Fall 2001
Vol. 7.3
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| Kansas Geological Survey | Fall 2001 |
Vol. 7.3 |
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Kansas Earthquakes
CONTENTS Kansas Earthquakes–page 1
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Now
and then, Kansans are reminded that earthquakes don’t always happen
somewhere else. Just last July, a minor quake measuring 3.0 on the Richter
scale was centered between Augusta and El Dorado in Butler County. Compared
to the devastating earthquakes on the west coast of the U.S. and in other
parts of the world, Kansas quakes are fairly mild, and most Kansans figure
they don’t have anything to worry about. But according to some seismologists,
this complacency could be risky. The July 24th quake in Butler County was just strong enough to be felt.
The quake shook computer screens at the city hall in Augusta and rattled
several residences in the area, but no damage or injuries were reported. The Butler County earthquake is associated with a deeply buried feature
known as the Nemaha uplift that extends from Omaha, Nebraska, to Oklahoma
City, Oklahoma, passing roughly through Manhattan and El Dorado. This
uplift formed about 300 million years ago, and the faults associated with
it are still slightly active today, especially the Humboldt fault zone
that flanks the eastern edge of the uplift. Very little evidence of these
faults appears at the surface. Earthquakes associated with the Nemaha
uplift are probably a result of minor adjustments in deep-seated rocks. The largest recorded Kansas earthquake hit the Manhattan area in 1867,
toppling chimneys, cracking walls and foundations, and inflicting several
minor injuries. The estimated magnitude 5.5 earthquake was felt as far
away as Dubuque, Iowa. Since 1867, more than 25 earthquakes with origins
in Kansas have been felt; before then, earthquakes generally went unreported. While most earthquakes that occur in Kansas are mild, a moderate to large
earthquake in east-central Kansas is possible. And with critical structures
such as major dams and power plants, even a low-probability earthquake
has to be taken seriously. Currently, Tuttle Creek Dam is being studied by seismologists at the
U.S. Army Corps of Engineers. The study found that a magnitude 5.7 earthquake,
located 12 miles from the dam (at the Humboldt fault), is the smallest
earthquake that could potentially cause significant damage. This hypothetical
earthquake would probably not cause consequential movement of the dam,
but it could damage relief wells that control the flow of water under
the dam and lead to dam failure. A seismic event of this nature has a
probability of occurrence of about once in 1,800 years. On the other hand, a magnitude 6.6 earthquake would cause the sand deposits under the dam to liquefy, or turn to quicksand, and lose their ability to support the dam. This, in turn, would allow the base of the dam to spread and the top to drop, and cracking would significantly reduce the ability of the dam to hold water. Although the top of the dam would probably not drop below the lake level, cracking and deformation could allow water to seep through the dam, leading to internal erosion, and eventually, uncontrolled release of the lake. This scenario has a very low probability of occurrence of about once in 10,000 years. The U.S. Geological Survey’s National Earthquake Information Center
maintains a website at http://neic.usgs.gov/neis/.
This site has locations and information about recent earthquakes, information
about earthquakes in Kansas, and historical earthquake data. For more information about Kansas earthquakes, see Kansas Earthquakes (KGS Public Information Circular 3), available online at http://www.kgs.ku.edu/Publications/pic3/pic3_1.html. Additional information on the U.S. Army Corps of Engineers’ Tuttle Creek Dam Safety Assurance Program is available at http://www.nwk.usace.army.mil/tcdam/index.htm.
Regional subsurface features associated with earthquakes in east-central Kansas. |
| Online February 10, 2003 Comments to: lbrosius@kgs.ku.edu Kansas Geological Survey URL:http://www.kgs.ku.edu/Publications/GeoRecord/2001/vol7.3/Page1.html |
