A scale to measure the intensity of earthquakes. It is determined from the logarithm of the amplitude of ground waves, as recorded by seismographs. After adjustments for the variation in the distance between the various (carefully calibrated) seismographs and the epicentre of the earthquake a whole numbers and decimal fractions indicating the magnitude can be calculated.
A magnitude of 2 or less is called a microearthquake, they cannot even be felt by people and are recorded only on local seismographs. Events with magnitudes of about 4.5 or greater are strong enough to be recorded by seismographs all over the world. But the magnitude would have to be higher than 5 to be considered a moderate earthquake, and a large earthquake might be rated as magnitude 6 and major as 7. Great earthquakes (which occur once a year on average) have magnitudes of 8.0 or higher (British Columbia 1700, Chile 1960, Alaska 1964). The Richter Scale has no upper limit, but for the study of massive earthquakes the moment magnitude scale is used. The modified Mercalli Intensity Scale is used to describe earthquake effects on structures.
The actual ground motion for a magnitude 5 earthquake is about 1.1 mm at a distance of 10 kilometres from the epicentre.
Developed by Charles Richter of the California Institute of Technology in 1935 as a mathematical device to compare the magnitude of earthquakes, initially called the ML Scale (Magnitude Local).