Solid as a Rock? Maybe Not
Most of the time, buildings and homes are built on solid earth. But, building a home also happens on what only seems like solid ground. Yet, underneath a layer of compact sediment, there might be a surprise. Liquefaction occurs when water logs loosen soil known as sand into quicksand. Have you ever looked closely at sand? If you did you would see it is made up of lots of tiny rocks. Because of the shape of these tiny rocks, there is space between them. This space is often filled with water. When we step on sand, the tiny rocks press together and make it stable. This is called compression. In the long-term compression actually creates sedimentary rocks from sediment. But, this occurs over millions of years. We build on this land because, most of the time, it behaves as a solid.
Earthquakes and Liquefaction
But in an earthquake, these tiny rocks move around so the water between them cannot flow away. Therefore, the ground cannot compress. This results in the water taking the weight of whatever building is on this land. Water, as we know, is not a good spot for a solid object to be. Thus, sinking occurs. When the earthquake stops, the ground turns back into a solid. This means that whatever sank is now stuck. The topography of the ground will become very uneven. Crevices and gaps open up. Roads and sidewalks break into pieces and become hazardous. Yet, they could sink right into the earth during an earthquake!
What Else Happens?
These are not the only effects of liquefaction. While structures will sink into the ground, other things will rise up. Loose, sandy soil or the water saturated soil underneath the sediment can make its way up to the surface. The columns of rising sandy soil are called sand dikes. When the sand emerges on the surface it is called a sand boil. These look like small, sandy volcanoes.
The results of this liquefaction phenomenon are often seen after high magnitude earthquakes. Street pavement will break apart, and sand boils can be seen on roadways and other areas. Sewer channels underneath the streets can rise up during liquefaction. As a result, the manhole cover you see every day on your way to school could be several inches to a couple of feet higher than the surface of the road. This is a common side effect of liquefaction.
Other underground structures can break also; important things like gas and water lines. Many studies have been conducted to find the places where liquefaction might happen.
Other Great Resources:
Earthquakes and Liquefaction:
Liquefaction Hazard Maps: https://earthquake.usgs.gov/hazards/urban/sfbay/liquefaction/sfbay/
Preventing the Damage Caused by Liquefaction: https://www.sciencelearn.org.nz/resources/343-liquefaction