But understanding this movement of the earth is not simple, scientists say. Even within the same city, some areas may be sinking at a faster rate than others.
“If we look at the data, we don't see a city as a whole, uniform society. We generally see hotspot areas where you can see parts of cities subsiding depending on what's going on,” said NASA scientist David Bekaert, who has analyzed land subsidence at the United States and worldwide.
Some subsidence, Bekaert said, is related to deep natural processes over long periods of time, such as responding to plate tectonic activity or the retreat of glaciers since the last Ice Age. Other subsidence is linked to human activity, including the extraction of oil, water or minerals from underground. In cities, buildings can also add weight and push the land down.
Many of the world's fastest-sinking places occur in populated areas in southeast Asia, largely due to groundwater extraction, but the United States also faces significant subsidence. There, you might not notice land settling around you in your daily life, but scientists have found that many places are sinking faster than sea levels are rising – increasing the risk of flooding in coastal cities. (Sea level is rising at a global average rate of approx 3.4 millimetersor 0.13 inches, per year.)
The areas with the most precipitation in the United States are mostly along the East and Gulf Coasts, but here we've selected a few hot spots across the country.
The western Gulf Coast experiences some of the fastest rates of sedimentation in the United States.
Analyzing sedimentation rates in large coastal cities, Bekaert et al found Houston he's got the fastest peak precipitation rates — about 17 millimeters (0.67 inches) per year from 2014 to 2020 — in the United States. Another investigation showed parts of Houston lost more than 10 feet in elevation in some areas since 1917.
Such subsidence can exacerbate damage during extreme weather events. Researchers found that the majority of areas that flooded during Hurricane Harvey in 2017 showed relatively high sedimentation rates.
Land subsidence in the Houston-Galveston area is largely caused by groundwater withdrawal. According to US Geological Survey, the removal of water from fine-grained silts and clays underground compresses the water table and lowers the land surface. Some areas experience high rates of sedimentation due to local oil withdrawal and natural gas reserves.
A smaller portion of the subsidence can be attributed to fault movement in the area.
Parts of New Orleans also experience high rates of subsidence, due to both man-made and natural processes. Research showed that rates are highly variable across the city, ranging from 150 to 500 millimeters (6 to 20 inches) over the past 20 years.
Previous data from 2009 to 2012 showed the highest rates of subsidence along the Mississippi River near industrial areas in Norco and Michoud — sinking as much as 50 millimeters (2 inches) a year. East of Norco, the Bonnet Carré Spillway, intended to reduce flood risk for New Orleans, has sunk as much as 40 millimeters (1.6 in) a year since construction.
NASA said the primary factor is groundwater extraction. Subsidence deeper into the ground is also caused by shifting faults, according to the city of new orleans.
While Houston and New Orleans are notable retreat locations, other places in the Gulf also have high rates. In a large area north of Tampa Bay, subsidence rates have been clocked at up to 6 millimeters (0.24 in) per year, about twice the rate of global sea level rise, from 2015 to 2020 due to groundwater pumping waters. The Tampa Bay area is also relatively flat, which means that rising seas can inundate large areas of the low-lying area.
East Coast: New York
With more than 8 million people, New York is the most populous city in the United States. It also sinks about 1 to 2 millimeters (0.03 to 0.08 in) per year, on average. People living in cities like this may see higher sea level rise — rates up to four times faster — than more stable areas.
“If now the water is rising and your land is falling, then your houses will be flooded,” said Matt Way, a geophysicist at the University of Rhode Island. “That's the point”.
Wei said much of the land subsidence in the city is responses to glacial retreat after the most recent Ice Age. During the last Ice Age, ice sheets weighed heavily on the earth and caused the crust beneath places like Canada and the northeastern United States to stretch and sag. Places on the periphery of these relaxing regions, however, such as around New York and the Chesapeake Bay, bulged upward. When the ice sheets began to recede about 12,000 years ago, these hanging areas began to rise again while the bulging areas receded.
Wei said to think of it like a balloon. If you push down on a balloon, the parts near your hand will go down while other parts that are further away will go up. Once you remove your hand, the balloon will try to bounce back to its original shape, causing some sections to rise and others to fall. He said the same process is happening on Earth except “on a much larger scale and also at a much slower speed.”
For the most part, Wei said land subsidence in New York City is in line with what researchers expected from this glacial recovery. However, some parts of the city experienced higher than expected subsidence rates. In a new study, Wei and his colleagues found that the weight of buildings around New York City actually pushes the land in some areas, further contributing to land subsidence. While the average rate in the city is 1 to 2 millimeters per year, some areas experience around 4.5 millimeters (0.18 in) per year.
“New York is sinking not because of the weight of the buildings. It's mainly because of the glacier's recovery,” Wei said. “But there are places that suggest the weight of the buildings [have] contributed to the accelerated pace.”
Norfolk has the highest rate of sea level rise on the east coast. Sedimentation plays no small role. The Hampton Roads area of Virginia sinks approx twice as fast as the waters rise. In a study from 2020Bekaert and colleagues found that Norfolk and Virginia Beach were sinking at more than 3.5 millimeters (0.14 in) per year, with some sections sinking more slowly or faster.
Like New York, parts of the Mid-Atlantic are also affected by glacial retreat. Norfolk also faces subsidence from a meteorite which created a hole millions of years ago. Softer dirt entered the crater and has since been compressed and displaced downward, causing subsidence to land around the crater.
Groundwater withdrawals are also exacerbating rates locally. Bekaert said historically, paper mills near Norfolk did a lot of pumping, but operations have been halted by new regulations. The city is experimenting a new project which will restore water to the aquifer and reverse long-term pumping.
Norfolk may stand out on the east coast, but many areas along the coast are also battling high rates of subsidence and sea level rise. The Chesapeake Bay is experiencing about 4.6 millimeters (0.18 in) per year of sea level rise, exacerbated by land subsidence.
West Coast: Central Valley of California
The subsidence along the west coast is relatively small compared to the east coast, but this does not mean that subsidence is not a problem.
In the Pacific Northwest, Bekaert said a major driver on a larger scale is plate tectonic activity, where one plate subducts or goes under another plate. Depending on where you are, he said, you may experience uplift or gradual subsidence. As you move further down the coast, the plates start moving past each other, which doesn't provide much vertical movement. Instead, anthropogenic processes dominate.
Overall, the figures along the west coast show that the rates are mainly less than 3 millimeters (0.12 inches) per yearexcept for areas like San Diego, which has experienced large amounts of human-caused subsidence.
Further inland, in California's Central Valley, over-pumping of groundwater for agriculture has caused significant land subsidence. Since the 1920s, parts of the San Joaquin Valley have sunk as much as 8.5 meters (28 ft). Groundwater pumping becomes even more problematic during drought, exacerbating subsidence problems. Data from 2015 showed that some spots receded at rates of up to 600 millimeters (2 ft) per year.
“There can be hot spots within cities where you see different rates of subsidence,” Bekaert said. “Often, they're related to human-caused local fingerprinting.”
This article is part of it hidden planet, a column that explores the amazing, unexpected and unusual science of our planet and beyond.
