WHO’S IN CHARGE? …NATURE!

According to a report by Raymond R. Anderson
and Paul E. Van Dorpe:




Earthquakes occur primarily along fault zones, tears in the Earth's
crust, along which stresses build until one side of the fault slips,
generating compressive and shear energy that produces the damage.
Heaviest damage generally occurs nearest the epicenter, that point on
the Earth's surface directly above the point of fault movement. The
composition of geologic materials between these points is a major
factor in transmitting the energy to buildings and other structures on
the Earth's surface. Earthquake strength is recorded by a SEISMOGRAPH
and is described using either the RICHTER SCALE, which is a measure of
the intensity of energy produced by an earthquake, or the MODIFIED
MERCALLI SCALE, which describes earthquake intensity by the damage that
results.






While geologists often refer to the Midwest as the “stable
mid-continent,” because of its lack of major crustal movements, there
are two regions of active seismicity, the NEMAHA RIDGE and the NEW
MADRID FAULT ZONE. The Nemaha Ridge in Kansas and Nebraska, associated
with the Humboldt Fault, is characterized by numerous small earthquakes
that release stresses before they build to dangerous levels. The area
is not considered a threat to Iowa. The New Madrid Fault Zone, on the
other hand, has a greater destructive potential. It is located along
the valley of the Mississippi River, from its confluence with the Ohio
River southward, and includes portions of Illinois, Kentucky,
Tennessee, Missouri, Arkansas, and Mississippi. The Earth's crust in
the mid-continent is older, and therefore thicker, cooler, and more
brittle than that in California for example. Consequently, earthquake
shock waves travel faster and farther in the Midwest, making quakes
here potentially more damaging than similar sized events in other
geologic settings. The strongest historic earthquakes in North America
occurred as a series of four shocks along the New Madrid Fault Zone
between December 16, 1811 and February 7, 1812 and were centered near
the town of New Madrid, in the boot-heel area of Missouri. Based on
historic eyewitness accounts, scientists have estimated the intensity
of the earthquakes using the Modified Mercalli Scale and estimated an
equivalent Richter magnitude of 8.3 to 8.7. These devastating
earthquakes were felt from the Atlantic seaboard to the Rockies and
felled trees, opened fissures, destroyed log buildings, erupted sand
and water, created REELFOOT LAKE in western Tennessee, and reportedly
caused the Mississippi River to temporarily reverse flow in places.
Because of the low population density, fatalities were relatively few
with most occurring on or near the Mississippi River.




Earthquake prediction is an inexact science. Even in areas that are
well monitored with instruments, such as California's San Andreas Fault
Zone, earthquakes are only rarely predicted by scientists. There are
always people who will profess some special knowledge of natural events
that transcends traditional science and who will attract media
attention. The public will do well to look at the credentials of those
who make such dramatic statements and to seek authoritative sources for
dependable information.




Seismologists attempt to forecast earthquake size and frequency based
on data from previous events. In the New Madrid Fault Zone, this
analysis is difficult because there are few historic moderate to large
earthquakes, and the active faults are too deeply buried to monitor
effectively. Based on recurrence intervals for small earthquakes,
scientists estimate a 40 to 63 percent chance of a Richter magnitude
6.0 earthquake between 1985 and 2000, rising to a 90 percent chance by
2040. Estimated recurrence intervals for larger earthquakes,
approaching the size of the 1811-1812 events, vary from about 175 years
to greater than 700 years. Will we get any warning prior to an
earthquake? Maybe. Our understanding of earthquakes is increasing, and
the future may bring a better forecasting system.




Estimated effects of a 6.5 Richter magnitude earthquake along the New
Madrid Fault Zone suggest that Iowans in four southeast counties could
experience trembling buildings, some broken dishes and cracked windows,
movement and falling of small unstable objects, abrupt opening or
closing of doors, and liquids spilling from open containers. About 29
other counties, from Page to Polk to Muscatine, could experience
vibrations similar to the passing of a heavy truck, rattling of dishes
and windows, creaking of walls, and swinging of suspended objects.
These effects will vary considerably with differences in local geology
and construction techniques.




The effects in the Midwest of another severe earthquake like those of
1811-1812 would constitute a major disaster. The New Madrid Fault Zone
is densely populated, with Memphis, Little Rock, Birmingham, Nashville,
Louisville, and St. Louis all less than 250 miles from the most
seismically active part of the area. In Iowa the direct physical
effects would likely be minor to moderate, with structures built on
poorly consolidated materials (such as river valley alluvium) nearest
the epicenter suffering the heaviest damage. This could include fallen
chimneys and cracked or broken walls and windows; disruption of local
gas, water, sewer, and electric utilities;  fluctuation of water
levels in wells, springs, reservoirs, and streams; local landslides
along steep slopes; liquefaction along floodplains; pressure changes in
gas-storage facilities; and even land subsidence and sinkhole collapse.
Additional consequences could include medical and other evacuations
from damaged areas to facilities in Iowa, aid from Iowa sent to
stricken areas, and increased east-west traffic through Iowa
compensating for routes severed at the Mississippi River in the
earthquake area.




Successful planning and mitigation efforts for earthquake events start
with a knowledgeable public. Agencies such as the Federal Emergency
Management Agency, the Central United States Earthquake Consortium,
Iowa's Office of Disaster Services, and local emergency response teams
are all concerned with providing aid in the event of natural disasters.
Experts stress that most efforts to protect lives and property during
earthquakes work equally well for any disaster, and these basic
measures are usually not expensive and take little time to implement.
Anyone can and should obtain information ranging from where to take
cover during an earthquake, to upgrading home utility connections, and
preparing an emergency survival kit. Midwesterners are well aware of
how to protect themselves during tornadoes. How well we respond to an
earthquake at home, at work, at school, or travelling will depend on
how well we understand the risks and how well we prepare.


 

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Adapted from Iowa Geology 1991, No. 16, Iowa Department of Natural Resources




For more information go to: www.igsb.uiowa.edu




For KIDS see: www.geology.com and www.energyquest.ca.gov




www.earthquake.org and http://geology.cr.usgs.gov/




As quoted by Diane Ackerman, “All around me: planet, moon, sun,
riverbed, marsh: grew out of cataclysms galore; nothing ever sprang
whole, stays put.  I feel the earth beneath my feet suddenly shale
away….”