The magnitude of an earthquake is a number that relates to the amplitude of the earthquake. Earthquake magnitude scales are logarithmic (i.e. a 1 unit increase in magnitude corresponds to a 10 fold increase in amplitude). Scientists can only estimate the true magnitude of an earthquake by measuring its effects, this leads to earthquakes appearing to have different magnitudes depending on what method is used for estimating the magnitude and which datasets have been used to make this estimate. Press reporters love the Richter scale and will report any earthquake magnitude as a ‘magnitude on the Richter scale’, however for any large earthquake that has made the news it is very unlikely that a Richter (or
Local magnitude) is an appropriate scale. Due to the complexities of the calculations the reported magnitude of events can change as more data
gets analysed. For the devastating Dec 24th 2004 event in Sumatra the original magnitude of Mw 9.0 was later recalculated to Mw 9.3 as more
data was analysed.