It turns out the biggest killer in the Palu earthquake on the island of Sulawesi, Indonesia, may not have been the tsunami after all — but liquefaction. Two thousand victims of the earthquake and tsunami are confirmed but 5,000 people remain missing, many of them presumed swallowed up in extraordinary ground deformation and mudflows, which took off when the underlying solid ground liquefied. Some buildings were transported hundreds of meters, others were ripped apart, many collapsed into fragments that then became absorbed into the mud. Media reports state that in Balaroa, just a few kilometers from Palu City, many of the 1,747 houses in the village appear to have sunk into the earth. In Petobo, a village to the east of Palu, many of the village’s 744 houses have disappeared.

What we have witnessed at Palu merits the term “ultra-liquefaction”, as witnessed in the 2011 Christchurch, New Zealand earthquake when perhaps half the total insurance loss costs were a consequence of liquefaction. For Christchurch, in the eastern suburbs it was single storey houses, ripped apart by the ground movements. In the Central Business District (CBD), many mid-rise buildings had to be demolished because underlying liquefaction had led to one corner of the structure sinking by ten or twenty centimeters (four to eight inches).

The phenomenon of liquefaction occurs in water-saturated silt or sand when the particles have settled out of still water, as when a river flows into a lake or the sea; or in land-reclamation, when seafloor sediment is pumped onshore. The sand particles touch at the corners, so that the surface load of a building, for example, gets transferred through the grains and the material behaves as a solid.

Should the sediment be shaken, however, the particles jostle and pack themselves more tightly together. This leaves less space for the intervening water, raising the water-pressure that pushes the grains of sand apart. The material now behaves like a liquid, which can flow and into which a surface load will sink — like a quicksand. The excess water pressure may be relieved if the water can escape to the surface, transporting grains of sand and leaving surface “sand volcanoes” — the hallmark of underlying liquefaction. However, even with the most gentle slope, surface soils and buildings will slide sideways on the liquefied layer, which can extend down to ten meters (32 feet) below surface.

The phenomenon of ultra-liquefaction changes everything around how we model earthquake impacts and even how earthquake scientists have attempted to classify the damage from past shocks. For a long time, it was convenient to assume that liquefaction supplemented the impacts of ground shaking, as if it made the ground shaking a little stronger. But the reality is that this is a completely different damage mechanism. As in the Christchurch CBD, buildings can be completely undamaged by the shaking, but still a 100 percent insurance loss from liquefaction.