In 1989 a powerful earthquake struck San Francisco and caused massive damage to the city. In 1995 an earthquake in Kobe Japan killed 5096 people and devastated the city.
Damage for both of the earthquakes was widespread, but was considerably greater in certain parts of the cities. This experiment is designed to demonstrate one reason why earthquake damage varied in different parts of the cities.
Because of the high demand for housing, and the limited area to build it, both cities have extended their development out over the nearby estuary areas. By trucking in dirt and filling in near-shore wetlands the cities have been able to build out over these areas. Therefore both cities have sections built over "dry" soil and bedrock, and sections built over "wet" soil and bedrock. One of the "wet" areas of San Francisco is called the Marina district.
THE EXPERIMENT
In the experiment the two tubs of sand are used to represent the two different parts of each city. A tub of dry sand represents the older part of the city built on dry soil and bedrock. A tub of wet sand represents the newer sections of each city that have been built on infilled in parts of the bay. A brick is used to represent a skyscraper. A sander is used to make a simulated earthquake. In the experiment we will put the brick in each tub, turn on the earthquake and watch what happens.
Materials--Two small washtubs, enough sand to half-fill the tubs, power sander, brick, trowel, bungee cord, water, timer.
Hypothesis Formulation--Ask the students to formulate a hypothesis about in which tub the brick will fall down faster. This can be prefaced with more general questions about what they think will happen. Usually students guess that the brick will fall faster in the dry sand.
Procedure--The simulated skyscraper (the brick) is placed on the sand in the dry sand tub, a simulated earthquake (a sander attached to the tub with a bungee cord) is applied and timed. This is repeated at least 3 times (to provide an average time). The procedure is then repeated using the tub with wet sand (Note: the more water in the wet sand the better-- water should form small pools at the surface. Use the trowel between each repetition of the wet sand experiment to stir the water into the sand). Observe what happens in the tub.
Starting the earthquake.
Side view of experiment.
RESULTS
The brick should, on the average, fall down faster in the wet sand (1-2 seconds) than in the dry sand (2-20 seconds), This is because the "earthquake" causes liquefaction-- in other words the earthquake makes quicksand. The features you may have observed in the wet sand tub during the experiment-- small water spouts, the brick slumping into the sand-- were actual features reported observed in the Marina District during the 1989 San Francisco Earthquake.
Download a student lab sheet for this experiment as a Word Document(34K) or PDF.
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