Science Puzzle
How Can Solid Ground Turn to Liquid?
In the 1964 earthquake in Niigata, Japan, several four-storey blocks of flats tipped over, one lying at a steep angle on its side. Inside, they were hardly damaged. They had been built to survive shaking, and they did. The ground beneath them didn't.
What happened to the ground?
The Answer
The ground turned to liquid. The buildings stood on loose, waterlogged sand, and the shaking made it behave like a thick fluid for a while, so the heavy buildings sank into it and toppled over.
Normally, loose sand can hold up a building because its grains press against each other, and the friction between them stops them sliding. The gaps between the grains are full of water. Shaking makes the grains try to settle more tightly, but the water in the gaps can't escape fast enough. Its pressure rises and pushes the grains apart until they're barely touching. With no friction between the grains, the sand and water flow like a liquid. This is called liquefaction.
For a minute or so, the ground can't hold anything up. Heavy buildings sink and tilt, and water and sand may spurt up through cracks. When the shaking stops, the water drains, the grains settle and the ground is solid again, leaving the buildings stuck at whatever angle they ended up.
You can feel a tiny version at the beach: stand on wet sand and wiggle your feet, and they sink. The same happened in Christchurch, New Zealand, in 2011. Engineers now compact or drain such ground, or support buildings on deep piles that reach firmer layers below.
The principle: Liquefaction. Shaking waterlogged sand raises the water pressure between the grains until they stop pressing on each other, so for a while the ground flows like a liquid.