California Subsidence Maps
Subsidence is a common issue throughout the United States, being documented in 45 states. Many issues can cause subsidence; however, the most common issue in the United States is groundwater pumping. The wells we rely on to provide us water can leave soil dryer and more compacted. This is especially true when there are droughts that make it harder for the underground water supplies to naturally replenish.
According to a United States Geological survey reported that more than 80% of the subsidence in the America is directly correlated to groundwater pumping. While we can’t map causes such as water runoff, development, poor compaction of soil under homes, or other exceptions, we can provide you with maps for the most common causes. The maps below are based on USGS surveys and show areas where subsidence is known to be an issue and has been measured over the last twenty years.
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What Causes Settlement or Subsidence?
There are several potential causes of subsidence or settlement of the ground underneath a property. Some causes are man-made while others are natural geological changes. Natural causes such as fault movement or sediment compaction can cause subsidence. More commonly, however, the cause is man-made. Groundwater pumping, mining, oil and gas extraction, river channelization, and other manipulation of the ground can cause soil disruption that leads to subsidence.
Soils such as saline, non-cohesive soils, gypsum, silt, and clay are not as stable as some other soils. These soils can dry out due to lowering water tables and, when the water is not replenished, it can cause the soil to compact (subsidence). In other cases, soils can breakdown and vertically compress due to the weight of the structure on the soil combined with other factors (settlement).
Several things can disrupt the soil under your property and cause either settlement or subsidence. Explore the most common causes below.
Groundwater Pumping
Groundwater pumping lowers the water table that naturally holds up the soil. This can dry out soils and cause subsidence in various soil types, especially in hotter weather and droughts where the water table takes longer to replenish.
Soil Types
Clay and silt are classed as cohesive soils, meaning that when wet and dry, they have a capacity to swell and shrink. Soils such as sand and gravel are what are known as non-cohesive soils and they are made up of tiny particles which can erode and wash away over time.
Unstable Foundations
In some cases, homes can experience settlement as the soils underneath the foundation have a reduction in volume. This can be caused by the weight of the building over the years. Some foundation settlement is perfectly normal over time, but substantial or quick settlement can be a concern.
Drainage Issues
Many subsidence issues are the result of leaks from pipes or drains. Water can seep through and affect the foundations by either making the ground softer, causing the supporting soil to lose structural support or washing away non-cohesive soil particles and removing volume from the soil.
Tree Roots
Trees, plants, and shrubs can be detrimental to your home’s foundation. Larger trees especially are guilty of taking moisture away from the supporting soil under your foundation if planted too close to the foundation.
Collapsing Cavities
In more rare cases, areas of the ground can collapse. This type of localized collapse is often triggered by severe groundwater level and collapses are rare because they tend to be associated with specific rock types such as salt or gypsum. Additional information can be found from the USGS.
Download Your Free California Subsidence Map Now
The United States Geological Survey (USGS) agency, universities, and agriculture agencies have studied subsidence in the state of California for decades. Large areas of subsidence were first documented in the early 20th century and studies have been done ever since. The most common causes of subsidence in California is excessive groundwater pumping and soil erosion or compaction.
We've created a comprehensive map that reflects the risk of subsidence in various areas throughout California. Shaded areas reflect a medium- to high-risk area. While the maps fon’t cover every possible cause of subsidence (e.g., tree roots, foundation issues, or water runoff), they can help you understand your risk as it related to the most common causes of subsidence.
Sources: To view the source maps on the United States Geological Survey (USGS) website or view the interactive maps on the UC Davis site here.
