As Indonesia attempts to recover from the 7.5 magnitude earthquake and tsunami that devastated the island of Sulawesi in late September, the process of liquefaction has been blamed for the huge scale of the destruction.

Officials have confirmed 1,234 people died, mostly in the city of Palu, as a result of the disaster, many killed by buildings collapsing as the soil shifted beneath them.

Here’s an introduction to the phenomenon.

What is liquefaction?

The term refers to saturated sand and silt – that is, ground in which the space between particles is completely filled with water – taking on the form of liquid as a result of the aggressive disturbance the earth is subjected to by a powerful quake. Areas with shallow water tables close to the sea or rivers are particularly susceptible.

The quake increases water pressure, causing particles in the soil to lose contact with one another and sandy ground to ooze apart, potentially creating landslides and rattling buildings, trees and telegraph poles as the shifting earth slides about beneath them.

The reduced stability decreases the capability of the ground to support foundations, making the collapse of residences, blocks and bridges highly likely.

Liquefied soil sliding towards standing structures, like walls, can also place increased pressure on them and cause them to topple.

The term was first employed by American hydrological expert Allen Hazen, investigating the fall of California’s Calaveras Dam in 1918.

“If the pressure of the water in the pores is great enough to carry all the load, it will have the effect of holding the particles apart and of producing a condition that is practically equivalent to that of quicksand,” he wrote.

What happened in Indonesia?

In Sulawesi, approximately 1,700 houses were swallowed up in the Palu neighbourhood of Balaroa while an entire new development in Petobo was wiped away as though it had never been there.

“When the quake hit, the layers below the surface of the earth became muddy and loose,” said Sutopo Purwo Nugroho, spokesman of Indonesia’s national disaster mitigation agency.

(EPA)

“Mud with such large mass volume drowned and dragged the housing complex in Petobo so that most of them became as if they were absorbed. We estimate 744 units of houses are there.”

Where else has the phenomenon occurred?

The problem was last experienced on such a serious scale in Urayasu, Japan, following the 2011 Tohuku earthquake and tsunami.

Indonesian earthquake: rescue efforts continue Show all 30 1 /30 Indonesian earthquake: rescue efforts continue Indonesian earthquake: rescue efforts continue Residents and rescuers walk in front of a washed out passenger ferry in Wani following the September 28 earthquake and tsunami. Aid poured into disaster-ravaged Palu after days of delays as efforts ramped up to reach 200,000 people in desperate need AFP/Getty Indonesian earthquake: rescue efforts continue Indonesian rescuers try to free a 15-year-old earthquake survivor, Nurul Istikhomah from the flooded ruins of a collapsed house in Palu EPA Indonesian earthquake: rescue efforts continue A rescue team searches for bodies under the ruin of a house in Balaroa EPA Indonesian earthquake: rescue efforts continue A ship stranded on the shore EPA Indonesian earthquake: rescue efforts continue Indonesians stand among debris at Mamboro village in Palu EPA Indonesian earthquake: rescue efforts continue Residents carry a victim AFP/Getty Indonesian earthquake: rescue efforts continue Residents trying to salvage belongings from their homes which collapsed AFP/Getty Indonesian earthquake: rescue efforts continue Indonesian soldiers bury quake victims in a mass grave in Poboya AFP/Getty Indonesian earthquake: rescue efforts continue A damaged mosque AFP/Getty Indonesian earthquake: rescue efforts continue A man sits on the rubble of a house AFP/Getty Images Indonesian earthquake: rescue efforts continue Rescuers walk past debris AFP/Getty Images Indonesian earthquake: rescue efforts continue Jumadil, 5, reacts at seeing his mother Susi Rahmatia, 26, after he went missing for seven days AFP/Getty Images Indonesian earthquake: rescue efforts continue A woman builds a temporary toilet AFP/Getty Images Indonesian earthquake: rescue efforts continue Rahmat Raidi, 26, shows portraits of his missing family members AFP/Getty Images Indonesian earthquake: rescue efforts continue Residents queue before receiving humanitarian aid in Palu AFP/Getty Images Indonesian earthquake: rescue efforts continue Indonesian villagers offload aid from a military helicopter in Proo village, Lindu district AFP/Getty Images Indonesian earthquake: rescue efforts continue Residents stand among houses destroyed by a landslide that was triggered by the earthquake in Petobo outside Palu EPA Indonesian earthquake: rescue efforts continue People queue for fuel at a petrol station in Palu Reuters Indonesian earthquake: rescue efforts continue Members of the Indonesian Red Cross attend to a victim at a crushed house at Talise beach in Palu EPA Indonesian earthquake: rescue efforts continue Residents walk among houses destroyed by a landslide that was triggered by the earthquake in Petobo, outside Palu EPA Indonesian earthquake: rescue efforts continue An Indonesian Police Officer uses a dog to search for victims amongst rubble in Petobo, outside Palu Reuters Indonesian earthquake: rescue efforts continue An Indonesian rescue team searches for victims and survivors amongst the rubble of the Roa Roa hotel in Palu Reuters Indonesian earthquake: rescue efforts continue A crowd gathers at the airport in Palu in the hope of fleeing by plane Getty Indonesian earthquake: rescue efforts continue Members of an Indonesian rescue team look for survivors in a collapsed home in Palu AFP/Getty Indonesian earthquake: rescue efforts continue Bhayangkara hospital in Palu Reuters Indonesian earthquake: rescue efforts continue Members of an Indonesian rescue team look for survivors in a collapsed home in Palu AFP/Getty Indonesian earthquake: rescue efforts continue An earthquake damaged bridge in Palu Reuters Indonesian earthquake: rescue efforts continue People drive amongst the damage in Palu Reuters Indonesian earthquake: rescue efforts continue Indonesian women stand on a tsunami devastated area of Talise beach Palu EPA Indonesian earthquake: rescue efforts continue Members of an Indonesian rescue team look for survivors in a collapsed home in Palu AFP/Getty

As much as 86 per cent of the city’s land was liquefied as a result of the disaster, which killed 15,896 people in total, taking six years to repair the damage to its subterranean sewerage, water and gas facilities.