Sulawesi quake: When earthquakes liquefy soil, devastation can follow

An aerial view of liquefaction, or shifting ground, on Oct 1, 2018, following an earthquake in Palu, Central Sulawesi, Indonesia.
An aerial view of liquefaction, or shifting ground, on Oct 1, 2018, following an earthquake in Palu, Central Sulawesi, Indonesia.PHOTO: REUTERS

NEW YORK - Liquefaction, which Indonesian officials said had destroyed several thousand homes in and around the city of Palu during the earthquake last Friday (Sept 28), 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 7.4-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.

The unstable, watery soil can also flow as mud. Even gently sloping land can collapse and slide. Videos taken in the aftermath of Friday's earthquake appeared to show such shallow landslides, and in some footage the entire landscape appeared to be slowly moving because of the liquefied soil.

In strong earthquakes the pressurised water can be forced out of the sediments. In the 9.2-magnitude 1964 Alaska quake, for example, residents of Valdez - a town built on loose glacial sediments - reported seeing sheets of water spouting 6m into the air as the land shook. Sometimes watery sand can be ejected as well, a phenomenon called a sand boil.