Subsidence
Based on Wikipedia: Subsidence
The Ground Beneath Your Feet Is Sinking
In Long Beach, California, the ground dropped nine meters over thirty-four years. That's nearly thirty feet—the height of a three-story building. The culprit wasn't an earthquake or some dramatic geological catastrophe. It was oil. As petroleum companies pumped crude from beneath the city, the earth slowly collapsed into the void they left behind. The damage exceeded one hundred million dollars before anyone figured out how to stop it.
This phenomenon has a name: subsidence. It's the quiet, gradual sinking of the Earth's surface, and it's happening right now beneath cities around the world. Unlike landslides or earthquakes, subsidence doesn't make headlines. It works slowly, imperceptibly, often for decades before anyone notices. By then, buildings have cracked, roads have buckled, and the damage is done.
What Makes the Ground Sink
Think of the Earth's crust as a layer of rigid material floating on something softer beneath. That image isn't far from reality. The crust sits atop the asthenosphere, a region of hot, slowly flowing rock. Everything maintains a delicate balance—add weight somewhere, and the crust sinks a bit to compensate. Remove support from below, and it collapses.
Subsidence happens through several distinct mechanisms, each with its own character.
The most widespread cause today is groundwater pumping. Aquifers—underground layers of rock and sediment saturated with water—act like wet sponges. The water pressure helps support the weight of everything above. When cities pump out this water faster than nature can replenish it, the sponge compresses. The sediments compact. The surface drops.
One study estimates that eighty percent of serious subsidence problems worldwide stem from excessive groundwater extraction. This makes it a signature problem of rapid urbanization. Cities grow, populations rise, water demand increases, and nobody regulates the pumping until the damage becomes visible.
But there's a crueler twist. In low-lying areas like river deltas and coastal plains, people drain the land to make it habitable. This exposes organic material in the soil—particularly peat—to oxygen for the first time. The peat oxidizes and decomposes. The ground shrinks. So communities lower the water table again to maintain dry ground, which exposes more peat, which decomposes further. The process feeds itself, dropping the land surface by as much as five centimeters every year.
When the Roof Caves In
Limestone dissolves in water. This is a slow process, measured in geological time, but it creates an underground landscape of caves, channels, and hollow spaces. Geologists call these karst terrains, and they exist across much of Florida, Kentucky, the Yucatan Peninsula, and many other regions worldwide.
Usually the roof of an underground cave holds. But sometimes it doesn't.
When a cave roof fails, everything above it crashes down suddenly. The result is a sinkhole—sometimes appearing overnight, sometimes swallowing cars, houses, or entire sections of road. These can be hundreds of meters deep. Unlike the gradual subsidence from groundwater pumping, cave collapses happen without warning. One moment you have solid ground; the next, you don't.
The Price of Extraction
Mining creates subsidence by design. Several extraction methods—particularly longwall mining for coal and block caving for ore deposits—deliberately allow the mined-out void to collapse. The surface follows.
Mining engineers can actually predict this with reasonable accuracy. They know roughly how much the ground will drop, where the effects will reach, and what kind of damage to expect. The vertical drop itself usually isn't the worst problem. What destroys buildings and infrastructure are the stretching, compressing, and tilting that accompany it. Imagine the ground not just sinking but warping—pulling apart in some areas, squeezing together in others, tilting slopes where things once sat level.
When mining is planned, these effects can be managed. Mines can be designed to minimize surface damage. Preventive measures can protect critical infrastructure. Repairs can follow extraction. The key is cooperation between mining companies, regulators, and surface property owners. When that cooperation breaks down—or when people build over abandoned workings from a century ago—the results can be sudden and catastrophic.
Natural gas extraction creates similar problems. Gas fields exist under enormous pressure, sometimes reaching sixty megapascals—about six hundred times atmospheric pressure at sea level. This pressure helps support the rock and sediment above. Extract the gas, and that support disappears. The ground compacts and drops.
In the Netherlands, the Slochteren gas field has been producing since the late 1960s. Over an area of two hundred fifty square kilometers, the ground has dropped by up to thirty centimeters. In a country where much of the land already sits below sea level, that's more than an inconvenience. The extraction has also triggered earthquakes, turning a geological resource into an ongoing liability.
When the Earth Shakes
Earthquakes cause subsidence through two distinct mechanisms.
The first is simple displacement along faults. When rock masses shift during a quake, one side can drop relative to the other. The 2011 Tōhoku earthquake in Japan—the one that triggered the Fukushima disaster—caused immediate subsidence across a wide area. The city of Rikuzentakata dropped eighty-four centimeters. The Oshika Peninsula sank by one point two meters while simultaneously moving over five meters horizontally.
The second mechanism is compaction. Unconsolidated sediments—loose sand, silt, and clay that haven't been compressed into solid rock—shake loose during an earthquake. The particles settle more tightly together. The surface drops.
This helps explain why earthquakes often cause worse damage in areas built on filled land or former wetlands. The ground isn't just shaking; it's compacting and sinking while the shaking continues.
Floating on a Sea of Rock
The concept of isostasy sounds abstract but explains much of how the Earth's surface behaves. The crust floats on the mantle like ice floats on water—not literally floating on liquid, but responding to weight the same way. Add weight somewhere, and the crust sinks. Remove weight, and it rises.
Lake Bonneville, the ancient predecessor of Utah's Great Salt Lake, demonstrated this principle dramatically. At its peak during the last ice age, the lake covered an enormous area and held an immense weight of water. The crust beneath it sank nearly two hundred feet to compensate. When the lake dried up, the crust slowly rebounded. Today, the former lake center sits about two hundred feet higher than its former edges—the geological equivalent of a foam mattress returning to shape after you get out of bed.
This rebound continues for thousands of years after the weight is removed. Scandinavia is still rising, centimeters per century, recovering from the ice sheets that melted ten thousand years ago. Hudson Bay is doing the same thing. This is subsidence's opposite: isostatic rebound, the Earth slowly returning to equilibrium.
The Peculiar Problem of Clay
Many soils contain substantial amounts of clay, and clay behaves strangely with water. The particles are extraordinarily small, and they swell when wet and shrink when dry. This creates seasonal subsidence—the ground surface drops during dry periods and rises during wet ones.
For buildings, this cycle creates stress. If foundations don't extend below the zone affected by seasonal moisture changes, they move with the soil. This movement creates cracks—distinctive tapering cracks that grow and shrink with the seasons.
Trees make this worse. A mature tree can extract enormous amounts of water from the soil around it, drying the ground far more than seasonal weather alone. Over years, this drying accumulates. The soil shrinks progressively as the tree grows. Cut down the tree, and the reverse happens—water returns, the soil swells, and the ground rises. This heaving can last twenty-five years, long after anyone remembers why the tree was removed in the first place.
The Weight of Cities
Here's a subsidence cause that sounds almost too simple to be real: heavy buildings push the ground down. But it's happening. New York City, the San Francisco Bay Area, and Lagos are all experiencing measurable subsidence simply from the mass of construction pressing down on the underlying soil.
This adds an uncomfortable feedback loop to urban development. Cities grow because people move to them. Buildings rise to house those people. The buildings compress the soil beneath them. The ground sinks. In coastal cities, this brings the surface closer to sea level and increases flood risk—making the city less habitable just as it becomes more populated.
The Consequences Add Up
Subsidence doesn't just mean the ground is lower than it used to be. The effects cascade.
Flooding worsens. River floodplains and delta areas naturally sit just above typical water levels. Drop that land surface by even a few centimeters, and floods that once stayed within bounds now spread across neighborhoods. Raise sea levels simultaneously—as climate change is doing—and the combined effect multiplies.
Earth fissures appear. When an aquifer compacts unevenly, stress concentrates in the sediments. Eventually the stress exceeds what the sediment can hold, and cracks appear at the surface. These aren't subtle hairline fractures. They can be several meters deep, several meters wide, and extend for kilometers. They swallow roads, rupture pipelines, and make land unusable.
Structures fail. Subsidence rarely happens uniformly. One part of a building's foundation might drop more than another, creating what engineers call angular distortion. Past a certain threshold, walls crack, floors tilt, and buildings become unsafe.
Watching the Ground Drop
Scientists have developed increasingly sophisticated ways to measure subsidence. Traditional surveying—measuring elevations at fixed points over time—still works but requires physically visiting each location. Borehole extensometers, instruments installed in deep holes, measure compression at different depths underground.
The Global Navigation Satellite System, commonly called GPS, can track tiny changes in position at monitoring stations. More powerful still is Interferometric Synthetic Aperture Radar, known as InSAR. This technique uses radar satellites to measure surface elevation across entire regions, detecting changes as small as a few millimeters.
Comparative studies have found InSAR offers the best combination of coverage, cost-effectiveness, and measurement frequency. A single satellite pass can survey vast areas, and repeated passes can track subsidence over time. Traditional methods like surveying and temporary GPS stations typically provide only one or two measurements per year. InSAR can do far better.
This matters because subsidence moves slowly enough to go unnoticed for years, but fast enough to cause serious damage within decades. The sooner it's detected, the sooner something can be done—whether that's stopping the groundwater pumping, injecting water back into depleted reservoirs, or simply designing infrastructure to cope with a sinking surface.
What Happens Next
The Long Beach story has a modest happy ending. Engineers halted the subsidence by pumping water into the oil reservoir, essentially replacing the extracted petroleum with something else to maintain underground pressure. The technique, called secondary recovery, stabilized the ground and stopped the sinking.
But that solution only works when someone recognizes the problem and has the resources to address it. Around the world, groundwater pumping continues to accelerate as populations grow and water demand rises. The Netherlands has spent centuries fighting water from above; now it faces land sinking from below. Coastal cities from Jakarta to New Orleans to Shanghai are racing subsidence against rising seas.
The ground beneath our feet seems eternal, unchanging, solid. It isn't. It responds to what we do, compressing under our cities, collapsing into our mines, settling as we drain it dry. Understanding subsidence means understanding that the Earth is not simply a stage on which we build—it's a participant in everything we construct, and it has its own physics to follow.