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About the Data

About New Madrid earthquakes

Most earthquakes (ground shakings) are caused by relatively fast movement of one fault block past another fault block along the planar break between the two blocks of rocks (the fault). If the fault blocks move relatively fast (a few centimeters or decimeters in tens of seconds), ground shaking will be produced; there is a direct cause and effect between fault movement and earthquakes.

The size of the earthquake can be measured in many independent ways. For example, the Mercalli scale is based upon human response and structural damage, the Richter scale is based upon the amplitude of the ground shaking, and the Moment scale is based upon the amount of fault movement and the area of fault rupture or breakage. The latter two scales are machine based and hence more objective than the somewhat subjective but socially relevant Mercalli scale. None of these three scales are directly comparable to each other.

Earthquakes may or may not be predictable, depending upon which seismologist you ask and how you define "predictable." Regardless, one of the first and most important tasks is to quantify the frequency distribution of earthquakes in your area or along your fault of interest. How often do earthquakes of a given size occur? The data given in the table and the diagram represent approximately 200 years of earthquake activity in the New Madrid area of southeastern Missouri and adjacent states. The magnitude of the earthquakes is based on the Richter scale (magnitude = log (size)); the sizes of older, pre-instrumental earthquakes have been estimated. The frequency of each magnitude has been expressed in terms of reverse cumulative frequency; that is, the frequency of an earthquake of magnitudes greater than or equal to size X.

The data fall on a straight line in log-log space, indicating an excellent fit to a power law with a negative exponent. Students can determine the parameters of the line through linear regression, then calculate the corresponding parameters of the power law using algebra. As the period of record is known (167 years), the recurrence interval of earthquakes of a given size can be calculated. Students can calculate the recurrence interval of a large quake that is not part of the data set; a magnitude 8, for example.

Can the recurrence interval be used to predict earthquakes? Students should think about the sizes of auto accidents on the local freeway; can the recurrence interval of auto accidents be used to predict the next auto accident?

The Center for Earthquake Research and Information's website is a rich source of data and information, including an online customizable New Madrid earthquake database: http://www.ceri.memphis.edu/

Johnston AC and Nava SJ (1985), Recurrence rates and probability estimates for the New Madrid seismic zone; J. Geophysical Research, v. 90, pp. 6737-6753.

Johnston AC and Shedlock KM (1992), Overview of research in the New Madrid seismic zone; Seismological Research Letters, v. 63, pp. 193-208.

Johnston AC and Schweig ES (1996), The enigma of the New Madrid earthquakes of 1811-1812; Annual Review of Earth and Planetary Science, v. 24, pp. 339-384.

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 Earthquakes in the New Madrid Fault System, Missouri Center for Earthquake Information and Research Earthquakes from 1816-1983 Richter magnitude and/or estimated RM for older quakes logarithm of the reverse cumulative frequency (quakes > magnitude X) magnitude log (rev. cum. freq.) 1.66 1.89 1.75 1.83 1.84 1.72 1.95 1.61 2.10 1.52 2.12 1.43 2.22 1.32 2.33 1.15 2.50 1.08 2.57 1.04 2.67 0.94 2.78 0.79 2.85 0.69 2.98 0.61 3.05 0.56 3.19 0.49 3.32 0.16 3.40 0.31 3.52 0.20 3.63 0.15 3.72 0.12 3.82 -0.05 3.94 -0.10 4.04 -0.15 4.10 -0.21 4.36 -0.43 4.46 -0.55 4.52 -0.69 4.69 -0.72 4.79 -0.98 4.86 -1.15 4.95 -1.23 5.24 -1.37 5.45 -1.49 5.63 -1.55 5.72 -1.70 5.98 -1.89

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