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About the frequency of large earthquakes

Earthquakes (ground shaking) are caused by relatively fast movement along breaks in the Earth's crust, known as faults. In general, the more movement or displacement along a fault during an event, the greater the amount of ground shaking. Ground shaking is also affected by the properties of the Earth materials through which the vibrations pass; weak soils or loose sediment can enhance ground shaking.

There are several different methods of measuring the size of an earthquake; a popular measure of earthquake size is the Richter magnitude. The Richter magnitude is based upon the amplitude (height) of ground shaking as recorded on a machine (a seismometer). Earthquakes vary in size over many orders of magnitude, and therefore the Richter scale is logarithmic to accommodate this wide variation in earthquake sizes. On the logarithmic Richter scale, a magnitude 7 earthquake is 10 times bigger than a magnitude 6 earthquake. Earthquakes that are magnitude 7 or above are often (though not always) very damaging earthquakes.

The United States Geological Survey has compiled information on the number of earthquakes around the globe that have equaled or exceeded magnitude 7. The data are expressed as the number of earthquakes per year for each year starting in 1900. Note that the older data in this table are somewhat suspect, because seismometers did not systematically record earthquakes in a standardized fashion with good spatial coverage until the 1930's or later.

What is the recurrence interval for large earthquakes globally? How often do large earthquakes occur? These questions are fundamental to understanding the impact of earthquakes on the human population. We can better visualize the data using a histogram of the frequency of large earthquakes per year. This particular graph is a little challenging, as it shows the frequency of rates of earthquakes.

The data are unimodal, with a mean around 20 large earthquakes per year on Earth. There is a wide variation in earthquake rate, from a low of 6 quakes per year in 1986 to a high of 41 quakes per year in 1943. The distribution of earthquake rates is skewed slightly to the right.

Students could be queried also about the temporal variation in large earthquakes.  General information on earthquakes can be found at the US Geological Survey's website: http://www.usgs.gov/ .

Reference: United States Geological Survey, published in the New York Times. We haven't found the ultimate source yet.

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 year number of quakes >7.0 1900 13 1901 14 1902 8 1903 10 1904 16 1905 26 1906 32 1907 27 1908 18 1909 32 1910 36 1911 24 1912 22 1913 23 1914 22 1915 18 1916 25 1917 21 1918 21 1919 14 1920 8 1921 11 1922 14 1923 23 1924 18 1925 17 1926 19 1927 20 1928 22 1929 19 1930 13 1931 26 1932 13 1933 14 1934 22 1935 24 1936 21 1937 22 1938 26 1939 21 1940 23 1941 24 1942 27 1943 41 1944 31 1945 27 1946 35 1947 26 1948 28 1949 36 1950 29 1951 21 1952 17 1953 22 1954 17 1955 19 1956 15 1957 34 1958 10 1959 15 1960 22 1961 18 1962 15 1963 20 1964 15 1965 22 1966 19 1967 16 1968 30 1969 27 1970 29 1971 23 1972 20 1973 16 1974 21 1975 21 1976 25 1977 16 1978 18 1979 15 1980 18 1981 14 1982 10 1983 15 1984 8 1985 15 1986 6 1987 11 1988 8 1989 7 1990 13 1991 10 1992 23 1993 16 1994 15 1995 25 1996 22 1997 20 1998 16 1999 11

 Bin Frequency 5-10 6 10-15 16 15-20 27 20-25 28 25-30 14 30-35 5 35-40 3 40-45 1

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