Trembling in our Books

Yesterday we did three activities related to plate tectonics: making a
model of continental motion and generating predictions from it;
locating earthquakes; and radioisotope dating of rocks.  The second
activity followed roughly the reasoning outlined here.  However, I
didn't want to get into S and P waves, so instead of measuring the
distance from the epicenter to the seismograph by analyzing the wave
form, I decided to "simplify" and give students the time of arrival at
the three seismographs.  Only after we started the activity did I
realize that although the timing information I gave was sufficient,
some serious algebra would be required to solve the problem with just
that information.  So I ended up giving them the distance from the
earthquake to one of the seismographs, just to get them started.

Using timing information to solve for a location is an important
problem with many real-world aspects.  For example, GPS uses exactly
the kind of reasoning shown in the last figure of the page linked to
above, but in full 3-d with satellites distributed around the Earth,
to solve for your full 3-d location.  So I like the pure-timing aspect
of my version of the activity, but I have to find a way to make
workable for 5-7 graders.

Still, I don't think the kids noticed all this scrambling going on
behind the scenes.  They got the main ideas: the intersections of two
circles are the candidate epicenters based on two seismographs, and a
third seismograph can be used to resolve the ambiguity.  And they had
fun finding the mystery location of the epicenter.  I think we took
about 40 minutes on this activity, including a 5-minute opening
discussion on the link between earthquakes and our previous activity.