Liquefaction, which Indonesian officials said Monday had destroyed several thousand homes in and around the city of Palu during the earthquake on Friday, is one of the most devastating effects of earthquakes. In addition to causing soil to flow like a liquid — in this case, leading to mudslides — it also can make land slump and sink.

For liquefaction to occur, soil must be relatively loose (when it has a lot of sand in it, it is especially vulnerable); it must be waterlogged (which is often the case in tropical, rainy Indonesia); and it must be shaken violently (as happened during the 7.5-magnitude quake).

In waterlogged soil, the microscopic spaces around the grains of sediment are filled with water. Shaking causes the grains to compact, which increases the pressure of the water. The ground becomes unstable and can sink unevenly. It loses its ability to support structures built on or in it, including houses, piers and bridge foundations, often with deadly results.

In the 1906 San Francisco earthquake, for example, much of the damage occurred in reclaimed land that was filled with sediment from San Francisco Bay. Liquefaction of the fill caused buildings to collapse.