How does a seismograph work?
A seismograph works by recording earthquake waves. When an earthquake occurs, three types of waves are generated. The vibrations are detected by a delicately balanced beam; the movements of the beam are recorded on a moving tape.
The three types of waves arrive at different times after the quake. By comparing the arrival times of the waves at different seismographs throughout the world, it is possible to determine exactly where the earthquake took place.