Vast Delta on Shaky Ground: New Maps Reveal Where Bengal Basin Soil Could Turn to Liquid

Home to roughly 220 million people, the Bengal Basin — spanning Bangladesh and parts of the Indian states of West Bengal, Jharkhand and Odisha — sits atop the largest fluvio-deltaic sedimentary system on Earth. Now, a comprehensive new study published in Discover Geoscience has produced the first basin-wide assessment of just how vulnerable that ground is to liquefaction, the terrifying phenomenon in which water-saturated soil temporarily loses its strength and behaves like a liquid during earthquake shaking. The findings suggest that a striking proportion of this densely populated delta could face severe liquefaction in future seismic events.

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