How does a seismograph detect vibrations from the passing wave front of an earthquake?
1) The instrument vibrates with the ground as it rolls along a track above the hypocentre
2) The wave front seeks out the instrument which stops vibrating when the wave front reaches it
3) The instrument is anchored to the ground or buried in a vault so that it is stationary with respect the the earth. When the instrument moves, the suspended mass inside is relatively stationary due to inertia. the relative motion measures the quake
4) The whole instrument moves up and down or back and forth on the ground so that is starts jiggling a little weight inside that trips the detector to record the earthquake