Soil liquefaction
When shaking makes waterlogged sand lose its strength and behave like a liquid, so buildings tilt and sink.
Soil liquefaction is what happens when strong shaking turns firm, waterlogged ground into something that behaves like a thick liquid. It mostly affects loose sand and silt saturated with water, such as reclaimed land, river deltas and low-lying coasts.
In such ground the spaces between the grains are completely filled with water. Normally the grains rest on each other and carry the weight of whatever is above, so the soil is firm. Earthquake shaking jostles the grains and tries to pack them more tightly, but the water cannot escape fast enough. Its pressure, the pore-water pressure, rises until the grains are no longer pressed firmly against each other and can move freely. The soil loses its strength and flows.
The effects can be dramatic. In the 1964 Niigata earthquake in Japan, apartment buildings tipped over almost intact as the ground beneath them gave way. Liquefied soil pushes harder on retaining walls, can set off underwater landslides and lets buried tanks and pipes float upwards. Sand and water often burst out at the surface in small fountains called sand boils.
You can feel a gentle version on a beach: stand on wet sand and wriggle your feet, and you slowly sink as the sand around them turns soft.
Engineers reduce the risk by compacting or draining the ground, or by driving foundations down to firmer layers below.
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