\nThis large region borders the much more seismically active New Madrid seismic zone on \nthe seismic zone's north and west. The Illinois basin - Ozark dome region covers parts of \nIndiana, Kentucky, Illinois, Missouri, and Arkansas and stretches from Indianapolis and \nSt. Louis to Memphis. Moderately frequent earthquakes occur at irregular intervals \nthroughout the region. The largest historical earthquake in the region (magnitude 5.4) \ndamaged southern Illinois in 1968. Moderately damaging earthquakes strike somewhere \nin the region each decade or two, and smaller earthquakes are felt about once or twice a \nyear. In addition, geologists have found evidence of eight or more prehistoric earthquakes \nover the last 25,000 years that were much larger than any observed historically in the \nregion.\n
\n\nEarthquakes in the central and eastern U.S., although less frequent than in the western \nU.S., are typically felt over a much broader region. East of the Rockies, an earthquake \ncan be felt over an area as much as ten times larger than a similar magnitude earthquake \non the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many \nplaces as far as 100 km (60 mi) from where it occurred, and it infrequently causes \ndamage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as \nfar as 500 km (300 mi) from where it occurred, and sometimes causes damage as far \naway as 40 km (25 mi).\n
\n\n\nEarthquakes everywhere occur on faults within bedrock, usually miles deep. Most \nbedrock in the Illinois basin - Ozark dome region was formed as several generations of \nmountains rose and were eroded down again over the last billion or so years. \n
\n\nAt well-studied plate boundaries like the San Andreas fault system in California, often \nscientists can determine the name of the specific fault that is responsible for an \nearthquake. In contrast, east of the Rocky Mountains this is rarely the case. The Illinois \nbasin - Ozark dome region is far from the nearest plate boundaries, which are in the \ncenter of the Atlantic Ocean, in the Caribbean Sea, and in the Gulf of California. The \nregion is laced with known faults but numerous smaller or deeply buried faults remain \nundetected. Even the known faults are poorly located at earthquake depths. Accordingly, \nfew earthquakes in the region can be linked to named faults. It is difficult to determine if \na known fault is still active and could slip and cause an earthquake. As in most other \nareas east of the Rockies, the best guide to earthquake hazards \nin the Illinois basin - Ozark dome region is the earthquakes themselves.\n
\n