what is liquefaction in earthquakes?

Answer 1

Liquefaction takes place only in soils that are saturated with water. It can be triggered by earthquakes, or by construction activities such as blasting. The process is caused by an increase in pore water pressure that reduces the friction and stability of sand grains in the soil. Where a large building rests on this soil, the support can become unstable and buildings will fall over and collapse can be caused by unequal support under foundations. Movement of the soil can also disturb buried gas lines, water lines, and even utility poles. It is also a danger in some water dams.

Many urban areas near the coast are built on sand deposits that are part of a river delta. These sand deposits are especially vulnerable to liquefaction, as they also commonly have a shallow water table. Beach front property shares the same vulnerability in many cases. In these areas there are often efforts made to anchor the foundation on underlying bedrock, which can eliminate much of the risk from liquefaction. Many older buildings and residential structures do not have this sort of engineering precaution. In some places the bedrock may be so deep that it is not possible to reach it.

To see a MPEG film showing how the grains in a soil move during an earthquake, and another that demonstrates how a building can collapse go to this website:
http://www.ce.washington.edu/~liquefaction/html/what/what1.html

This is probably the best site explaining liquefaction on the internet, at the University of Washington Civil Engineering Department. The main page for this is at:
http://www.ce.washington.edu/~liquefaction/html/main.html

Answer 2

If you are at a sandy (or silty?) beach just above the waters edge you can move your bare foot up and down and the sand will become suspended in water and even look wet. During an earthquake if there is enough water in the soil the sand or clay will become suspended and can flow. This often results in a landslide where an entire layer of soil flows downhill.

Answer 3

Some cities are not built on bedrock, but on a soil with a high water content, that when violently shaken quickly loses the ability to support weight. It turns from firm to liquid. Buildings lose their foundations and break up, and sink.

Answer 4

Liquefaction is when sediment, such as sand, or clay is moving/shaken in such a way that it acts as if it were a liquid.

Answer 5

The water is forced up because of the shaking, this means that the sand/soil becomes less saturated, and more muddish.