The research, led by Arpita Biswas of the Indian Institute of Technology Kharagpur together with Sankar Kumar Nath of the University of Burdwan, drew on an extraordinary geotechnical and geophysical database of approximately 6,000 investigation sites across the basin. These included Standard Penetration Test boreholes, downhole seismic surveys, multichannel analysis of surface waves, ambient noise measurements and resistivity surveys, supplemented by laboratory tests on soil samples for grain size, moisture content, plasticity and density. The sheer scale of the dataset allowed the team to move beyond the city-level case studies — focused previously on Kolkata, Dhaka and Chittagong — that had dominated liquefaction research in the region.

The geological setting could hardly be more favourable for liquefaction. The basin contains the world’s deepest Neogene-to-recent sedimentary accumulation, exceeding 18 kilometres in thickness, and about 77 percent of the study area is blanketed in Quaternary alluvium — loose silt, sand, clay and gravel deposited by the Ganges, Brahmaputra and Meghna rivers. Statistical analysis of the borehole data revealed that the upper 15 metres of basin sediment is very loose to medium dense, with corrected penetration values averaging between 6 and 35 depending on depth, and a mean fines content of just 36.8 percent, indicating predominantly coarse-grained soils. Grain-size curves for sites in Dhaka, Rajshahi, Kalyani and Bhubaneswar showed that most soil layers fall within the internationally recognised boundaries of ‘most liquefiable’ and ‘potentially liquefiable’ soils.

Groundwater compounds the danger. Post-monsoon water tables across the basin range from just 0.27 to 4.71 metres below the surface, and shallow groundwater dramatically lowers the effective confining stress on sediments, creating ideal conditions for pore-pressure buildup during shaking. Historical precedent is sobering: the 1885 Bengal earthquake of magnitude 6.8, the 1897 Shillong earthquake of magnitude 8.1, the 1918 Srimangal earthquake of magnitude 7.6 and the 1934 Bihar-Nepal earthquake of magnitude 8.1 all triggered documented liquefaction, leaving ground fissures, sand boils, lateral spreading and soil slumping in their wake. Even moderate events such as the 2003 Kolabunia earthquake of magnitude 5.6 produced ground rupture along riverbanks in Rangamati.

Methodologically, the study is a tour de force of integrated hazard modelling. Because instrumental recordings are absent for the great historical earthquakes, the team used the EXSIM finite-fault stochastic simulation technique, which discretises a fault plane into numerous subsources and sums their contributions with realistic rupture propagation and travel-time delays. The resulting bedrock motions were then fed into two-dimensional nonlinear finite element site response analyses in PLAXIS 2D, capturing soil behaviour that simpler one-dimensional equivalent-linear methods miss. The simulated ground motions were validated against Modified Mercalli intensities derived from historical isoseismal maps, yielding a root-mean-square error of 0.91 — a respectable agreement for regional-scale work.

The probabilistic component is equally ambitious. Combining a comprehensive probabilistic seismic hazard model with fourteen ground-motion prediction equations, the researchers computed bedrock peak ground acceleration for a 475-year return period — the conventional design-basis earthquake — ranging from 0.08 to 0.58 g. Multiplying these values by spatially interpolated site amplification factors, which ranged from 1.30 to as much as 4.60 in the soft coastal deposits of the Sundarbans, produced surface-consistent hazard values reaching a remarkable 1.40 g near Sylhet in the northeast, close to the Dauki fault and the Indo-Burman subduction zone. Cities such as Mymensingh and Sylhet fell into the ‘severe’ category, while Dhaka, Kolkata and Khulna were rated ‘moderate’ and Bhubaneswar and Dhanbad ‘low’.

With these ground motions in hand, the team evaluated liquefaction using the stress-based framework of Idriss and Boulanger, computing factors of safety for every soil layer, then aggregating them into the Liquefaction Potential Index, the Liquefaction Risk Index, the probability of liquefaction and the probability of ground failure. The results are stark: under the probabilistic scenario, roughly 42 percent of the basin’s area falls into the ‘severe’ liquefaction potential class, with a further 35 percent rated ‘high’. Dhaka, Sylhet, Mymensingh, Chittagong, Barisal, Khulna and Rangpur all exceed the severe threshold of LPI greater than 15, while Rajshahi and Kolkata sit in the high class. The risk index map places nearly half the region — about 48 percent — in the ‘extremely high’ risk zone. At representative sites including Dhaka, Chittagong, Haldia and Kalna, probabilities of liquefaction exceeded 65 percent between depths of 5 and 20 metres, and cities from Dhaka to Barasat showed probabilities of ground failure above 0.9 — ‘extremely high to absolutely certain’.

Crucially, the modelled hazard aligns with reality. The predicted severity zones show an increasing trend in the number of documented liquefaction occurrences, providing qualitative validation of the approach. The spatial pattern also makes geological sense: the northeast around Sylhet and Mymensingh combines high seismic loading from nearby active faults with young floodplain deposits and shallow groundwater, while the southwest, underlain by older, stiffer sediments near the Indian Shield, fares considerably better. Notably, the study shows that strong shaking alone does not guarantee severe liquefaction — the Chittagong Fold Belt’s uplifted, folded sediments partially offset high seismic demand, whereas the unconsolidated Holocene deltaic plains of Khulna and Barisal remain highly susceptible despite more moderate loading.

Perhaps the most practically valuable contribution is the back-analysis of target penetration resistance. By reversing the standard liquefaction triggering equations, the researchers calculated the improved SPT-N values — denoted N-improved — needed at each site to guarantee no liquefaction under the probabilistic scenario, assuming a target factor of safety of 1.5. Mapped at depth intervals from 5 to 20 metres, these values range from as low as 1 in the stable west to 50 at 20-metre depth in the most demanding northeastern locations. Such maps give engineers, contractors and planners a first-order estimate of the densification or reinforcement required at specific sites, bridging the persistent gap between regional hazard assessment and ground-level engineering decisions.

The authors are candid about limitations: the empirical triggering relationships were calibrated on global case histories rather than Bengal Basin-specific data, uncertainties were not formally propagated through the entire framework, and the maps represent regional trends rather than site-specific design criteria. Ground improvement techniques — vibro-compaction, stone columns, gravel drains, grouting, dewatering and reinforcement — each have their place depending on geology, land use and economics, and the study suggests densification suits the young deltaic south while targeted foundation measures may be more realistic in congested megacities like Dhaka. Still, as a unified, basin-scale portrait of liquefaction threat across one of the world’s most populous regions, the work offers an essential foundation for risk-informed urban planning — and a sobering reminder that beneath the streets of Dhaka and the fields of Sylhet, the ground itself may be the region’s most insidious seismic hazard.

Subject of Research: Assessment of earthquake-induced soil liquefaction potential and mitigation targets across the Bengal Basin using probabilistic seismic hazard analysis and historical earthquake scenarios

Article Title: Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies

Article References: Biswas, A., & Nath, S. K. (2026). Probabilistic seismic hazard and historical scenario-based assessment of liquefaction potential in the Bengal Basin for liquefaction mitigation strategies. Discover Geoscience, 4(1), Article 293. https://doi.org/10.1007/s44288-026-00672-6

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00672-6

Keywords: Bengal Basin, soil liquefaction, seismic hazard, liquefaction potential index, probabilistic seismic hazard analysis, ground motion simulation, site amplification, Standard Penetration Test, ground improvement, earthquake engineering, Ganges-Brahmaputra delta, geotechnical investigation

The setting alone makes the work compelling. Bangladesh sits atop the Bengal Basin, one of the world’s great sedimentary repositories, where strata eroded from the rising Himalaya and carried by the Ganges, Brahmaputra and Meghna rivers have buried the ancient crust under hundreds to thousands of metres of Late Permian to Quaternary sediment. In the northwest, however, the Precambrian basement rises unusually close to the surface. The shallowest known occurrence lies at just 128 metres depth at Maddhapara, where diorite-tonalite-granodiorite rocks have been dated at 1.64 to 1.73 billion years using U-Pb SHRIMP analysis of zircon. That age places the Dinajpur Block firmly within the Paleoproterozoic, an era when the planet’s landmasses were assembling into the supercontinent Columbia, also known as Nuna.

Until now, drill hole GDH-62 had never been examined petrographically. The team collected nine core samples from the Geological Survey of Bangladesh repository and prepared twenty-seven thin sections, two or three per sample to capture mineralogical variation along and across the core axis. Under a standard polarizing microscope, they identified minerals and counted them along twenty-five to thirty-five grid-based traverses per section, estimating modal compositions visually with an uncertainty of about plus or minus five percent for major minerals. The approach is deliberately classical, but it is exactly the kind of careful, quantitative petrography that turns a stack of grey core boxes into a readable geological history.

The results split cleanly into two rock families. The felsic rocks, which occupy about 20.2 metres of the core or roughly 73 percent of the studied interval, are granodiorite: massive, grey to greenish grey, leucocratic to mesocratic, and fine to medium grained. Their modal composition ranges from 33.4 to 49.2 percent plagioclase, 11.8 to 27.0 percent quartz, 6.6 to 27.2 percent biotite and 6.7 to 11.7 percent K-feldspar, with secondary chlorite and epidote and accessory muscovite, zircon, hematite and opaque minerals. Plotted on the standard Quartz-Alkali Feldspar-Plagioclase discrimination diagram, every felsic sample falls within or adjacent to the granodiorite field, some near the granodiorite-tonalite boundary. The textures are hypidiomorphic and inequigranular, with fine- to medium-grained interlocking fabrics that speak of slow crystallization in a deep plutonic environment.

The microscopic detail is where the rocks begin to tell their deformation story. Quartz grains show wavy, undulose extinction, a classic signature of strain accumulated in the crystal lattice, and locally the quartz has recrystallized into finer equant aggregates. Biotite occurs as blade-like, euhedral to subhedral crystals with light brown to yellowish brown pleochroism, and many grains are twisted, bent and partially foliated. Aligned biotite and chlorite along recrystallized quartz grain boundaries define a weak foliation whose intensity, notably, decreases with depth in the section, hinting at a gradient in deformation intensity that the authors caution cannot be quantified from petrology alone. K-feldspar displays textbook Carlsbad twinning and hosts inclusions of acicular muscovite, biotite and rare epidote, the latter possibly pointing to hydrothermal alteration. Plagioclase shows weak to moderate sericitization, and much of the biotite is altered to chlorite, evidence that fluids, possibly of hydrothermal origin, percolated through these rocks long after they crystallized.

Cutting through the granodiorite near the base of the hole is the study’s most eye-catching discovery: an appinite dyke, massive, greenish grey to very dark green, and dominated by hornblende at 67 to 73 percent of its mode, accompanied by biotite, plagioclase, chlorite, hematite and accessory opaque minerals, with calcite forming micro-veins and veinlets. Appinites are an unusual and petrogenetically significant class of rock, typically hornblende-rich mafic intrusions associated with hydrous, mantle-derived magmas in convergent settings. In thin section the appinite shows hypidiomorphic, inequigranular and locally poikilitic textures, with hornblende enclosing plagioclase crystals. Hornblende exhibits the characteristic amphibole cleavage angles of 56 and 124 degrees with extinction angles between 12 and 34 degrees, while reddish brown biotite suggests a titanium-rich composition that the authors flag as potentially significant for petrogenesis. A tentative identification of orthopyroxene, based on pleochroism and parallel extinction, remains unconfirmed without optical or chemical constraint on its 2V angle.

The contact between the dyke and its host is sharp and non-gradational, with no chilled margin, and the authors interpret it as evidence that the appinite was emplaced after the felsic host had consolidated, most likely as a hypabyssal dyke within the main acid to intermediate association. From granodiorite to appinite, quartz and plagioclase decrease while biotite increases, tracking the shift from felsic to mafic chemistry. What petrography cannot constrain, the researchers emphasize, is the emplacement depth, a limitation they state plainly rather than paper over. That honesty matters in a field where over-interpretation of thin-section evidence is a chronic hazard.

Perhaps the most technically rich part of the study is its analysis of vein microstructures, which function as a kind of strain gauge for events long after the magmas cooled. In the granodiorite, lensoidal quartz veins with smooth vein-wall interfaces and lensoid wall-rock fragments point to a fracture-controlled vein system formed by syntaxial growth, in which crystals grow from both walls toward the vein centre. The small angle between the growth phase and the crack wall suggests a low growth rate, and the absence of fluid inclusions indicates that the vein crystals were deformed under non-hydrostatic stress during or after formation. In the appinite, by contrast, monomineralic calcite veins show slightly curved growth and selvages at their margins, hallmarks of antitaxial growth in which crystals expand outward from the vein centre, recording progressive, stress-related deformation over time. Minor calcite veins oriented obliquely and sub-parallel to the major vein hint at changing stress directions and possible fracture reactivation across the life of the intrusive complex.

The comparison the authors draw with the West African Craton gives the work its global resonance. Paleoproterozoic granitoids of the Bas Draa inlier in Morocco’s Anti-Atlas, dated around 2.1 billion years, share calc-alkaline, subduction-related characteristics with the Dinajpur Block, and both regions feature sodic TTG-style suites derived from partial melting of hydrated mafic sources. Mafic and appinitic dykes in GDH-62 resemble the lamprophyric dyke swarms of the West African Craton, though the Bangladeshi examples point more strongly to metasomatic and mantle-wedge influences than to plume-related processes. The similarities support the interpretation of GDH-62 as a fragment of Paleoproterozoic arc crust, a piece of a convergent-to-collisional margin that helped stitch together Columbia. For a country whose geology is usually described in terms of deltas, floods and gas fields, the message is striking: beneath the youngest sediments lies a two-billion-year-old arc, complete with dykes, veins and deformation histories, waiting in core boxes for anyone patient enough to grind the thin sections.

Subject of Research: Petrographic characterization of Precambrian crystalline basement rocks in drill hole GDH-62, Dighipara, northwest Bangladesh

Article Title: Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh

Article References: Zahan, H., Ameen, S. M. M., Hossain, M. S., & Hossain, M. S. (2026). Petrographic characterization of the crystalline basement in Dighipara, Northwest Bangladesh. Discover Geoscience, 4(1), Article 332. https://doi.org/10.1007/s44288-026-00703-2

Image Credits: AI Generated

DOI: 10.1007/s44288-026-00703-2

Keywords: petrography, crystalline basement, granodiorite, appinite, Bangladesh, Dinajpur Block, Bengal Basin, Precambrian, vein microstructures, Columbia supercontinent, drill core, hornblende