How do seismologists locate an earthquake? Teachers' notes

Students investigate the effect of different 'earthquake' positions on the signals received by two 'seismic stations', using two microphones connected to a PC with sound-editing software.

Learning objectives

Students will:

  • compare the sound signal traces on the computer screen to a seismogram created by an earth tremor
  • investigate the effect of source distance from the microphones on signal strength
  • investigate the effect of source position on the arrival time of the signal at each microphone.


This is intended to be a relatively short activity to model how the size and arrival times of seismic waves vary with distance from the source. The comparison of traces is meant to be one of overall appearance, and does not need detailed knowledge of what a seismogram shows. There are two aspects to look at: amplitude and arrival time.

  • The difference in amplitude should be evident on the default images;
  • To look at the difference in arrival time, you need to zoom in on the traces.

Additionally, students might examine the time difference between the two signal peaks and relate this to the different distances travelled (using distance = speed x time, and speed of sound in air is roughly 330 m/s).

The examples in task B introduce a simplified model of how seismologists use the records from several stations to establish exactly where an earthquake occurred. This could be done as a demonstration or as a small-group activity.

It is important to check that the software is already installed on the computer(s) you plan to use and to confirm any further set-up details before you use this activity in class: in this case, you need to be able to see two channels of input on screen (i.e. stereo input). You can find details of how to download and use Audacity (free, open source software) in the pdf 'Downloading and using the Audacity software'.

It is important to make sure that the sound is travelling through the air - not through a table - to both microphones, so clapping is preferable to tapping the bench.

Resources needed

Each group will need: