Grades 6–8 · Earth & Space
Why can wet ground lose strength during strong earthquake shaking?
Loose sandy ground can support weight through contacts between its grains. If the spaces are filled with water, the water also responds when shaking disturbs the structure. Under some conditions, that changes how the load is carried.
In the diagramPore pressure can reduce grain-contact forcesBefore shakingDuring strong shakinggrains stay solid; load sharing changes
Earthquake shaking can make grains seek a denser arrangement. If water cannot drain away quickly, pressure in the spaces rises. Higher pore-water pressure reduces the effective squeezing force holding grain contacts together.
The soil can then lose much of its strength and deform easily. Buildings may settle or tilt, and sloping ground may move sideways. This behavior is called liquefaction even though the mineral grains have not melted.
The risk depends on soil type, packing, saturation, drainage and shaking. Dense or well-drained ground behaves differently from loose saturated sand. The key change is a temporary redistribution of forces between solid contacts and trapped water, rather than a chemical transformation of soil into a new liquid.
About this explanation
Starting knowledge: Know that soil contains grains and spaces between them, which may hold water.
Sources
- U.S. Geological Survey — What Are the Effects of Earthquakes? Ground Failure