Many cities around the world are sinking because too much water is extracted from groundwater reservoirs, leaving coastal cities more exposed to rising seas as the climate warms. New research demonstrates that this process of land subsidence can actually be reversed when depleted groundwater reservoirs are replenished, allowing urban areas to rise. Researchers tracking these changes in Osaka discovered that tectonic fault lines play an unexpected role in shaping where groundwater recovery and land uplift happen fastest.
The Global Crisis of Sinking Coastal Cities
Land subsidence—the gradual sinking of the ground surface—affects many of the world’s major urban centers where groundwater is pumped faster than it can naturally refill. In Indonesia, parts of the capital city Jakarta are sinking more than ten centimetres each year, prompting government plans to relocate the city entirely. Meanwhile, Tianjin, China, home to 15 million people, faces a future where 15% of its population could be underwater by 2120 if current sinking rates continue unabated. Similar subsidence stories are playing out from San Diego to major cities across Iran. In Aotearoa New Zealand, recent studies indicate that roughly 80% of the urban coastline is sinking, with groundwater identified as a contributing factor in locations such as Christchurch and Wellington.
Cities are frequently built directly above natural groundwater reservoirs known as aquifers. These urban areas are held up, in part, by the water stored beneath them. When excessive groundwater is pumped out, the underground aquifer acts like a deflated water balloon, causing the ground above to sink. This downward movement is particularly dangerous at the coast, where sinking land combines with rising seas to accelerate the impacts of climate change.
Osaka as a Natural Laboratory for Ground Uplift
To understand how the ground responds when long-term water aquifers recover, researchers examined Osaka, Japan. Intense extraction of groundwater from the 1920s through the 1960s dropped subsurface water levels beneath the metropolis by as much as 30 metres, which led to portions of the terrain dropping over two metres. In response, the Japanese government introduced strict groundwater regulations in the early 1960s, allowing groundwater levels to recover steadily in the decades since.
To track these changes, investigators analyzed data from 44 monitoring wells—some reaching as deep as 500 metres and stretching back to 1985—combined with interferometric synthetic aperture radar (InSAR) to measure millimetre-scale elevation changes. Across greater Osaka, home to approximately 15 million people, the ground is now rising rapidly. Uplift averaged about four millimetres a year, reaching up to 12 millimetres annually in certain areas where groundwater levels climbed by up to a metre a year.
Did You Know? Between the 1920s and 1960s, heavy groundwater pumping lowered water levels beneath Osaka by up to 30 metres, causing the land to sink by more than two metres in some places before strict government regulations were introduced in the early 1960s.
How Tectonic Faults Act as Underground Dams
The mapping of Osaka’s uplift revealed a patchy pattern where one city block might rise quickly while a location less than 100 metres away moved much slower. These abrupt changes form narrow, continuous corridors that map almost perfectly onto known tectonic fault lines. Rather than only functioning as fractures associated with earthquakes, faults act like vertical curtains cutting through aquifers to block the sideways flow of water.
Measurements show that subsurface water tables climb at a rate three times higher on one side of the Uemachi Fault in Osaka than they do on the opposite side. This same pattern repeats at other faults across the city, where water dams on the upstream side drive faster groundwater recovery and quicker land uplift, leaving the downstream side lagging behind.
Groundwater extraction and aquifer recovery affect coastal city subsidence
Why are many coastal cities sinking?
Many cities are sinking because groundwater is extracted from underground reservoirs faster than it can refill, causing the aquifers to deflate and the ground above to subside.
Can the sinking of coastal cities be reversed?
Yes, recent research shows that when groundwater is replenished and aquifers recover, land subsidence can slow down and, in some places, the ground can actually rise.
What role do tectonic fault lines play in land uplift?
Fault lines can act like underground dams that block the sideways flow of water. This traps water on the upstream side, leading to faster groundwater recovery and accelerated land uplift in those specific corridors.
How might your local coastal community factor underground water management into future sea-level rise planning?
Related reading