This webpage
details the model made by Ross Stein and demonstrated most recently
at the May 2000 USGS Open House by the Earthquake Hazards Team.
The website has been constructed due to the great public interest
shown at the Open House, particularly by teachers.
This website
includes a TV documentary film clip, animation
and photographs
of the model 'in action', a lengthy model
description, and associated diagrams. It also includes technical specifications
(bottom of this page) to aid those wishing to build their
own model.
 |
See the earthquake machine in action
Click on image to see the earthquake machine in action.
Video from: Eye on the Bay, The Science of Predicting Earthquakes,
courtesy of Brian Hackney (CBS 5) |
Earthquake
hypotheses that can be explored with the model
Hypothesis 1: Earthquakes
are periodic (in other words, all of the same slip, and all
separated by the same amount of time). There is some evidence
for this, particularly among very small earthquakes on creeping faults.
Hypothesis
2: Earthquakes are 'time-predictable' (this means that the
larger the slip in the last earthquake, the longer the wait until the next
one.) This idea was formulated in the 1980's by Shimazaki and Nakata in
Japan, and has been widely used.
Hypothesis 3: Earthquakes occur randomly in
time and and have randomly varying size. (This 'Poisson' hypothesis
is also widely used, particularly
when little information about a fault and its past earthquakes is
available).
These hypotheses are briefly explained in:
- R. S. Stein,
- Parkfield's unfulfilled promise (News & Views), Nature,
419, pp. 257-258, 19 September issue, doi:10.1038/419257,
2002.
- [Printable
article (300 kb)]
Model
Description
The following is
a description of Ross Stein's "spring and rider" (also known as
the "brick and bungee") earthquake simulation machine.
The apparatus consists
of a wooden board 3-4 feet in length with a winch on one end. You can buy a small trailer winch from an auto parts store, a trailer supply store, or perhaps at Home Depot. There is a pulley leading from the winch to the brick that is
oriented so that the force acting on the brick has no vertical
component; this is not required but is helpful. The board has a strip of adhesive-backed sandpaper down half of it
to increase friction. Buy the nonskid at Home Depot type stores. Don't use the rubber-surfaced non-skid; instead it should have sand grains in it. The best material for the bungy cord is surgical rubber tubing, which you can buy from some pharmacies. If you can’t find this, you can use standard bungy cords.
Two types of material
connect a brick to the winch. The first is a non-stretching cord,
which is used to ensure that all of the accumulated stress is
transferred into the second material - surgical rubber tubing.
This tubing is connected directly to the brick and is extremely
elastic, which allows stress to build when the crank on the winch
is turned. The brick also has a strip of sandpaper on one
side, for additional friction. This is the basic set-up
of the experiment.
With this, students
can mark off "rupture length" during each "earthquake" by seeing
how far the brick slips. They likely will find that the
lengths are not consistent. They may also wish to time the
"earthquakes" assuming a constant speed by the person turning
the crank. Again, time is not always consistent either.
If they turn the crank slowly as the cord nears "failure" they
may hear the sandpaper crackle a moment before the brick moves,
thus simulating a foreshock.
There are some accessories
to this model as well. The first is talcum powder, which
can be sprinkled on the board next to the sandpaper. When the
brick is placed on the powder (the side of the brick without the
sandpaper) students will observe an almost constant rate of motion.
This simulates creeping faults, such as is found on part of the
San Andreas. An additional brick is included in the demonstration
as well. This may be stacked atop the first brick to produce
larger "earthquakes." It can also be removed from atop the
first brick just at the moment of failure to show that slippage
can occur even without additional stress, just by removing the
additional brick.
Finally, the second
brick is equipped with surgical tubing as well, so the two can
be placed on the board in tandem. In this way, the machine
demonstrates how earthquakes "talk" to each other. When
stress is sufficient, the first brick moves forward, increasing
stress on the second brick. Eventually the second brick
slips, reducing the backwards force on the first brick, and the
first brick can slip again.
One additional accessory
for the machine is a mass balance that can be used as a strain
gauge. Although it was previously shown that earthquakes
are not consistent with time or in their rupture length, the brick
tends to slip at the same reading on the gauge. With this set-up
and the various accessories, this demonstration can show a variety
of earthquake concepts. If the students use the mass balance
to weigh the bricks, they can calculate the coefficient of friction
on the board, and predict what force is necessary to cause an
"earthquake."
Despite being relatively
simple and elegant, the machine is remarkably true to the
actual earth. Thus students get a fun, hands-on look at
stress and rupture in the laboratory.
Earthquake machine diagrams
Technical
Specifications
FOR RESIDENTS OF THE S.F. BAY AREA, the USGS library maintains
a wonderful teacher resource facility with materials any teacher
can check out. Tel 650 329 5026 or 5028, USGS Library on Survey
lane off 345 Middlefield Road, Menlo Park (map http://online.wr.usgs.gov/kiosk/mparea3.html)
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