Liquefaction occurs when loose, saturated soils lose strength during an earthquake, transforming from a solid to a liquid state. This can damage buildings and bridges through sinking and toppling. The 1995 Great Hanshin Earthquake in Japan with a magnitude of 6.9 caused significant damage to bridges from liquefaction. To prevent liquefaction, it is best not to build on vulnerable lands like old swamps and marshes. Soil treatment methods like vibroflotation and vibro-replacement can also be used to increase soil density and reduce liquefaction hazards.