‐stratigraphic evolution of Shillong Plateau and its adjoining regions on the basis of previous works, to try to understand the distinct tectonic and environmental factors. Early Permian continental rifting to the south‐west of the plateau and south‐east and south‐west of the Malda High presumably separated the Shillong Plateau from Chotanagpur Granite Gneiss Complex (CGGC). Extrusion of Rajmahal Traps west of the Malda High and Bengal‐Sylhet‐Mikir volcanism along an Early Cretaceous geofracture probably accelerated the east‐northeast migration of Shillong Plateau with respect to the CGGC. Since the Late Cretaceous, sedimentation at the southern fringe of Shillong Plateau commenced with an intracratonic basin which gradually transformed into a passive margin configuration during the Late Palaeocene‐Eocene Epoch during growth of the Indian Ocean. Differential subsidence in southern Shillong Plateau caused clastic sedimentation without considerable transportation. At the beginning, the Late Cretaceous intracratonic basin had received volcanoclastic, fluvio‐lacustrine sediments in a series of graben/half‐grabens associated with extensional faulting and volcanism, as represented by the Lower Mahadek and Jadukata formations in the plateau. The upper Mahadek and later Palaeogene sequences along the southern margin of the plateau record marine sedimentation, initiated with an estuarine‐marshy environment during the Late Cretaceous to Palaeocene, followed by a platformal marine succession since the Mid‐Eocene. The Calcutta‐Mymensingh Eocene Hinge Zone that separates a stable shelf from the deep basinal part might be traced through the south‐eastern fringe of Shillong Plateau and presumably continues further north‐east between the Naga and Disang thrusts in the eastern part of Dhansiri Valley.

Geospatial assessment of active tectonics using SRTM DEM-based morphometric approach for Meghalaya, India

Meghalaya, situated over the Shillong Plateau in the northeastern region of India, is considered a seismo-tectonically active region because of the Indian-Eurasian convergence and rising since the Cenozoic era. The present study aims to identify active deformation zones due to tectonic activity following a morphometric approach. Based on DEM data and GIS techniques, morphometry parameters along with geomorphic indices are obtained which allows the analysis of geomorphic processes responsible for the region’s landscape evolution, and indices of relative active tectonics (IRAT) are calculated by combining them. High drainage density, larger ruggedness number, intermediate to high hypsometric integral values with S/convex-type hypsometric curve, lower valley floor width-to-height ratio and stream sinuosity index values suggesting the region is tectonically active and according to IRAT, the Khasi and Jaintia Hills region (the central and eastern part) of the Meghalaya possesses relatively higher tectonic levels. The variation of tectonic activity is not uniform in the same structural feature and is found to be lower in the western portion and higher towards the east along the same Dauki Fault.

Correction to: Soft sediment deformation features in Dauki Fault region: evidence of paleoearthquakes, Shillong Plateau, NE India

Correction to: Soft sediment deformation features in Dauki Fault region: evidence of paleoearthquakes, Shillong Plateau, NE India

Soft sediment deformation features in Dauki Fault region: evidence of paleoearthquakes, Shillong Plateau, NE India

The northeastern region (NER) of India has several complex regional geological structures, out of which the Dauki fault (DF) is a prominent one. The E–W trending reverse DF, which is inferred to go through the southern margin of Shillong Plateau (SP), has played major role in the regional deformation of the adjoining areas and was believed to be active during the Late Quaternary time. Previous paleoseismological studies conducted on the eastern and western part of the DF, Bangladesh, revealed that the fault ruptured in AD 849–920 and AD 1548, respectively. However, there were no studies on the DF from southern side of the SP, India. For the first time, from Indian side, soft sediment deformation structures (SSDS) are reported from five trenches in and around the DF zone, SP. Close to the Dauki village, five trenches in the eastern part of the DF, SP, show micro faulting, sand dykes, disturbed strata, and water escape structures. The detailed investigation of SSDS indicates that the origin of deformation is seismic trigger. The 14C AMS dating of deformation structures generated by earthquakes suggests three seismic events occurred between 130 and 920 year BP, 5415 to 9140 year BP, and at about 4285 year BP. This study confirms that DF is indeed active, at least, since the mid-Holocene. More trenching and dating of seismically induced deformation features are needed to accurately calculate the recurrence interval of significant earthquakes that can strike the fast-expanding urban areas in India and Bangladesh.

Drainage Basin Morphometry Analysis in Parts of South Shillong Plateau, Meghalaya: Implications for Landscape Development

The Shillong Plateau is characterised by multiple phases of deformation and number of prominent sets joints/fracture system. The Southern Shillong Plateau unlike the northern part is highly dissected by the scarp faces which are mostly erosional. The area is characterised by deep incising river networks which cuts across many lithological units. To understand the factors influencing the drainage characteristic of the area and landscape development, the present study has been done in context of morphometry, geomorphology and geology of the drainage basin. Fifteen sub basins of 4th order have been selected from the Um Sohrynkew River basin. Drainage morphometry, indices of active tectonism, geology and tectonics and landform features have been worked out for better evaluation of drainage characteristics. The study area forms part of the Meghalaya Precambrian province of upper Proterozoic age. The southern border of the Shillong Plateau is demarcated by Dauki (Also known as Dawki) fault which is a prominent structural lineament. It consists of at least four E-W trending normal faults with occasional reversal. The segment of the Dauki fault in the study area is believed to be active. The present study focuses on the drainage network and the landscape development of the study area where a very strong relationship has been observed between the lithology and structure together with the tectonic activity influencing the drainage pattern in the area.

Multi-dimensional earthquake vulnerability assessment of residential neighborhoods of Mymensingh City, Bangladesh: A spatial multi-criteria analysis based approach

The assessment of the vulnerability of a city to different natural hazards is the prerequisite to achieving sustainable urban resilience. The city of Mymensingh is located in the most earthquake vulnerable zone of Bangladesh and surrounded by multiple fault lines including the Madhupur Blind Fault, Dauki Fault, and Sylhet-Assam Fault lines. The city also achieved the title of the 8th divisional city in 2015, thereby opening the door to unrestrained future development. Therefore, it is of paramount importance to assess the earthquake vulnerabilities of Mymensingh city and this research is envisioned to evaluate this vulnerability by combining 23 parameters derived from four major dimensions-geological, socio-economic, structural and systematic dimensions. The MCDM techniques of Analytical Hierarchy Process and Weighted Linear Combination coupled with GIS-based spatial analysis were applied in this study to assess the residential neighborhood-level earthquake vulnerability. The findings show that out of the 241 residential neighborhoods of Mymensingh city, 51 are observed to be highly vulnerable, while 123 are exposed to medium vulnerability and 67 to low vulnerability. According to the analysis, the topmost factors that make Mymensingh City highly earthquake-vulnerable are the soil type, proximity to the fire station, elderly population levels, Peak Ground Acceleration, child population levels and distance to healthcare facilities. The simplistic, efficient, and reasonably accurate methodology and findings of this research are expected to be helpful for policymakers in low-income countries to prioritize special consideration areas, resource targeting, and deciding on appropriate planning and development control interventions for disaster management.

Late Cenozoic transpression at the plate boundary: Kinematics of the eastern segment of the Dauki Fault Zone (Bangladesh) and tectonic evolution of the petroliferous NE Bengal Basin

The Sylhet Trough in the Bengal Basin (Bangladesh) hosts a number of gas fields. The evolution of the trough is closely associated presumably with the growth and the tectonics of the Dauki Fault Zone (DFZ). Hence understanding the transpression tectonics of the NE Bengal Basin is crucial in petroleum geoscience. The eastern segment of the DFZ in the NE Bengal Basin transpressed dextrally and activated since the latest Miocene up to the earliest Pliocene due to compression along the north, east and southeast. The structural and the tectonic elements of the area are assessed from geomorphic features and deformation structures. Nature of faults and comparison of fault-slip stress field with modern geodetic measurement allow establishing geometric and kinematic relationship between different tectonic elements with the DFZ. Bedding attitude measurements from the study area indicate sub-horizontal east–west-trending antiform fold axis gently plunging towards west confirming the existence of a large-scale monocline. Out of the two interpreted compression directions N–S and E-W, the former explains the orientation of the monocline while the later may cause the dextral transpression in combination with the former compression direction. Fault kinematics and incremental strain axes indicate a bulk north-trending subhorizontal shortening and vertical thickening of the fold and faults approximately perpendicular to the east-west oriented DFZ. The timing of the compressional deformation and fault activation in the study area is inconclusive. Paleostress analyses results match with the present day stress regime and this implies that all the deformations are genetically linked with the DFZ.

Geomorphological Study of Jaflong Area near Dauki Fault Using Remote Sensing and Geographic Information System

Geomorphology is closely related to geology, soil science, hydrology and environmental science and is being increasingly applied in planning, mining and hydrological sectors, and within environmental consultancy and tourism. The study area lies in the north-eastern part of the country and tectonically this area belongs to Surma Basin situated into Bengal Fore deep zone of Bengal Basin also known as Sylhet Trough. Under this study an attempt was made to establish the relationship between geomorphic unit and existing landuse based on remote sensing data. In the study area nine landuse categories were identified through remote sensing and GIS techniques. Two different physiographic units which are North-Eastern Terrace Land and Surma-Kushiyara Flood Plain are belongs to the study are. Moreover two surface geological units include Young Gravelly Sand and Marsh Clay and Peat are also there. From analysis no relationship has been found between landuse classes with its physiography and surface geology.&#x0D; Environ. Sci. &amp; Natural Resources, 12(1&amp;2): 151-155, 2019

Assessment of soil liquefaction potential: a case study for Moulvibazar town, Sylhet, Bangladesh

Liquefaction can intensify the destruction caused by an earthquake; thus, a region with high liquefaction potential could be more disastrous. Bangladesh is surrounded by the Indo-Burma Folded Belt in the east, the Dauki Fault and Himalayan Syntaxis in the north that are known to have occurred high magnitude earthquakes (e.g., Mw > 7) in the past. Therefore, assessing seismic hazards in the regions that are economically growing fast is of great interest. Among many other hazard assessment parameters, soil liquefaction potential index (LPI) can be used to assess seismic hazards. In this study, we have assessed the seismic hazard potential for a small town (Moulvibazar) in the northeast Bangladesh documenting liquefaction potential indices for different surface geological units using an earthquake of moment magnitude Mw 8 having a peak horizontal ground acceleration (PGA) of 0.36 g. Twenty-five standard penetration test (SPT) boreholes were completed within the study area to obtain SPT-N values for two surface geological units: (1) Holo–Pleistocene low elevated terrace deposits (Zone 1) and (2) Holocene flood plain deposits (Zone 2). Using the SPT-N values, the LPI values have been calculated for the soil profile of each borehole. The LPI values in the town vary from 0 to 42.33, whereas values from 1.42 to 7.52 are in Zone 1 and values from 0 to 42.34 are in Zone 2. It has been predicted that 42% and 78% areas of Zone 1 and Zone 2, respectively, might exhibit surface manifestation of liquefaction. The results of this study can be used for seismic risk management of Moulvibazar town.

Kinematics of faults in Bengal Basin: Constraints from GPS measurements

The crustal deformation of Bengal Basin is constrained by motion on faults within the basin and those along the basin boundaries on all sides except the southern margin. Eocene Hinge Zone (EHZ), the most prominent structure within Bengal Basin, running through Kolkata-Ranaghat-Mymensingh, shows present day reactivation by hosting 8 earthquakes of magnitude 5.3 to 7.2 on or near its vicinity since 1906. Analysis of GPS derived velocities of stations around the hinge belt reveals a change in crustal kinematics across the belt. The predicted fault model that best fits the observed GPS station velocities on the surface shows that EHZ is a low angle east dipping fault which is locked to a depth of 10 km below the alluvium cover with a maximum slip rate deficit of 1.6 ± 0.7 mm/yr in its northern part at a shallow depth from surface while the deficit in the southern part of EHZ is 1.0 ± 0.6 mm/yr. The Garh-Moyna Khandaghosh fault running almost parallel to the western basin-shield margin does not show significant slip rate deficit. The basin-bounding faults are more active than the faults within and therefore they are greater sources of seismic hazard in the basin. The model results reveal that the Sylhet trough region in the northeastern corner is most vulnerable to future earthquakes as it undergoes deformation due to a maximum slip rate deficit of 9.5 ± 2.4 mm/yr on the shallowly dipping locked megathrust along the eastern margin of the basin plus that of 7.4 ± 2.6 mm/yr on the over thrusting Dapsi thrust and Dauki fault in the north.

Probable intensity of Sylhet city from an event like 1918 Srimangal earthquake

An intensity map plays an important role and is required for construction, so in this study, we presented a map of intensity for every ward of the city of Sylhet. Another objective of this research is to create a methodology to develop the intensity map for Sylhet with the equation readily available. Sylhet is Bangladesh’s most seismically susceptible region owing to its location in the southeast portion of Dauki fault. In the last 101 years, almost 184 massive earthquakes in the Sylhet area have taken place between the magnitude of 4 and 7.2. Among these earthquakes, the 1918 earthquake of Srimangal was selected as it was the first one to be felt and recorded in Bangladesh as a destructive earthquake. The probable intensity map for the Sylhet city of Bangladesh using Geographic Information Technology has been made considering the 1918 Srimangal earthquake as a scenario case. For this purpose, 157 boreholes data were collected to calculate the shear wave velocity. The SPT-N value of most of the borehole data is as deep as 15 m. Then it was converted to the required amount at a depth of 30 m. In addition, Shear wave velocity was also converted to the Peak Ground Acceleration using McGuire equation. The highest Amplification Factor was used among the three equations for converting the Peak Ground Acceleration into the Surface Acceleration. Finally, the Surface Acceleration has been converted into the Modified Mercalli Intensity scale to make it simple to comprehend. The findings show that the minimum and maximum Peak Ground Acceleration are both 0.164 g and 0.175 g, and the Amplification Factors vary between minimum and maximum from 2.027 to 3.40. Also, surface acceleration ranges from 0.347 to 0.578 g. The intensity is estimated at 27 wards in the city of Sylhet. This demonstrates that 81.48% of the intensity IX areas are impacted and 18.52% of the intensity VIII areas as a scenario case for the Srimangals 1918’s earthquake. Since the town of Sylhet does not have an intensity map, this research aims to prepare a map for all Sylhet wards.

Interpretation of gravity anomaly to delineate thrust faults locations at the northeastern part of India and its adjacent areas using source edge detection technique, tilt derivative and Cos(θ) analysis

The Northeast India and its adjacent areas converge among the three different plates, viz. Eurasia, India and Sunda plates. The tectonic interaction of Northeast India and underlying dynamics of the Himalayas as well as the Indo-Burma Ranges might cause the Assam Syntaxis. The area of study is located between latitude 23°–28°N and longitude 88°–96°E and situated in one of the most seismically active tectonic provinces in the world with seismic zone-V. This area had demonstrated several thrust faults activities and tectonic evident accomplishments during the recent past. The complicated geotectonic setups inspirits various smaller magnitude earthquakes, and the current seismicity shows seismic activities are still enduring in the Shillong Plateau, Arakan-Yoma fold belt, Bengal Basin, Naga Hills, Mikir Hills, Upper–Lower Brahmaputra Valley and Mismi Hills of Himalayan foothills. It is imperative to obtain wide-ranging learning tectonic configuration, thrust faults delineation for improved geoscientific study. Parts of the areas are extremely unreachable, and very limited thrust faults were marked by studying GIS map received from the various agencies and field geological study. During the past studies, most of the prominent lineaments/thrusts are marked; however, many active and hidden thrust faults are still unidentified. Seismic data can provide better information about the thrust faults locations, but due to small number of seismic data, the information is not adequate. In this paper, attempt has been made to study and reinterpret the available ground gravity data of northeastern parts of India for understanding thrust fault locations using various applications of gravity derivatives like analytical signal, horizontal gravity gradient, tilt derivative, horizontal tilt angle derivative and Cos(θ) analysis. Source edge detection technique has also been premeditated to categorize thrust fault locations. It is understandable that the low gravity is observed at Assam Valley which contributed sediment accumulations and higher gravity anomaly observed at Shillong Plateau and Bengal Basin containing denser formations. Bouguer gravity data is used after isostatic correction assuming Airy’s isostasy root depth model and first-order trend removal using least square technique. The derived thrust fault locations from the present study are superimposed with the existing thrust-fault locations for correlation. Some additional thrust faults are narrated which are not previously mapped. It is also suggested that Brahmaputra Thrust, Dauki Fault, Naga Thrust, Disang Thrust and Kopili Fault have key responsibility for high seismicity and tectonic movement causing upliftment and depression that encouraged some anticlockwise rotation in the area.

Active tectonic deformation of the Shillong plateau, India: Inferences from river profiles and stream-gradients

The Shillong plateau in northeast India constitutes an actively deforming anticlinal basement uplift in the foreland of the Himalayas in the north and the Indo-Burman ranges in the east. The study aims to document zones of active crustal deformation in the Shillong plateau through the interpretation of the geometry and the quantitative analysis of stream profiles derived from 30-m SRTM DEM.Longitudinal profiles of the streams draining the plateau exhibit conspicuous convex upward reaches, mainly to the upstream of faults and in areas of deep river incision. Analysis of river profiles using Hack’s stream-gradient index (SL-index) reveals perturbations in profiles and steep channel-gradients in the middle reaches of the streams.Spatial distribution of the SL-index brings out extensive zones of anomalously high stream-gradients along the monoclinal flexure at the southern margin of the plateau adjacent to the Dauki fault and along the northern margin, where multiple en echelon faults define step-like topography. In the western part of the plateau, spatial association of zones of high stream-gradients with well-defined fault-scarps is suggestive of active crustal deformation by differential vertical movement along faults.The observed spatial distribution of anomalously high stream-gradients is uncorrelated with the lithological variations and hence, perturbations in river profiles and abrupt changes in stream-gradients are suggestive of differential tectonic uplift of the plateau. The study suggests that the rate of active tectonic deformation in the plateau decreases from the east to the west.

A study on b-value and investigation of seismic hazard in Sylhet seismic region, Bangladesh using Gumbel’s extreme value distribution method

In Gutenberg–Richter (GR) earthquake frequency magnitude relationship the b-value plays an important role and is required for the seismic hazard analysis of any region. In this study, both the Gumbel’s extreme distribution and the GR methods are used to estimate the b-value for Sylhet region of Bangladesh. Due to the geological location this region is the most seismically vulnerable area of the country. Sylhet region is situated on the south eastern part of the Dauki fault. The frequency of seismic activity at the Dauki fault zone has lessened after the great Indian earthquake that occurred in 1897. Therefore, energy may be stored in this fault region, which leaves possibilities that Sylhet may encounter an earthquake. The Sylhet region has experienced nearly 184 earthquakes of magnitude ranging from 4 to 7.2 during the last 101 years. In this study, the probability of recurrence of earthquakes of various magnitude has also been investigated using USGS catalogue which has been prepared by taking moment magnitude (Mw) as a homogenous magnitude. The least square regression method has been applied to compute the b-value by the GR relationship, which resulted in the b-value to be 0.75. Nevertheless, the regression analysis through Gumbel’s extreme value displays a satisfactory result. Further the regression analysis through Gumbel’s method estimated the b-value to be 0.91 and using this the earthquake recurrence probability and the return period for different magnitudes have been calculated. From the probabilistic analysis it has been found that the probability of recurrence of an earthquake (R) of Mw ≥ 6.5 in 100 years is 85% and its return period (Tr) is 53 years in this region. The earthquake recurrence probability of significant damaging earthquakes affecting Sylhet and their return periods have also been investigated. It has been observed that the probability of recurrence of an earthquake equal or greater than 1885 Bengal earthquake (Mw ≥ 7.0) in 100 years is 48.5% in this region and its return period (Tr) is 151 years.

Lithological control on the geomorphic evolution of the Shillong Plateau in Northeast India

The Shillong Plateau in Northeast India is a block of raised topography in the Himalayan foreland which consists of crystalline basement rocks partially covered by a Cretaceous to Miocene sedimentary succession. It is dominated by a mature, low relief landscape surrounded by high relief, fluvially dissected margins, particularly along its southern flank which is bounded by the Dauki thrust Fault. We use river profiles and geological relationships to show that the low relief plateau is a topographic expression of a re-exposed basement palaeosurface following the stripping of sedimentary cover by scarp retreat. We show that initiation of the wave of incision does not require surface rupture on the Dauki Fault or an increase in fault slip rate at the end of the Miocene, as suggested by previous studies. We propose that incision has been spatially controlled by the slope of the basement palaeosurface, likely moderated by an incision threshold. River profiles in the Shillong Plateau cannot be interpreted as simple records of surface uplift. The observed heterogeneous spatial pattern of steepness is a function of a dynamic landscape response to the erosion of layered lithology with contrasting erodibility. Such dynamics have implications for fluvial geomorphology, highlighting that near-horizontal lithological contacts can strongly influence river profiles and topography, even when no longer physically preserved. The topography of the northern Shillong Plateau is controlled by the structure of basement rocks and is reminiscent of stable cratonic interior landscapes, consistent with its surface exposure during late Cretaceous times.

Seismic damage assessment using RADIUS and GIS: A case study of Sylhet City, Bangladesh

Seismic risk of urban areas around the world is a major concern to stakeholders due to the potential losses of lives and extensive damage to infrastructure that can occur following an earthquake. As risk mitigation resources are limited, a method for assessing risk and prioritizing structures for seismic retrofit at a city or regional scale is required. Scenario-based seismic damage prediction can be used by decision-makers for identifying the riskiest structures that can be prioritized for risk mitigation to optimize the use of available resources. In this case study, for the first time, seismic damage estimation is performed for Sylhet City Corporation (SCC), Bangladesh by integrating Risk Assessment tools for Diagnosis of Urban areas against Seismic disasters (RADIUS) and Geophysical Information System (GIS). Building structures, infrastructure, and soil type information for SCC from a recent study performed by the local government under the framework of the Comprehensive Disaster Management Program (CDMP) are used. The building inventory and other information in the study area are divided into 1.0 × 1.0 square km grids and the information is mapped into RADIUS. Two scenario earthquakes are chosen to perform damage estimation of existing buildings and infrastructures. The study reveals that the two scenario earthquakes (Mw 8.0 from Dauki Fault and Mw 7.6 from Plate Boundary Fault 2) can result in extensive damage and a high number of casualties in the SCC area. The integrated GIS and RADIUS approach provides useful information for decision making in the seismic risk mitigation plan of the city.