Highway Seismograph—Bumping over the logs 
This activity is is used with permission from Paul Doherty
http://www.exo.net/~pauld/index.html or http://www.exploratorium.edu/faultline/basics/measure.html
Time: 10-minute homework
Materials
Target Grade Level: 6th grade and up
• A car, a driver, and a bumpy highway.
• A felt tip pen
• A pad of lined paper.
Content Objective
Students will learn about the Richter scale as they make
their own seismograms in a moving vehicle.
Introduction
As you drive down a highway, you can use an extended
arm and hand to model the operation of a seismograph.
To Do and Notice
While driving down the highway as a passenger in a car,
hold your arm straight out in front of you. Hold it near the
dashboard without touching the dash. Notice that as you
hit bumps your hand moves up and down relative to the
dashboard.
To record the motion, use one hand to hold a pad of paper
vertically against the dashboard. Hold the felt tip pen in the
other hand stretched out to arms length.
As you drive along, use the pen to trace the line from
left to right across the paper. Move the pen slowly. Try to
keep the pen line straight. Notice that when you hit a bump
the pen moves up and down making a seismogram-like
recording.
Sample of 1 to 10 on a
logarithmic scale.
What’s Going On?
When the car hits a bump it accelerates up. You are firmly
attached to the car and accelerate with it. Your hand and
arm are less firmly attached to the car and so their mass
and inertia cause them to lag behind the motion of your
body and the car. This is why your arm moves down
relative to the car.
In actuality your arm stays the same height above the earth,
but the dashboard moves up.
Math Root
Earthquakes are measured on a logarithmic scale. Let’s
create a logarithmic car bump scale called the “Pothole
scale.”
1. Measure the maximum amplitude of each recorded
bump.
2. Convert your measurement to micrometers.
3.Take the base 10 logarithm of the amplitude in
micrometers.
This is the bump rating on the pothole scale.
For example, a 1 cm amplitude bump is 104
micrometers or a 4 on the Pothole scale.
A Pothole 5 bump is 10 times larger than a Pothole 4.
So the pen motion for a pothole 5 is 10 cm.
A Pothole 6 bump is big enough to destroy your car!
So What ?
The Richter scale measures earthquakes. The Richter
magnitude takes the amplitude in micrometers of the
pen motion on a Wood-Anderson seismograph and takes
the base 10 logarithm of this amplitude. (The magnitude
is adjusted to make it what it would have been if the
earthquake were 100 km away.)
Section 2—Earthquake Activities
53