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Major earthquakes are those with magnitudes greater than 6, and great earthquakes have magnitudes greater than 8 on the Richter Scale. The Richter Scale is a continuous scale, so that an earthquake can have
a magnitude 5.7, 5.8, and so on. There are, on average, 20,000 earthquakes with a magnitude greater than 4 occurring somewhere in the world each day.
The Modified Mercalli Scale expresses the intensity of an earthquake’s effects in a given locality in classes ranging from I to XII. This ranges from I – not felt except by a very few people under especially favourable conditions, to XII – damage total.
The Richter Scale relies on seismograms, which
is why it is able to pick up the earthquakes worldwide. The Mercalli Scale relies on eyewitness reports and
is used only for populated areas. The maximum intensity experienced in the Alaska earthquake of 1964 was X; damage from the San Francisco 1906 and New Madrid 1812 earthquakes reached XI.
The reason why two scales are used is that large magnitude earthquakes do not necessarily cause the most intense surface effects. This is because the effect of an earthquake depends on the period of the waves and the materials they pass through. An area with unstable ground underneath it (sand, clay, or other unconsolidated materials), for example, will shake like a bowl of jelly, amplifying the waves, while an area equally distant from an earthquake’s epicentre firm ground, such as granite, underneath it will experience much smaller effects.
An earthquake’s destructiveness depends on
many factors. In addition to magnitude and the
local geological conditions, they include the focal depth of the earthquake.
The geology of an earthquake
The nature of earthquakes is most easily understood through an example. A relatively recent and very well- documented earthquake occurred in Los Angeles in 1994. It was called the Northridge earthquake.
The Los Angeles area (the San Fernando Valley) is a