Modified from a classroom lesson prepared by Wendy Gerstel,
Washington Division of Geology and Earth Resources
In California, the effects of an earthquake commonly are amplified in areas underlain by sandy unconsolidated sediments in. Common geologic settings for liquefaction include unlithified sediments in coastal regions, bays, estuaries, river floodplains and basins, areas surrounding lakes and reservoirs, and wind-deposited dunes and loess. When such sediments liquefy, they lose their structure and strength. During earthquake shaking, the individual grains of sand within a deposit collapse on each other. Any structure built on sandy sediments can sink or collapse. Picture a container of balls of different sizes: baseballs, golf balls, marbles. If they were transported by water into the container and then deposited, they would settle with spaces between them. Some of the spaces would be filled with water, some with air. When you shake the container, the balls settle against each other, and the water and air are forced to the surface. That is exactly what happens in a sediment-filled valley. The valley is a large "container" holding gazillions of "balls" or grains of sand. Shaking the container simulates an earthquake.
This experiment demonstrates what happens to sandy soils when they liquefy. You will create a model river valley, then watch how and why houses get damaged or collapse during an earthquake in a seemingly stable geologic environment.
- glass baking pan or plastic bin (so contents of pan can be observed)
- enough dry sand to fill pan 1-2 inches deep
- a few toy houses or blocks
- Evenly pour the dry sand into the pan.
- Mark the level of the sand on the side of the pan or bin (use a washable marker).
- Place houses or blocks gently on the surface.
- Slowly add water until about two-thirds of the thickness of the sand is saturated.
- Gently shake the table on which you have placed your pan (or gently shake the pan itself). If using a plastic bin, you can use a rubber mallet to tap the side of the pan 10 times).
You should see the following:
- The water will work its way to the surface, flooding the area around the houses.
- The houses will start leaning over and sinking into the sand.
- The volume of the sand should decrease by a small amount. Note where the surface is after shaking in relation to the mark denoting the surface before shaking.
Try the experiment using clay or gravel to separate sand layers and represent different types of sedimentary layers. Watch what happens to the water and the surface of your model of a river valley. Compare what happens to the water when using different materials.
Courtesy of Ohio Division of Geological Survey "HANDS ON EARTH SCIENCE" Webpage. Modified by the California Geological Survey.
Source:Earth Connections No. 2, published in Washington Geology, v. 27, no. 2/3/4, December 1999, Washington Division of Geology and Earth Resources.
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