Indonesia earthquake: What is liquefaction and what role did it play in the natural disaster?

Houses swallowed up by mudslides as increased pressure causes soil particles to lose stability

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.

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.

Liquefaction has been seen in previous Indonesian quakes as well as in the Great Alaska and Niigata earthquakes of 1964, in the San Fernando quake of 1971, California’s Loma Prieta earthquake in 1989, the Great Hanshin quake of 1995 and those that struck Christchurch in New Zealand in 2010 and 2011.