richter scale

and in case anybody is worried that a 5 sounds reasonably high on the richter scale
especially if a 7 or 8 is really really destructive
remember a 6 is 10 times bigger than a 5

for all you maths nerds, with thanks to wikipedia:~

The Richter magnitude test scale (or more correctly local magnitude ML scale) assigns a single number to quantify the size of an earthquake. It is a base-10 logarithmic scale obtained by calculating the logarithm of the combined horizontal amplitude of the largest displacement from zero on a seismogram. For example, an earthquake of magnitude 5 is ten times greater than one of magnitude 4 and an earthquake of magnitude 8 is 10(8 − 4) or 10000 times greater than one of magnitude 4.
Since the energy of an earthquake is proportional to the square root of the cube of the amplitude, each step of the Richter scale has an energy 103/2 (~ 31.6) times that of the previous step. So while a magnitude 9 has 10,000 times the amplitude of a magnitude 5 earthquake, it has a million times more energy.
The diminution of amplitude due to distance between the earthquake epicenter and the seismometer is corrected for by subtracting the common logarithm of the expected amplitude of a magnitude 0 event at that distance. This correction for distance is intended to make the local magnitude an absolute measure of earthquake size.
The magnitude of the earthquake, M, is given by:
M = log10A + 3log10(8Δt − 2.92)
where A is amplitude in millimeters and t is time in seconds.