Causes, Mechanisms and Rates of Land Subsidence in the Red River Delta and Warnings of Excessive Groundwater Exploitation

Quaternary sediment subsidence in the Red River Delta has been quietly occurring for 1600 ka BP. This paper introduces the causes, mechanisms, and rates of tectonic subsidence and subsidence due to sediment compression in three boreholes selected for detailed study: BH4-HN, BH02-HP, and BH56-ND, representing three key areas of the Red River Delta. The activities of the northwest-southeast co-sedimentary fault systems and the northeast-southwest post-sedimentary fault systems have created a weak subsidence zone in Hanoi and two strong subsidence zones in Hai Phong and Nam Dinh. The results of sequence stratigraphy analysis show that these 3 boreholes all have 5 sequences: (1) Q₁¹ : Early Pleistocene; (2) Q₁^2a : Early middle Pleistocene; (3) Q₁^2b : Late middle Pleistocene; (4) Q₁^3a : Early late Pleistocene; (5) Q₁^3b -Q2: Late late Pleistocene-Holocene. These 5 sequences are related to 5 glacial/interglacial cycles. Each sequence has 3 sedimentary systems tracts: lowstand (LST), transgressive (TST), and highstand (HST) systems tracts. The sequences and system tract of the 3 boreholes were calculated for the total of tectonic subsidence rate and sediment compression rate as follows: BH4-HN has Vav of 0.281 mm/year, BH02-HP: 0.63 mm/year, and BH56-ND: 0.799 mm/year. This has shown that the two strong subsidence zones of Hai Phong and Nam Dinh cause the strong estuaryization process at the Bach Dang estuary and the strong erosion of the Hai Hau coast.