Sinking Cities: understanding fundamental fluid flow in aquifers for the enhanced detection of earthquake faults in subsiding basins

Omissions in our current understanding of the fundamental behaviour of fluid flow in porous media and its interaction with impermeable barriers such as earthquake faults systems, means that aquifer systems are often analysed in complex models to try to characterise the behaviour of these systems. 

This project will develop and solve new mathematical models able to address the mechanics of fluid flow in aquifers to take advantage of an opportunity to use patterns of land subsidence to better detect hidden earthquake generating faults within sinking basins. This work is important because seismic hazard and hidden faults potentially affect many people in growing cities around the world, and the long-term impact of this research will aid the UN development goal of sustainable cities and reduced loss of life from hazards. 

Many seismically active faults remain to be discovered around the world, sometimes because they are hidden in the landscape, covered by sediments in basins. Cities are built on these basins because they offer fertile ground for agriculture and sources of water contained within the ground. However, extracting water from underground often causes the land at the surface to fall. Whilst the sinking land is a problem for housing and security of water supplies, it also provides an important opportunity to find faults beneath the city that may pose a future earthquake risk. This is because the faults modify the flow of the fluids within the subsurface and consequently affect the pattern of land subsidence observed at the surface. Using satellites, it is possible to measure the spatial and temporal pattern of this ground motion to high precision. However, improvements to our understanding of the behaviour of fluids within a porous medium and the interaction with faults is required to fully exploit these observations. 

Figure 1: City of Tehran (population 8.7 million) experiences severe land subsidence due to water extraction for agriculture. However, these patterns of land subsidence offer the chance to find earthquake faults beneath the city that will enable improved assessments of seismic hazard and risk. To improve this detection, our understanding of the fundamental controls on the behaviour of fluids and faults in the subsurface is required.  

Figure 2: Map of average subsidence rate (mm/yr) around Tehran (northern Iran) based upon time series analysis of the Sentinel-1 interferometry for the area (2015-mid 2020). The four main basins around Tehran are each subsiding by over 100 mm/yr. White lines denote the known major active fault traces in the area. Patterns of differential subsidence within the basins offers the chance to detect new hidden faults buried in the sediments that may pose a future hazard to the city. 

Figure 3: Annualised death rates from disasters resulting from natural hazards, grouped by decade from the beginning of the 20th Century (e.g., 1900 is the average annual rate of deaths for the complete years 1900– 1909, coloured by hazard type). A decline is observed in the number of deaths from climatological and hydrological disasters attributed to drought and flood. However, deaths due to earthquake disasters have persisted. Source: International Disaster Database, EM-DAT, CRED, UCLouvain, Brussels, Belgium. http://www.emdat.be.