<P> The first scale for measuring earthquake magnitudes was developed by Charles F. Richter in 1935 . Subsequent scales (see seismic magnitude scales) have retained a key feature, where each unit represents a ten-fold difference in the amplitude of the ground shaking, and a 32-fold difference in energy . Subsequent scales are also adjusted to have approximately the same numeric value within the limits of the scale . </P> <P> Although the mass media commonly reports earthquake magnitudes as "Richter magnitude" or "Richter scale", standard practice by most seismological authorities is to express an earthquake's strength on the moment magnitude scale, which is based on the actual energy released by an earthquake . </P> <P> It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation . About 100,000 of these can be felt . Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in El Salvador, Mexico, Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, India, Nepal and Japan, but earthquakes can occur almost anywhere, including Downstate New York, England, and Australia . Larger earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5 . In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are: an earthquake of 3.7--4.6 every year, an earthquake of 4.7--5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years . This is an example of the Gutenberg--Richter law . </P> <P> The number of seismic stations has increased from about 350 in 1931 to many thousands today . As a result, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes . The United States Geological Survey estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0--7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable . In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical fluctuation rather than a systematic trend . More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey (USGS). A recent increase in the number of major earthquakes has been noted, which could be explained by a cyclical pattern of periods of intense tectonic activity, interspersed with longer periods of low - intensity . However, accurate recordings of earthquakes only began in the early 1900s, so it is too early to categorically state that this is the case . </P>

Where did the first large earthquake occur and what was its magnitude