NPR reports on the effects of liquefaction in Japan during and after last month’s earthquake. Of course, in Oregon we have to be a little self-interested when we see how things went down in the Japanese disaster.
Liquefaction happens when an earthquake shakes groundwater up through the soil and effectively turns hard ground into liquid.
You can test it out yourself with a bucket of sand and water. The sand is hard when it’s separated from the water, but when you shake the bucket, the two mix to form a gloppy liquid (perfect for making drip castles, not so much for supporting an actual castle). Some areas are more susceptible to liquefaction than others, as NPR reports in Japan:
“The quake itself did remarkably little damage to buildings in most areas, thanks to strict construction codes. But in a few places, particularly around Tokyo, buildings didn’t fare as well. These are places built on land reclaimed from the sea.”
Here’s a less-than-comforting description of liquefaction from Oregon’s Department of Geology and Mineral Industries (which has many maps of liquefaction risk areas):
“If an earthquake induces liquefaction, several things can happen: The liquefied layer and everything lying on top of it may move downslope. Alternatively, it may oscillate with displacements large enough to rupture pipelines, move bridge abutments, or rupture building foundations. Light objects, such as underground storage tanks, can float toward the surface, and heavy objects, such as buildings, can sink. Typical displacements can range from centimeters to meters. Thus, if the soil at a site liquefies, the damage resulting from an earthquake can be dramatically increased over what shaking alone might have caused.”
According to DOGAMI, the risk areas largely follow river and stream drainages, marshes and lakes – or places that have been filled by natural or human-caused processes. And even a few inches of settlement will damage buildings.
Here’s the best demonstration I could find on YouTube: