When vibrations (ground motion)from a distant earthquake reach a seismograph, the inertia of the suspended mass keeps it relatively stationary, while the Earth and the case of the instrument move, oscillating around it.

Question

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This is the principle behind how a seismograph works. The stationary mass of the seismograph allows it to detect the ground motion caused by an earthquake.

When Vibrations (Ground Motion)from a Distant Earthquake Reach a Seismograph