How does a seismograph work

Earthquake for students

How earthquakes are measured

Earthquakes are measured with the help of seismometers. These devices are so sensitive that they can record the slightest vibrations in the ground that we humans do not feel. The magnitude of an earthquake is determined with the help of seismometers.

In principle, a seismometer works very simply. A heavy steel ball is suspended from a spiral spring that is attached to the top of the cross brace of a housing. When the earth shakes, the movements are transferred to the case, but not to the steel ball at the end of the hairspring. This remains in one place due to the inertia. The relative movement between the steel ball and the housing can be recorded by attaching a pen to the steel ball, which draws on a sheet which is attached to the housing and vibrates with it. In this way, the relative movement of the ground can be recorded as a change in length over time. This can then be used to determine the amplitude and magnitude of the earthquake oscillations. The device on which the vibrations of the seismometer are recorded is called a seismograph. The associated graphic is the seismogram. In the past, large drums with a sheet of paper clamped onto them were common. An ink needle transmitted the vibrations of the earthquake to the paper as the drum turned. So many vibration lines were recorded side by side and you only had to change paper once a day.

There are also modern acceleration sensors that can be used to register earthquakes. Such sensors are built into a smartphone, for example. While they are not as sensitive as a seismometer, they can be used to record stronger earthquakes. In order for this to work with a smartphone, you need a special app, which is available free of charge.