Magnitude is a measure of the energy released during an earthquake, i.e. its strength. In principle, the greater the magnitude of an earthquake, the stronger the ground motion it will cause. Unlike an earthquake's intensity, which will vary with the distance from the earthquake, its magnitude is independent of its location (see the question "What is intensity?“).

Magnitude is a logarithmic value, and an increase of one magnitude signifies a roughly 30-fold increase in energy. So a magnitude 6 earthquake is thirty times stronger than a magnitude 5 earthquake and 900 (30 x 30) times stronger than a magnitude 4 earthquake. In other words, an 0.2 increase in magnitude signifies an earthquake of double the energy. Consequently, a magnitude 3.4 earthquake is roughly twice as powerful as a magnitude 3.2 earthquake.

The magnitude of an earthquake is usually determined by records called seismograms generated by seismic measuring devices called seismometers (see the questions "What is a seismogram?" and "What is a seismometer and how does it work?"). The first magnitude scale was developed by physicist and seismologist Charles Richter in 1935. Even today, the size of an earthquake in Switzerland is usually specified using the Richter scale. However, over time it became apparent that the Richter scale is only suitable for earthquakes of certain magnitudes occurring within specified distance ranges, so other magnitude scales have been developed (see the question "What types of magnitude are there?").

Earthquakes can be felt above a magnitude of approximately 2.5. Quakes with a magnitude of roughly 4.5 to 5.5 can be expected to cause minor damage to individual buildings, though in rare instances they may also cause serious damage. The impact of an earthquake depends very much on a building's distance from the hypo- and epicentre, how it was constructed and the properties of the local subsoil.

The most powerful earthquake ever measured, with a magnitude of 9.5, occurred in Chile in 1960. At the other end of the scale, some earthquakes have magnitudes of less than zero (see the question "What does minus magnitude mean?").

Comparing earthquakes' magnitudes

Comparing earthquakes' magnitudes

Potential consequences of earthquakes of different intensities and magnitudes

Potential consequences of earthquakes of different intensities and magnitudes