Land subsidence is a poorly constrained but potentially dominant contributor to relative sea-level rise in the Ganges-Brahmaputra (GBM) delta, where more than 100 million people live within a few meters of sea level. Karpytchev et al. (2018) estimated, using laterally uniform viscoelastic Earth models, that Holocene sediment and ocean loading alone generate up to about 1.6 mm/yr of the delta's present-day subsidence, while observed local rates reach up to about 18 mm/yr and also include sediment compaction, groundwater extraction, and tectonic deformation. The open problem is to close the delta's subsidence budget: quantitatively partition the observed present-day subsidence into loading-driven, compaction, anthropogenic, and tectonic components with quantified uncertainties, using an Earth model whose laterally varying lithosphere-mantle structure beneath the Bengal Basin is constrained by geophysical and geodetic observations rather than assumed, improved estimates of Bengal fan sediment volume and Bay of Bengal Holocene sea-level history, and validation against Holocene relative sea-level and stratigraphic records. Later work produced a first three-dimensional loading model (2-3 mm/yr, amplified toward the densely populated eastern delta) and a semi-quantitative parsing of subsidence by depth and timescale, but no closed, uncertainty-quantified budget with an independently constrained rheology exists.

Background

The Ganges-Brahmaputra-Meghna (GBM) delta, also called the Ganges-Brahmaputra delta (GBD), is the world's most populous delta: roughly 100-150 million people live on its low-lying plain where the Ganges, Brahmaputra, and Meghna rivers meet the Bay of Bengal. Its habitability depends on the balance between sea-level rise, land subsidence (sinking of the ground surface), and sediment deposition. Subsidence matters because it converts climate-driven sea-level rise into faster relative sea-level rise experienced at the coast, worsening flooding, storm-surge exposure, and salinity intrusion. Several processes contribute, operating at different depths, spatial scales, and timescales. (1) Sediment and ocean loading: the delta's enormous sediment mass - especially the early-Holocene pulse that began about 11,000 years ago, when a strengthened South Asian monsoon roughly doubled sediment discharge from the Himalaya (Goodbred & Kuehl, 2000) - together with the weight of ocean water added by deglacial sea-level rise, depresses the crust and mantle. Because the deep Earth behaves as a Maxwell viscoelastic solid (elastic at short timescales but flowing over thousands of years - the same physics as glacial isostatic adjustment, the ongoing rebound of the crust after ice-sheet melting), this sediment isostasy continues to drive subsidence today, millennia after deposition. (2) Compaction: young, water-rich sediments lose porosity under their own weight, fastest in shallow, muddy, organic-rich deposits. (3) Anthropogenic drivers, notably groundwater extraction (the water table beneath Dhaka has fallen more than 70 m). (4) Tectonics: the delta overlies a locked, oblique subduction-collision boundary whose elastic interseismic deformation produces several mm/yr of subsidence in the east. Karpytchev, Ballu, Krien, Becker, Goodbred, Spada, Calmant, Shum, and Khan (2018) quantified component (1) for the GBD for the first time, using a viscoelastic spherical Earth model (the TABOO code used in glacial-isostatic-adjustment modeling) driven by the observation-based Holocene sediment budget of Goodbred & Kuehl (2000) and the global eustatic (ice-volume-equivalent) sea-level curve of Lambeck et al. (2014). Lacking seismic and gravity constraints on the deep structure, they bracketed the answer with laterally uniform (one-dimensional, depth-dependent) viscosity profiles and obtained up to about 1.6 mm/yr of present-day loading-driven subsidence along the delta coast - a minimum estimate, comparable to twentieth-century global mean sea-level rise. Meanwhile, Interferometric Synthetic Aperture Radar (InSAR, satellite radar measurements of millimeter-scale ground motion) and GNSS (Global Navigation Satellite System geodesy) show local present-day rates of 0-18 mm/yr, largely attributed to compaction and groundwater pumping (Higgins et al., 2014), and stratigraphy-based reconstructions average about 0.2 mm/yr near the delta's inner Hinge Zone (the Eocene shelf edge marking the northern limit of the subsiding delta plain) increasing to about 4-5 mm/yr near the coast over the Holocene (Grall et al., 2018). Subsequent regional analyses bound present-day rates at a few mm/yr regionally (Becker et al., 2020) but explicitly cannot identify which process causes them. The unresolved question is how all of these pieces add up: which fraction of the observed subsidence is the irreducible, centuries-persisting viscoelastic response to past loading, which fraction is manageable compaction and anthropogenic subsidence, and what Earth structure beneath the Bengal Basin actually is.

Problem Statement

Determine the present-day subsidence budget of the Ganges-Brahmaputra delta: quantitatively partition the observed present-day land subsidence, as a function of position across the delta and with quantified uncertainties, into (i) the viscoelastic response to Holocene sediment and ocean loading, computed with a laterally varying lithosphere-mantle structure beneath the Bengal Basin that is constrained by geophysical and geodetic observations (subsidence, seismic, and gravity data) rather than assumed from a range of plausible profiles; (ii) sediment compaction, resolved as a function of depth and sediment age; (iii) anthropogenically driven subsidence such as groundwater extraction; and (iv) tectonic deformation - and validate the loading component against Holocene relative sea-level and stratigraphic records, using improved Bengal fan sediment volumes and a regional Bay of Bengal Holocene sea-level history in place of the global eustatic curve, so that the irreducible loading-driven contribution to relative sea-level rise can be distinguished from the manageable compaction and anthropogenic contributions.

References

  1. Karpytchev, M., Ballu, V., Krien, Y., Becker, M., Goodbred, S., Spada, G., Calmant, S., Shum, C. K., & Khan, Z. (2018). Contributions of a strengthened early Holocene monsoon and sediment loading to present-day subsidence of the Ganges-Brahmaputra Delta. Geophysical Research Letters, 45(3), 1433-1442. https://doi.org/10.1002/2017GL076388
  2. Krien, Y., Karpytchev, M., Ballu, V., Becker, M., Grall, C., Goodbred, S., Calmant, S., Shum, C. K., & Khan, Z. (2019). Present-day subsidence in the Ganges-Brahmaputra-Meghna Delta: Eastern amplification of the Holocene sediment loading contribution. Geophysical Research Letters, 46(19), 10764-10772. https://doi.org/10.1029/2019GL083601
  3. Grall, C., Steckler, M. S., Pickering, J. L., Goodbred, S., Sincavage, R., Paola, C., Akhter, S. H., & Spiess, V. (2018). A base-level stratigraphic approach to determining Holocene subsidence of the Ganges-Meghna-Brahmaputra Delta plain. Earth and Planetary Science Letters, 499, 23-36. https://doi.org/10.1016/j.epsl.2018.07.008
  4. Becker, M., Papa, F., Karpytchev, M., Delebecque, C., Krien, Y., Khan, J. U., Ballu, V., Durand, F., Le Cozannet, G., Islam, A. K. M. S., Calmant, S., & Shum, C. K. (2020). Water level changes, subsidence, and sea level rise in the Ganges-Brahmaputra-Meghna delta. Proceedings of the National Academy of Sciences, 117(4), 1867-1876. https://doi.org/10.1073/pnas.1912921117
  5. Steckler, M. S., Oryan, B., Wilson, C. A., Grall, C., Nooner, S. L., Mondal, D. R., Akhter, S. H., DeWolf, S., & Goodbred, S. L. (2022). Synthesis of the distribution of subsidence of the lower Ganges-Brahmaputra Delta, Bangladesh. Earth-Science Reviews, 224, 103887. https://doi.org/10.1016/j.earscirev.2021.103887
  6. Steckler, M. S., Jaman, M. H., Grall, C. J., Goodbred, S. L., Wilson, C. A., & Oryan, B. (2024). Contribution of campaign GNSS toward parsing subsidence rates by time and depth in coastal Bangladesh. Frontiers in Earth Science, 12, 1354686. https://doi.org/10.3389/feart.2024.1354686
  7. Higgins, S. A., Overeem, I., Steckler, M. S., Syvitski, J. P. M., Seeber, L., & Akhter, S. H. (2014). InSAR measurements of compaction and subsidence in the Ganges-Brahmaputra Delta, Bangladesh. Journal of Geophysical Research: Earth Surface, 119, 1768-1781. https://doi.org/10.1002/2014JF003117
  8. Milne, G. A., & Mitrovica, J. X. (2008). Searching for eustasy in deglacial sea-level histories. Quaternary Science Reviews, 27, 2292-2302. https://doi.org/10.1016/j.quascirev.2008.08.018
  9. Goodbred, S. L., & Kuehl, S. A. (2000). Enormous Ganges-Brahmaputra sediment discharge during strengthened early Holocene monsoon. Geology, 28(12), 1083-1086. https://doi.org/10.1130/0091-7613(2000)28<1083:egsdds>2.0.co;2
  10. Lambeck, K., Rouby, H., Purcell, A., Sun, Y., & Sambridge, M. (2014). Sea level and global ice volumes from the Last Glacial Maximum to the Holocene. Proceedings of the National Academy of Sciences, 111(43), 15296-15303. https://doi.org/10.1073/pnas.1411762111
  11. Henry, A., Grall, C. J., Becker, M., Karpytchev, M., & Chaumillon, E. (2026). Present estimations of subsidence induced by sediment isostasy and glacial isostatic adjustment in the Ayeyarwady Delta (Myanmar). Journal of Geophysical Research: Solid Earth, 131. https://doi.org/10.1029/2025JB032264
  12. Ghosh, T., Hazra, S., Malick, K., & Talukdar, T. (2025). Monitoring ground deformation in recent years in Western Ganga delta India, using persistent scatterer interferometry on multi-temporal ALOS-2 L band imaging radar data. Natural Hazards, 121, 21551-21581. https://doi.org/10.1007/s11069-025-07654-w
  13. Chadwick, A., Steckler, M., Wilson, C., Goodbred, S., Rana, M., Akter, S., Bhuiyan, M. A. H., Camargo, S., Larochelle, S., Hossain, M. J., Mahmud, S. S., Tanvir, A., Ahmed, Z., & Mim, A. (2025). Future projections of compaction-induced subsidence on the Ganges-Brahmaputra Delta. Geological Society of America Abstracts with Programs. https://doi.org/10.1130/abs/2025am-5530