While we watching movies about the layers of the earth last week, I heard one of the students ask, “How do we know that?” Which is a great question because we haven’t dug down to the center of the earth, heck, we haven’t even gotten through the crust yet. So how do we know there is an iron core in the center of the earth surrounded by a molten outer core? Earthquakes! Earthquakes generate waves that travel through the earth and we can measure them with seismographs. Just like we use radar or sonar to detect things we can not see, we can use waves from earthquakes to learn about the center of the earth. There are two main types of body waves (waves that travel through the body of the earth), P – waves (primary or push/pull waves) and S-waves (secondary or shear waves).
The P waves travel faster than the S waves so they arrive at the seismograph first.
By looking at the difference in the arrival times of the two types of waves you can determine how far away the epicenter of the quake was from the seismograph. If you do this for 3 different seismographs you can triangulate the exact location of the epicenter. This is the lab the kids did this week, I gave them seismograms from three locations for a particular earthquake and they had to measure the arrive times off the data, figure the difference and then use a graph to find the distance from the epicenter. Then they took a compass and drew a circle of that radius on the map. Once they had done that for all three locations they could locate the epicenter where the three circles intersected. Sometimes they get a small triangle instead of a point and that’s ok. The quality of your end result really depends on your drawing compass, we didn’t have any locking compasses and it would have made this lab much easier if we had.
We watched the videos above as well as the ones below during class.
The lab we did was from a college course I taught many years ago but I found this updated version on the web. I skipped the bit about magnitude since my students are pretty young and most haven’t heard about logarithms but they did know that an earthquake with magnitude 7 is 10x worse than a 6, and 100x worse than a 5.