How does a seismograph work?

A seismograph is an instrument that detects and records the vibrations of the ground caused by earthquakes and other events. Its key principle relies on inertia: a heavy mass is suspended, for example from a spring, so that when the ground shakes, the frame of the instrument moves with the earth while the suspended mass tends to stay still because of its inertia. The relative motion between the moving frame and the stationary mass is recorded, traditionally by a pen tracing on a rotating drum of paper, and today by sensitive electronic sensors that produce a digital record called a seismogram. By analysing the timing and size of the recorded waves at multiple stations, scientists can determine an earthquake location, depth, and magnitude. Networks of seismographs around the world constantly monitor seismic activity, aiding earthquake science and detecting underground nuclear tests.