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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