Dictionary:Magnitude of earthquake
A logarithmic measure of the strength of an earthquake or of the strain energy released by it, as determined by seismographic observations. Magnitude is a rating of an earthquake independent of the place of observation. The Richter scale of magnitude indicates the base-10 logarithm of the maximum amplitude observed (or which might have been observed) on an instrument of specified type 100 km from the epicenter. Empirical tables correct observations at other distances. Each step of one in magnitude means multiplying the amplitude by 10. The largest earthquakes are of magnitude about 9. Zero represented the smallest recorded earthquakes when the scale was devised but microearthquakes can have negative Richter values. Various relations are used to give the order of magnitude of the energy released in an earthquake from the magnitude M. Roughly, the energy E in ergs is given approximately by logE=10+2M; Bath (1966) gives log10E=12.24+1.44 M ergs. Distinction is sometimes made between magnitude based on body-wave (Mb) versus surface-wave (Ms) measurements. See also seismic moment.