It was an undignified thing to happen to a block of flats. Japan is an earthquake-aware country, and when apartment complexes were built in the Kawagishi-cho area in the city of Niigata, they were sturdy reinforced concrete. But after a magnitude 7.5 shock on June 16, 1964, it was not the structures that gave way but the ground beneath them. Hundreds of buildings tilted like ships listing in heavy seas, victims of soil liquefaction. When the time came for repairs, some of the buildings were simply winched back upright without appreciable damage.

Photos of that spectacle are in all the textbooks. The liquefaction in Niigata was not deadly (only 26 died from the quake and tsunami), but it was still an expensive calamity, and a notorious indictment of civil engineering practices. Really important structures, not just buildings, collapsed with the liquefied soil�bridges and roads, airports and rail lines.

Here's what happens to your patch of beach sand. The sand is full of water and it's freshly deposited, so the grains are loosely packed. Shaking makes the sand grains settle down more densely, but the water can't get out of the way fast enough. In physics terms, the downward force of the sand's weight meets an upward force from the water, and when those two forces are nearly equal, the solid sand turns to a semiliquid. Geologists classify what happens next into several different kinds of ground failure.

• Flow failure happens when liquefied sediments are sitting on a slope. The result is some form of landslide.
• Lateral spread occurs on flatter terrain. The ground just oozes out sideways, no more than a few meters at the most. But that's enough to completely disrupt things like foundations, pipelines, railroads, and retaining walls. Lateral spread is common near riverbanks, and it's especially harmful to things like bridges, whose feet may sit in young river sediments.
• Ground oscillation affects flat ground. The liquefied sediment starts to slosh into waves as shaking continues. Whatever is on top of the sediment gets broken and thrown around. Cracks in the ground open and close, and water or mud may erupt from them.
• Settlement happens in all types of terrain as the soil compacts and the ground water dissipates. This can cause a lot of damage to natural features, such as drowning low-lying forests, and it can shift stream courses.

We can counteract liquefaction by fixing the soil�either draining it, to remove the ground water, or compacting it, so it won't settle any further. Exactly what work to do in a given place, and how much of it is enough, are matters for geotechnical engineers.

More important for planning and policy purposes is knowing where the susceptible ground lies. Geologists are part of that effort, and in many earthquake-prone regions they are mapping the kinds of deposits that could liquefy under the right amount of shaking. Seismologists, for their part, create maps of the same region showing the degree of earthquake shaking to be expected. Lay the two maps together, and planners can get an idea of where the worst places to build are.