By Philip Whiteside, international news reporter
Hundreds of buildings on the Indonesian island of Sulawesi collapsed due to a phenomenon called liquefaction.
The terms refers to the tendency of loose soils, sand and silt to take on the qualities of a liquid when they are shaken intensely, often when the water table is just below the surface.
In many cases, objects and structures above ground sink into the earth – with some swallowed completely.
Many of the pictures emerging from the region around the city of Palu show the characteristics of liquefaction.
Some show farmers’ fields disturbed to the point where the rice paddies which filled them have disappeared and been replaced by earth.
In other pictures, cars have fallen into furrows and cracks that have opened up in roads and tracks.
Footage shows trees falling down, with their roots unable to support their weight as the earth around them is furiously shaken.
The United States Geological Survey says liquefaction takes place when increased water pressure in saturated soil makes particles in the soil lose contact with each other.
The pressure increases due to the shock waves that travel through the ground after an earthquake and the phenomenon particularly affects sandy soils, which are common in river valleys.
But the effect is the same as that which can cause mudslides in hills and mountains during a tremor.
Palu and other towns affected badly by the quake are in low-lying areas where the highly fertile soils have been laid down over thousands of years by river sediment and occasionally floodwater.
It is only in recent times that areas like this have become heavily populated and built up.
Indonesia’s national rescue agency said about 1,700 houses in the Palu suburb of Balaroa were swallowed up when the earthquake caused soil to liquefy.
Satellite images of the Petobo neighbourhood, south of Palu’s airport, showed another major area of urban development where almost all the buildings were destroyed.
Sutopo Purwo Nugroho, a spokesman for Indonesia’s national disaster mitigation agency, said: “When the quake hit, the layers below the surface of the earth became muddy and loose.
“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.”
Among those killed by the consequences of liquefaction were 34 children at a Christian bible study camp, a Red Cross official said.
Liquefaction has followed earthquakes in other countries and delivered similar consequences, often in areas where land has been reclaimed from the sea or wetlands.
It was reported during several previous Indonesian earthquakes, the major quakes in New Zealand in 2010 and 2011, and the Great Alaska Earthquake of 1964.
The phenomenon is even referred to in the Bible, which analysts say could relate to the writers’ experiences in the seismic hotspots around what is now Israel and Jordan.
And the Lord God of hosts is he that toucheth the land, and it shall melt, and all that dwell therein shall mourn: and it shall rise up wholly like a flood
Toshitaka Kamai, professor at Kyoto University’s Disaster Prevention Research Institute, said: “Compared with what’s been formed naturally over a long period of time, what’s manmade has been constructed for a short period of time, where soil particles are only loosely connected to each other.”
During the massive magnitude 9.0 quake that hit Japan in 2011, the eastern city of Urayasu, where the majority of land had been reclaimed, saw 86% of its land affected.
Mark Quigley, associate professor of earthquake science at the University of Melbourne, said it can often occur in areas where earthquakes are common, like countries on the Ring of Fire.
He said: “People need to live in places that are habitable for a whole bunch of reasons.
“In the case of Palu it’s on a very nice, natural bay. It’s on a river that would have provided historically a source of water for them. There’s a lot of really mountainous jungle-y terrain all around it, it’s not like they had endless opportunity to just choose whatever site they wanted.”