What is a magnitude ?

The strength of an earthquake is measured by its magnitude, which is calculated from the ground vibrations recorded by seismometers. The magnitude of an earthquake is a measurement made at a distance, but it represents what happened at the epicentre: it is therefore identical wherever you are. It is a bit like the power of a light bulb, described by a number in Watts: this number does not change whatever the distance from the bulb!

For earthquakes, it would all end there if there was only one way to measure the ground motion…

…if one measures the maximum amplitude of the seismic P or S waves recorded by seismometers within a few hundred kilometres of the epicentre, we obtain the “local magnitude“, noted ML. This is what ayiti-seismes does routinely in the framework of the OSMOSE project!

…if one measures the maximum amplitude of the first P-waves recorded more than 1000 km from the epicentre, we obtain the “body wave magnitude“, noted mb or mB. It is not calculated for small earthquakes because they are too weak to be recorded so far away.

…if one measures the maximum amplitude of the surface waves (see figure), which are slower and later than the P and S waves, we obtain the “surface wave magnitude“, noted MS.

…if one is able to use the seismic recordings to determine the amount of fault slippage and size of the fault, one can then calculate the “moment magnitude“, noted Mw. This is related to the mechanical energy released by the earthquake.

There are therefore several types of magnitude which are all presented on the same scale, but measure slightly different aspects of earthquakes. This explains why the same earthquake can sometimes be presented with slightly different magnitude values, depending on the type of data available to seismologists.

In addition, the number and location of seismometers used to measure a given earthquake varies from one seismological centre to another. This also has an influence on the calculated magnitude.

Finally, the magnitude estimate may vary slightly in the first minutes or hours after an earthquake, while seismologists collect the maximum amount of data available.