A City on Sinking Ground
Parts of the coastal metropolis of Tianjin are sinking. The primary cause is the massive withdrawal of underground fluids, a process that has been happening for decades. The Bohai Bay Basin, where Tianjin is located, is a Cenozoic rift basin with oil and gas. It is also one of China's most important industrial and economic hubs. The combination of intense resource extraction and unfavorable geology has created a serious problem: land subsidence. By 2008, the maximum cumulative subsidence in Tianjin had reached 3.22 meters.
The mechanism is straightforward. Underground layers of rock and sediment are supported by the pressure of the fluids—water, oil, and gas—within their pores. When these fluids are pumped to the surface, the pressure drops. The weight of the overlying earth then compacts the now-emptier layers, causing the ground level to fall. This process is particularly pronounced in the Bohai Bay region due to its geology, which consists of thick layers of Cenozoic sediments that are highly susceptible to compaction. While groundwater extraction has been a major historical driver of subsidence since the 1920s, hydrocarbon exploitation in areas like the Dagang and Gudong oilfields contributes significantly to localized, rapid sinking.
Modern satellite technology, specifically Interferometric Synthetic Aperture Radar (InSAR), allows for precise measurement of this ground deformation from space. Studies using InSAR data have recorded alarming subsidence rates. Between 2020 and 2023, some areas in the wider Bohai Bay region sank at a maximum rate of 235 millimeters per year. In the Tianjin coastal area specifically, rates as high as 87 to 94 millimeters per year were recorded between 2003 and 2020.
Compaction, consequences, and countermeasures
For a low-lying coastal city like Tianjin, land subsidence presents a dual threat. As the land sinks, the relative sea level rises, dramatically increasing the risk of flooding, coastal erosion, and damage from storm surges. This human-caused subsidence exacerbates the effects of global climate-change-driven sea level rise. The consequences are severe, threatening urban infrastructure, railways, and the extensive port facilities that are important to the region's economy.
To combat the sinking, authorities are implementing several strategies. One of the most direct is to reduce the primary cause: fluid withdrawal. The South-to-North Water Diversion Project, which brings water from the Yangtze River, has helped decrease Tianjin's reliance on groundwater. This has led to a stabilization or even a slight rebound of the ground level in some areas since 2019.
In the oil fields, a different technique is used. To counteract the pressure loss from hydrocarbon extraction, treated seawater is pumped back into the depleted underground reservoirs through injection wells. This process re-pressurizes the rock formations, slowing the compaction and mitigating further subsidence. This method has been used in other sinking oil fields around the world, such as in Long Beach, California. These measures have slowed the overall rate, the problem is not solved. Some areas still show delayed subsidence as deep sediment layers continue to slowly compress.