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 An earthquake occurred 20 miles southwest of
Vincennes, Ind., and 125 miles southwest of Indianapolis, Ind., at
3:36 a.m. MDT Friday (4:36 a.m. CDT in Illinois). The magnitude and
location may be revised when additional data and further analysis
results are available. There have been no reports of damage.
Tectonic summary
Earthquakes in the Illinois basin-Ozark dome region
This large region borders the much more seismically
active New Madrid seismic zone on the seismic zone's north and west.
The Illinois basin-Ozark dome region covers parts of Indiana,
Kentucky, Illinois, Missouri and Arkansas and stretches from
Indianapolis and St. Louis to Memphis. Moderately frequent
earthquakes occur at irregular intervals throughout the region. The
largest historical earthquake in the region (magnitude 5.4) damaged
southern Illinois in 1968. Moderately damaging earthquakes strike
somewhere in the region each decade or two, and smaller earthquakes
are felt about once or twice a year. In addition, geologists have
found evidence of eight or more prehistoric earthquakes, over what
scientists estimate to be the last 25,000 years, that were much
larger than any observed historically in the region.
 Earthquakes in the central and eastern U.S.,
although less frequent than in the western U.S., are typically felt
over a much broader region. East of the Rockies, an earthquake can
be felt over an area as much as 10 times larger than a similar
magnitude earthquake on the West Coast. A magnitude 4.0 eastern U.S.
earthquake typically can be felt at many places as far as 60 miles
from where it occurred, and it infrequently causes damage near its
source. A magnitude 5.5 eastern U.S. earthquake usually can be felt
as far as 300 miles from where it occurred and sometimes causes
damage as far away as 25 miles.
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Faults
Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most
bedrock in the Illinois basin-Ozark dome region was formed as several
generations of mountains rose and were eroded down again over what scientists
estimate to be the last billion or so years.
At well-studied plate boundaries like the San Andreas fault system in
California, often scientists can determine the name of the specific fault that
is responsible for an earthquake. In contrast, east of the Rocky Mountains this
is rarely the case. The Illinois basin-Ozark dome region is far from the nearest
plate boundaries, which are in the center of the Atlantic Ocean, in the
Caribbean Sea and in the Gulf of California. The region is laced with known
faults, but numerous smaller or deeply buried faults remain undetected. Even the
known faults are poorly located at earthquake depths. Accordingly, few
earthquakes in the region can be linked to named faults. It is difficult to
determine if a known fault is still active and could slip and cause an
earthquake. As in most other areas east of the Rockies, the best guide to
earthquake hazards in the Illinois basin-Ozark dome region is the earthquakes
themselves.
[Text from
NewsEmergency.com]
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