# How do seismologists locate an earthquake?

Seismologists compare seismograms from several recording stations and use the difference in arrival times to work out the different distances the seismic wave must have travelled to each of the stations.

This activity uses two microphones to represent two monitoring stations.

## Task A: Exploring the kit

2. Open the sound-editing software: when you are ready to start, you should be able to see two input channels on the screen.
3. Get someone to clap once: what do you notice about the trace for each microphone? How is the response to a clap similar to a seismogram? Make sure you know which trace on your screen matches each microphone.
4. Now move the microphones so that they are as far apart as they can be without pulling the leads tight.
5. Get someone to clap just once again, standing the same distance away from each of the microphones.
6. What do you notice about the signal you get from each microphone?
7. What happens to the patterns you see on screen if the person who claps is standing much closer to one speaker than another?

The two diagrams each show simplified images of the two sound traces from microphones X and Y, produced when a student clapped at an unknown position (A, B or C) in the laboratory.

Use the sound traces to decide which location of A, B or C is the most likely for the source of the sound. How did you decide this?

Why can't you identify with certainty which position the source was at?

If you could also see the input from a third microphone at position Z, would you have been able to tell where the source was?

You could work out the difference in distance the sound travelled if you know the time scale on your screen and the speed of sound: suppose the separation of the two sharp peaks is equivalent to 0.005 seconds, and the speed of sound is 330 m/s, what is the difference in distance?

## Teachers' notes

How do seismologists locate an earthquake?

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.

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