Liquefaction! Do you picture the earth beneath your feet turning to water? Here's what happens, with a little help from Dr. Hazen, the first geologist to explain the phenomenon, back in the 20s.
When the soil is sandy and not well compacted, it's texture is grainy. If pressure is applied from above (for instance, a building is constructed on top of it) the soil can bear the pressure. Any water left in the sandy soil tends to be squeezed out. It moves from high pressure (under the building) to low pressure (anywhere not under the building).
But--if water is suddenly and repeatedly injected among the sand grains--instead of bearing the load, the grains roll around like well oiled ball bearings. The repeated shaking of unstabilized soil by earthquakes is one way water finds its way in. The sandy grains--separated from each other and the friction that holds them together--flow in a liquidy way.
This happened in San Francisco's Marina district in the 80s, in Alaska, in Kobe Port in the late 90s, and in New Zealand earlier this year. Now it is happening in Japan's Chiba prefecture.
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