How do seismographs turn ground motion into earthquake records?

A seismograph uses a mass suspended so that it tends to stay still while the ground moves beneath it. The relative movement between the ground and the suspended mass is recorded electronically or mechanically as a seismogram. Seismologists analyze the arrival times and amplitudes of P waves, S waves, and surface waves to estimate location and magnitude. The first practical seismographs were developed in the late 19th century, and today networks of digital instruments monitor the planet continuously. These records help scientists locate earthquakes and study Earth's interior.