Soil Stratum Tides

Abstract

AbstractRiver deltas, where ocean tides can often freely propagate into the river, are focal points of human settlement. Beneath the delta surface, groundwater stored in the soil strata fluctuates with river tides, resulting in pressure variations within the soil strata. Here, we introduce a novel ultra‐weak fiber‐optic instrument to test the hypothesis that tide‐induced groundwater level variations induce periodic soil stratum deformation. Using borehole deployments ranging over 70 m in the subsurface of the Yangtze Delta, we observe semidiurnal and spring–neap cycles of soil stratum deformation and reveal its dependence on lithology and depth. This allows us to coin the new term “soil stratum tide,” defined as the periodic deformation of soil strata in response to pressure fluctuations induced by tides in open water. High‐accuracy monitoring of the breathing of tidal deltas enables calibrating Earth observation systems, analysis of delta subsidence, and safeguarding of infrastructure jeopardized by soil stratum deformation.