The earth's surface is not one undifferentiated whole but is composed of a number of separate pieces called plates each of which is in constant motion relative to the others; and it is this constant movement that results in the diversity of phenomena that we have come to associate with the earth, including earthquakes.
An earthquake is a tremor of the earth's surface that is brought about by a release of underground stress. Most earthquakes occur along the lines where the great plates come into contact one with another; when in the course their movements stress is released, vibrations in the form of seismic waves are sent through the earth and it is these vibrations that we feel when we say there is an earthquake. More infrequently, earthquakes occur within the plates themselves rather than at the fault lines between plates; what causes these intraplate earthquakes, as they are called, is currently unknown. These types of earthquakes, known as tectonic earthquakes, constitute the vast majority of all earthquakes.
Other than these tectonic quakes, there are some quakes that are directly related to volcanic activity and come by as a result of the movement of magma deep within the earth that result in volcanic eruptions. Earthquakes happen constantly, which is not surprising given the fact that the plates are in constant motion but most of them are so small that they can be detected only by the most sensitive instruments.
To determine how severe a quake is, both the quake's magnitude and its intensity are used. Magnitude measures the severity of a quake by indirect measurement of the energy released by the quake, while intensity measures the effects that the quake has on the earth's surface and how it has impacted on humans, animals, structures, etc. The intensity of a quake can differ from point to point on the earth's surface since what is being measured is the relationship between the quake and humans and other factors, while magnitude ought to be the same, since the energy discharge is the same whatever effect it might have at a particular place. In practice though, measurements of magnitude may differ slightly from one measurement to another depending on the mode of measurement, the criteria used etc.
The local magnitude scale (M1), popularly referred to as the Richter scale, is a base 10 logarithmic scale whose measurements are determined by calculating the logarithm of the quakes combined horizontal amplitude, and it was developed in 1935 by Charles Richter and Beno Gutenberg of the California Institute of Technology in order to determine the relative magnitudes of Southern California earthquakes using a properly calibrated seismometer at a distance of up to 600 kilometres from the quake epicentre. So, an earthquake with a magnitude of 5 on the scale is about 10 times as strong as one with a magnitude of 4. However, at distances more than 600 kilometres the calculations are unreliable as regards the size of a quake and, furthermore, since the scale has an upper limit of around 6.8, all large earthquakes will have a local magnitude of this order irrespective of the actual severity of the quake.
In order to address the shortcomings of M1, the moment magnitude scale, (MMS or Mw), was devised by Tom Hanks and Hiroo Kanamon of Harvard University in 1979. The scale measures the seismic moment, M0, (i.e. the total amount of energy transformed during the event) of the quake and it is also a logarithmic scale which is consistent with M1 in as far as M1 goes, so that for medium sized quakes, both scales will deliver approximately the same readings. Further, Mw has no fixed upper limit to the possible magnitude of the quake being measured. Mw is now the scale that is used for all large earthquakes although it is not so commonly used for smaller quakes. The US Geological Survey, for instance does not use the scale for quakes that are less than 3.5 in magnitude. An increase of one step on Mw corresponds to over 30 times the amount of energy released.
M1 and Mw are not the only scales that are used for measuring the magnitude of quakes; scales have been devised to measure body wave, surface wave and duration magnitudes and each has been designed to give values similar to M1. However, because each focuses upon one particular aspect, there is a tendency to underestimate the total power of larger quakes. Also, all of these scales measure just magnitude and have nothing to do with intensity; there are several scales which have been developed to measure the intensity of earthquakes such as the Modified Mercalli Scale and the European Microseismic Scale.