A seismograph is an instrument used to measure and record ground vibration caused by explosions and earthquake shock waves. In the late 1800s, John Milne (1850–1913), an English mining engineer, developed the first precise seismometer, the sensor in a seismograph that detects and measures motion. Since then, seismograms, the data recorded by a seismograph, have helped seismologists predict much more than Earth movement. These devices have also led to discoveries about the nature of the earth's core.

The process of using a tool to detect ground motion dates back to the ancient Han Dynasty when Chang Heng, a first-century Chinese astronomer and mathematician, invented the first seismometer. He used a pendulum connected to an eight-spoked wheel in which each spoke was connected to a mounted dragon's head with moveable jaws. When the pendulum moved during an earthquake, a bronze ball in each of the heads would pop out if...