seismograph

An instrument that records the ground's motion during an earthquake — converting the shaking of the earth into a visual trace on paper or a digital signal. The principle is simple: a heavy mass suspended on a spring or pendulum remains still while the frame holding it moves with the ground, and the difference between the two is the record. Seismographs detect motion too faint for any human to feel, recording the earth's continuous, low-level trembling alongside the sudden violence of earthquakes.