9:30pm – UPDATE: According to the Chicago Tribune reportsthere was no reports of damage according to Illinois Emergency Management Agency spokeswoman Patti Thompson. The largest earthquake reported in the state was a magnitude 5.4 in southern Illinois in 1968.
5:55pm – Within the past hour, an earthquake has been observed in Southern Illinois. An intial strength of 3.6 on the Richter Scale. The quake occurred at a depth of 7.3 miles below the surface. There are no reports of damage and it is unlikely to have caused major damage. Developing…
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 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 ten 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 100 km (60 mi) 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 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).
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 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.
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