Geoscience Reference
In-Depth Information
Earthquake magnitudes
The Richter and Mercalli scales plot the magnitude of earthquakes. The former measures
the actual wave energy, while the latter charts the destructive effects. The Richter scale is
logarithmic, so, just using the numbers one to ten, it can accommodate everything from the
frequent daily tremors that go by almost unnoticed in a seismically active area to the largest
earthquakes recorded - to date, the largest has been on the coast of Chile in 1960, which
measured 9.5 on the scale. The difference in energy between each point on the scale is a
factor of 30. So, for example, a factor 7 quake is likely to be much more destructive than a
factor 6. Ironically, many of the personal records of Charles Richter, the Californian seis-
mologist who gave his name to the scale and who died in 1985, were destroyed in a house
fire following the magnitude 6.6 Northridge earthquake near Los Angeles in 1994.
20. The distribution of major crustal earthquakes (magnitude 5 and above) in the past
30 years. Most cluster along tectonic plate boundaries, but a few occur mid-continent.
The most famous crack in the world
In California earthquakes are almost a regular feature of life. The great Pacific plate is on
the move, not diving beneath the American continent but grating past it in what is called a
strike-slip fault. The junction is seldom a straight line, so kinks in the main fault line result
