An earthquake is the result of a sudden release of energy due to an
adjustment in the earth's crust. This adjustment causes the
ground to tremble and produces vibrations that radiate out from the focus of the quake. The focus is the location of the "point"
within the earth where the energy of the earthquake is released. The location of the "point" on the surface of the earth, directly
above the focus , is called the epicenter .
What Causes Earthquakes?
Earthquakes can be produced by sudden natural processes such as volcanic
explosions and meteorite impacts. However,
earthquakes are primarily caused by a sudden release of stress built up over time in rocks below the earth's surface. In South
Dakota, the most likely causes of this built up stress would be from plate movements along faults and isostatic (glacial) rebound.
The earth's crust bends and folds. In this process rocks break into blocks along zones of weakness. When two rock bodies or blocks slide by one another (horizontally and/or vertically), this motion is called faulting . The process of faulting may be a very large-scale or a small-scale phenomenon. Although some surface fault lines have been mapped in South Dakota, they appear to be old and inactive. The quakes we have experienced in South Dakota have been low intensity ones in which the fault movement is limited to the subsurface, with no displacement occurring along fault lines. The hypothesis that faulting has caused earthquakes in our state is based on data from places where it has been shown that these mechanisms have caused quakes.
Isostatic rebound, is another cause of earthquakes. The eastern half of South Dakota was covered by glaciers; the last glacier retreated from the state approximately 10,000 years ago. It has been estimated that the ice was 500 to 1000 feet thick at the southernmost edge of the glacier, along the present-day Missouri River valley. The weight of the glacier caused the underlying rocks and sediments to be compressed and pushed downwards. After the ice retreated, the rocks gradually rebounded upwards. This will continue until equilibrium of pressure in the earth's crust is attained.
How Are They Measured?
Seismic vibrations produced by an earthquake are recorded by a special instrument called a seismograph. Readings from a minimum of three seismographs in different locations must be measured to locate the focus of an earthquake and its epicenter . Seismographs also can be used to measure the magnitude of an earthquake.
The magnitude of an earthquake as derived from a seismograph is given a value on the Richter scale . The Richter scale is a logarithmic measure. This means that a quake of a magnitude of 3 has 10 times greater ground motion than a quake having the magnitude of 2.
The intensity of an earthquake may be measured or rated in everyday human terms and experiences, using the modified Mercalli scale . This scale relates the intensity of an earthquake at a particular location to its effect on people, our structures, and the earth's surface (see Table 1). On the right hand side of Table 1, the Richter scale magnitude is also given for comparison with the maximum intensity values of the Mercalli scale.
How Many Earthquakes Have Been Recorded In South Dakota?
Earthquakes are measured in South Dakota as a part of the United States Geological Survey (USGS) national seismograph network. The Golden Colorado office of the USGS monitors the seismograph stations in our region. Since the midwestern states are relatively stable in terms of earthquake activity, only a few seismograph stations are located in the area. The only one in South Dakota is in Rapid City and that has only been in operation since August 1991. Other stations near South Dakota are in Golden, Colorado and French Village, Missouri. There is also some equipment, which can sense earth movements, at the dams along the Missouri River
Between 1872 and 1991, at least 65 earthquakes have been recorded with epicenters in South Dakota (Hammond, 1992). As illustrated in Figure 1, the earthquakes occur primarily on the eastern and southern parts of the state. The numbers used to identify the location of each epicenter are in chronological order, from map listing 1 which occurred in 1872 to map listing 65 which occurred in 1991.
An individual listing of each South Dakota earthquake since 1872 is given in Table 2. This table includes the date and time of each event, its location as determined by latitude and longitude, and its intensity on the modified Mercalli scale . Most of the earthquakes in South Dakota are relatively weak, the strongest ones reported with intensities of VI on the modified Mercalli scale (Hammond, 1992). There has been no known major rerouting of streams or other major physiographic change in the state as the result of earthquake activity.
Epicenter - the point of the earth's surface directly above the
focus of an earthquake.
Faulting - a fracture or a zone of fractures along which there has been displacement of the sides relative to one another, parallel to the fracture.
Focus - the point within the earth which is the center of the earthquake.
Isostatic Rebound - the land surface rising in elevation, in delayed response to the melting and retreat of ice sheets that once covered the eastern half of S.D.
Logarithmic - usually referring to numbers that are exponents of 10. A 2 in a logarithmic scale means 10 to the second power, or 100. A 3 would be 10 cubed, or 1000.
Mercalli scale - an arbitrary scale of earthquake intensity, ranging from I (detectable only instrumentally) to XII (causing almost total destruction). It is named after Guiseppi Mercalli, an Italian geologist who devised it in 1902. Its adaptation to North American conditions is known as the modified Mercalli scale.
Physiographic - referring to the physical features and phenomena of nature.
Richter scale - a numerical scale of earthquake magnitude, devised in 1935 by the seismologist C. F. Richter. This scale is logarithmic so that a quake of 3 is actually 10 times stronger than a quake of 2.
Seismic - pertaining to an earthquake or earth vibration, including those that are artificially induced.
Glossary terms are adapted from Bates, R. L. and Jackson, J.A., 1987,
Glossary of Geology , American Geological Institute,
Hammond, R.H. 1992. Recorded Earthquakes in South Dakota, 1872-1991: South Dakota Geological Survey map.
Await the Quake , board game from Lawrence Hall of Science, Univ. of CA, Berkeley CA, 94720 (grades 1 to adult).
Numerous brochures and pamphlets are available from the United States Geological Survey, Rm. 408, Fed. Bldg.., 200 4th Street SW, Huron, SD 57350..
A pamphlet on Earthquakes in South Dakota, by Dr. Val Ansfield, will soon be published by the South Dakota Geological Survey, Akeley Science Center, USD, Vermillion, SD 57069.
(See Natural Source Directory for phone numbers.)
South Dakota Geological Survey, Akeley Science Center, USD, Vermillion, SD 57069.
United States Geological Survey, 200 4th Street SW, Huron, SD 57350.
S. D. School of Mines and Technology: Geology Dept., 501 E. St. Joseph Ave., Rapid City, SD 57701.
University of South Dakota: Earth Sciences and Physics Dept., 414 E. Clark St., Vermillion, SD 57069.
Sarah Chadima, S.D. Geological Survey, USD Vermillion, SD 57069. 1992.
Dr. Val Ansfield, Dept. of Earth Sciences, USD, Vermillion, SD 57069.
Publication of the South Dakota Earthquakes fact sheet was funded by the Prairie Pothole Joint Venture of the North American Waterfowl Management Plan.