How does a portable earthquake generator work?

Question: How does a portable earthquake generator work?
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Asked by Galaxies to Lidunka on 14 Jun 2015.
- Lidunka Vocadlo answered on 14 Jun 2015:
  
  we get a lump of rock, squash it, it fractures (breaks), and this releases “seismic waves” – sound waves which travel through the rock and which get picked up by microphones and transmitted to a computer.

Question: How does a portable earthquake generator work?