What is Subsidence? - Definition, Causes & Signs

Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

In some areas, residents have to be aware of the ground below them shifting. You read that right; the ground below your feet can move! This lesson will detail a shift in the ground downward, called subsidence, its causes, and signs that it has happened.

What is subsidence?

Imagine this: You have been in the market for a new home for a while now. Your career has really taken off recently and you can finally make this exciting purchase. You find the perfect house in the perfect neighborhood. There is just one problem: This area has been known to experience subsidence. What? What does that even mean? It's actually a much bigger problem than you might think; one that may affect your decision.

Subsidence beneath buildings.
Subsidence beneath buildings

Subsidence is the downward shift in the ground surface that can be caused by a variety of processes, both natural and human-related. Generally, subsidence is neither a sudden nor a drastic shift, but it can still be a very big deal to residents and business owners. Even a slight downward shift in the ground surface can cause the foundation of a building to be compromised. If the foundation cracks, much bigger (and expensive!) problems can arise.

Subsidence beneath a road.
Subsidence under a road

What Causes Subsidence?

Subsidence occurs whenever some part of the earth's crust (or something within the crust) either goes away or becomes compacted. This can be caused both by humans or natural processes.

  • Human processes that can cause subsidence include mining, natural gas removal, and overuse of groundwater.
  • Natural processes that sometimes result in subsidence include the erosion of bedrock, earthquakes, the faulting (breaking) of bedrock, and the shifting of groundwater reservoirs.

For this lesson, we'll focus on the groundwater-related causes of subsidence. Just remember, essentially anything that causes part of the ground (sometimes even very deep down) to lower can cause subsidence on the surface.

Groundwater-Related Subsidence

In many areas of the world, groundwater is used extensively for many purposes. From drinking to crop irrigation, groundwater serves an incredibly important purpose in our lives. However, there are numerous examples of how groundwater is used too quickly from reservoirs, which can cause subsidence.

Groundwater is found in reservoirs, which can be thought of as a container (although many are more like a sponge). The reservoir is filled as rainwater seeps into the ground, and is emptied as humans remove groundwater for drinking or irrigation. If the water is used too quickly, the ground above the reservoir could lower, leading to the subsidence of the surface. One study showed a maximum amount of subsidence (that has been measured) to be more than 28 feet in California. That's taller than most houses! This subsidence was caused by the overpumping of groundwater for the irrigation of crops. The area did not have much rain, so the reservoirs didn't refill, causing the ground to sink.

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