My Science Box

Teacher Background
P and S waves: Image courtesy of USGS.An earthquake has struck somewhere in California! Can you figure out where? If you learn to read a seismogram you can!

First some earthquake basics… (The following background information is provided in greater detail on the student handout.) There are faults (cracks in the Earth’s surface) that can suddenly move as pressure from the movement of the Earth’s crust builds up. This sudden movement is an earthquake.

An earthquake will generate different types of waves that travel through the earth and along its surface. Several different types of earthquake waves are triggered with every earthquake. Each wave makes particles in the soil move in different ways and travels at different speeds. For our purposes, we will focus on p waves and s waves.

P waves (primary waves) are side-to side compression waves and travel quickly through the Earth. S waves (secondary waves) are up-and-down waves and are typically more destructive. In an earthquake, s waves travel more slowly than p waves. Thus, even though p and s waves start at the same time from the epicenter of a quake, the farther they travel, the greater the delay between the p and s waves.

Seismogram: Image created by Crickett.Earthquakes are recorded on instruments called seismographs which make recordings called seismograms. The x axis represents time while the y axis represents amplitude. The time axis can show the lag between when the p and s waves arrive and can thus be used to calculate the distance between the epicenter and the location of the seismograph. The amplitude axis reflects the strength of the shaking and can be used to calculate the magnitude of the earthquake.

The Virtual Courseware program takes students step by step through these calculations. In the “Travel Time” activity, students learn the relationship between p and s wave lag time and the distance from the epicenter. In the “Epicenter and Magnitude” activity, students use seismogram recordings to determine the epicenter and magnitude of an unknown earthquake.

Student Prerequisites
It is recommended that students are familiar with seismographs, seismograms, the difference between p and s waves, and reading latitude and longitude from a map before using the Virtual Courseware software. The student handout has a quick summary of this information, but 5 minutes to illustrate p and s waves with a slinky and to show students a seismogram before letting a sub take over would be helpful.

The program does provide a tutorial section that will show students a seismogram being generated and the propagation fronts of a p and s wave as they travel outward from an earthquake epicenter (the “SP Lag Time” tutorial). There is a second tutorial describing how to read latitude and longitude information (the “Latitude/Longitude” tutorial). If you don’t have time to preteach these concepts, then the tutorials can serve as a prelude to the 2 activities.