Damage to Interstate 5, CA during the Northridge Earthquake, 1994. Unconsolidated sediments with a certain amount of water in it, will be shaken by seismic waves. This expels the water towards the surface.
Sediments become a mass of material saturated with water and with no strength.
It will start acting like a fluid, thus the term liquefaction. Extremely dangerous for structures. Causes complete collapse of buildings. Last aftershock shook an area of 2.5 million square km.