Douglas A. Harned
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.
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.
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.
Learn more about liquefaction