Seismologists use a seismograph to observe and study ground motion at a particular location. The instrument generally consists of two parts: a sensitive ground motion detector, called seismometer, and a recording system.

All seismometers are based on the same principle of inertia – a body in motion (or at rest) tends to stay in motion (at rest) unless acted upon by a force. Conventional seismometers consist of a mass and a spring that are suspended from a frame anchored in the ground, while the recording system can include a pen and a rotating drum attached to the mass and frame respectively.

If the earth moves, the frame moves along with the earth’s surface, whereas the mass remains stationary. The relative motion between the two is recorded and seismograms (graphs of ground motion over time) are obtained.