How do seismographs measure earthquakes?

A seismograph measures how the ground moves during an earthquake. In a classic design, a heavy mass remains relatively still while the Earth moves underneath, and the relative motion is recorded as a wavy line called a seismogram. Modern seismometers are highly sensitive electronic devices that can detect very small vibrations. By comparing records from multiple stations, scientists can locate the quake's epicentre and estimate its magnitude. The Richter scale was introduced in 1935, but today the moment magnitude scale is widely used for major earthquakes.