Subsidence and Uplift Including Shoreline Change

Primary reference(s)

BGS, 2020. Subsidence and shrinking and swelling soils. British Geological Survey (BGS). Accessed 27 September 2020.

Additional scientific description

Subsidence and uplift are caused by many factors, including the impacts of mining or tunnelling, consolidation, sinkholes, or of groundwater and moisture changes on expansive soils (BGS, 2020). Such near-surface, relatively shallow crustal, or humangenerated processes, are often localised. Several crustal scale processes also drive subsidence and uplift that tend to be more regional in scale. Crustal movements occur in response to several different mechanisms, including tectonic, glacio-isostatic (Milne et al., 2006), erosional isostatic (denudation; Watts, 2001) and hydro-isostatic (Watts, 2001) processes. These operate over different timescales and different wavelengths. Crustal movements, climate change-driven sea-level rise and erosionderived sedimentation can result in shoreline change. Tectonic uplift and subsidence are the distributed vertical permanent ground deformations (warping) that result from displacement on a dipping (inclined) fault (Styron, 2019). Earthquake surface ruptures and fissures are localised ground displacements that develop during and immediately after an earthquake, where the fault which hosted the earthquake intersects the Earth’s surface. Surface ruptures represent the upward continuation of fault slip at depth, while fissures are smaller displacements, or more distributed deformation in and around the rupture area (PNSN, no date). Volcanic uplift and subsidence are deformations of the ground associated with volcanic unrest and eruptions (Dzurisin, 2007). Hydro-isostatic and erosional-isostatic deformation occur in response to the stress changes induced by changing ground water levels and load (erosion). Hydro-isostatic movements are largely anthropogenic or climatic and therefore commonly seasonal.

Ground-level rise is commonly associated with plate subduction zones, such as the Himalayas where the Eurasian and Indian plates converge (USGS, 2015). Uplift can also be driven by swelling, or mantle plumes, such as the Iceland Plume form in higher temperature regions of the Earth’s mantle. Subsidence may be associated with plates moving apart, for example in rift valleys such as the Ethiopian rift valley. The relative motion of the crust on either side of faulting associated with earthquakes results in persistent or permanent deformation of the Earth’s surface. Surface ruptures, fissures, and uplift and subsidence are all manifestations of this longer-term deformation, and although less dramatic, may all pose hazards during and after earthquakes. Lithospheric flexure also responds to extensional and compressional tectonic forces, including movement associated with the formation of rift valleys (commonly associated with plate boundaries) and mountain belts as well as strike slip faults and fault zones (Watts, 2001).

In the coastal environment, as well as the potential tectonic impacts, sediment and global sea-level rise impact on shore-line change. Sediment loading can exacerbate regional subsidence, thereby increasing the relative sea-level rise. In coastal areas where accelerated glacial wasting has been reported, glacio-isostatic rebound results in a relative rise in ground level, as exemplified in the wasting of the Laurentide Ice Sheet (Simon et al., 2016).

Local-to-regional scale subsidence and uplift resulting from changes in groundwater or porewater pressures occur in areas that are underlain by compressible and elastically deforming soils responding to groundwater withdrawal. Cohesive soils commonly exhibit seasonal changes in moisture content that can be associated with local subsidence (e.g., Simic et al., 2015).

Anthropogenic impacts on ground level, primarily result from dewatering for potable supply or for subsurface mining or engineering (Cigna et al., 2017).

Metrics and numeric limits

Rates of uplift in the Himalayas are reported to be in the order of 1 cm/yr (USGS, 2015). Global sea-level rise is in the order of 1.8 mm/yr, but relative sea-level rise varies considerably in accordance with other processes such as sedimentation or subsidence (USGS, no date a). High rates of uplift in Iceland (25–29 mm/yr) have been related to glacial isostatic adjustment with a feedback on plume evolution as a consequence of reduced pressure increasing magma production rates (Schmidt et al., 2013).

The size and spatial extent of surface rupture, fissures and uplift/subsidence associated with earthquakes depends on the type, magnitude and depth of the earthquake as well as the distance from the earthquake (Biasi et al., 2006; USGS, no date b).

Tectonic uplift and subsidence are generally as large or larger than the displacement of the surface rupture; moderate to large earthquakes in the crust that do not rupture to the surface will still broadly warp the region. The magnitude of the displacement will decrease with increasing distance from the earthquake, but in the case of ruptures on inclined faults such as subduction zones (rather than vertical strike-slip faults) uplift or subsidence of at least 1 m may extend for more than 200 km from the fault trace for the largest earthquakes (Styron, 2019). Both effects will extend along the length of the earthquake fault, a distance of a few kilometres for Magnitude 6 earthquakes to more than 1000 km for Magnitude 9 earthquakes.

Ground-level response to groundwater dewatering is a global issue (USGS, no date c). Values of up to 53 mm/yr have been determined using InSAR monitoring of Kabul (Meldebekova, 2020).

Examples of drivers, outcomes and risk management

Surface ruptures and fissures can cause damage to buildings, roads, and utility infrastructure (e.g., gas and water lines). In addition to the immediate, local risk posed by collapsing infrastructure, this damage may hamper rescue and rebuilding efforts by impeding transportation and utility delivery. In the worst cases, damage to lifelines may cause local flooding (e.g., water lines), environmental impacts (e.g., oil pipelines) and even highly destructive fires (gas lines) that may be more damaging than the initial earthquake. Potential also exists for disruption due to flooding or re-routing of rivers if the river channel has been sufficiently modified (Holbrook and Schumm, 1999).

Shoreline change poses a threat to coastal settlements, businesses and tourism. There is also potential for impacts on coastal stability and groundwater resources, such as saline intrusion (USGS, no date d). Potential impacts are especially significant for atoll islands with shallow unsaturated zones. These islands are particularly susceptible to impacts on groundwater resources and populations (UNESCO, 2019).

While no technology exists for reducing these or other earthquake hazards, the risk to infrastructure posed by surface rupture and fissures can be mitigated to some degree by not building on known fault traces, seismic retrofitting of existing buildings, and engineering of pipelines with enough flexibility to absorb the displacement by bending and flexing, rather than breaking (e.g., USGS, 2003).

Coastal change can impact harbour water depth and damage infrastructure. In these zones, modelling to enable adaptive planning is the best form of mitigation (Steven et al., 2020).

In areas where stopping anthropogenic groundwater dewatering leads to rising ground levels, as well as potential impacts on infrastructure, consideration should be given to impacts on water quality, such as where mine water rebound results in the mixing of mining and potable water (Boak et al., 2007).