How can solid ground turn into a liquid in an earthquake?

Soil liquefaction is what happens when the ground, for a few seconds, stops acting like a solid. Water-soaked, loose sand under your feet gets shaken by an earthquake and begins to behave like a liquid. Things on top of it can tilt, sink or float.

Here is how it works. Loose sand is a pile of grains with water filling the spaces between them. The grains hold each other up where they touch. When the shaking squeezes the sand, the grains try to pack more tightly, but there is no room for the water to escape quickly. So the pressure of the trapped water goes up. If the shaking is rapid or repeated, the water pressure can build until it exceeds the force between the grains. The grains are held apart by water, like cars on a wet road floating on a film, and the sand loses its strength almost completely.

In 1964 an earthquake of magnitude 7.6 struck near Niigata, Japan. Much of the city sits on loose, water-saturated river sand. Shaking caused liquefaction over large parts of the city, and in one area most of the apartment blocks tilted, with one overturned completely. In a place like that, the shaking itself is not the whole story.

Liquefaction does not strike everywhere. The deposits most likely to liquefy are young, loose sands and silts saturated with water, often along streams, beaches and dunes. Dense sand, which tends to expand when squeezed, is far less of a danger.

Engineers now test the ground before they build, and can strengthen it, for instance by packing the soil denser.