Elastic waves that move through the Earth are called seismic waves. Want to study them more closely? SeisMac is a free application that displays the output from the Sudden Motion Sensor in recent Mac laptops as a real-time, three-axis, acceleration graph, or, more commonly, a seismogram.
Once your class has received some instruction
on seismic waves, the following activity can
leverage SeisMac technology to help students
understand how a seismometer records ground motions. Consider: What information does a threecomponent seismogram express? How are S and P waves identified on a seismogram? How do seismic waves travel through the Earth?
1. Put your laptop on a table and project SeisMac onto a screen.
Discuss: What do you think this program is
designed to do? The computer and software are
recording the “acceleration” of the floor in the
classroom (like a seismograph). Why might we want to know how the ground moves? View the Kobe earthquake video clip.
2. Discuss: Why are there three traces? Propose
experiments to explain the three traces. Try
experiments (e.g. bump the table in the X, Y and
What is the relationship between these physical
motions and the graph? Motion has three
components (X, Y, Z), and each is represented on
a separate trace on the graph.
3. Hand out a three-component seismogram
from an earthquake. Discuss: What does this
graph say? How is this graph similar to what
can be seen on the screen? The seismic data are represented in a three-component graph similar to the one produced by SeisMac.
Review seismic waves. Based on what we already know about waves, pick the P (the first signal to arrive) and S (the second) wave arrivals on the seismogram. In which component is most of the energy for the P wave? Or for the S wave? Why?
4. Try to replicate the motion shown in this
seismogram by moving the laptop. Replicate the
motion of the P arrival first, then the motion for
the S arrival. Finally, put the two together.
Discuss: How did the ground move for the P
wave? Or for the S wave? Based on what you
already know about particle motion compared
to the direction of travel for seismic waves (particles move parallel to the direction of wave propagation for P waves), how did the seismic energy arrive at the station that recorded this earthquake?
Draw a diagram. Waves from distant earthquakes follow a curving path through the Earth. As a result, they arrive at seismic stations from below, and thus most of the energy from the P wave is in the vertical component.
5. To connect the seismograms you have studied
to actual earthquake ground motion, watch the
earthquake video again and identify the arrivals
of the different seismic waves in the video.