Liquefaction essentially means that the soil is turned into a liquid. The key ingredient is a formation of loose, saturated sand. As seen in the figure, uniform sand grains can be packed either in a loose or a compact (dense) formation. Loose sand has usually been deposited gently underwater, either naturally, or sluiced into what is called hydraulic fill. The loose grains can support considerable weight, with the help of the water, which forms a good portion of the mass.
Once strong earthquake shaking begins, the grains are sheared into the more compact arrangement. The water, however, interferes, and the grains float in a liquid slurry. The excess water is squeezed out which causes the quicksand condition at the surface. If there is a soil crust or impermeable cap, then the sand boils out in the form of sand volcanoes.
Soil liquefaction can be dangerous if it leads to landslides or building foundation failures. Mapping the location of old liquefaction zones allows scientists to determine the strength and location of ancient earthquakes.