Magnitude 5.2 ILLINOIS
Friday, April 18, 2008 at 09:37:00 UTC
EARTHQUAKES IN THE WABASH VALLEY SEISMIC ZONE
The April 18, 2008 earthquake occurred in the Wabash Valley Seismic zone, which is defined by a zone of earthquakes that are scattered across a large area of southeastern Illinois and southwestern Indiana. The Wabash Valley fault system is the main structural feature associated with the seismic zone. The fault system consists of a network of normal faults that trend north-northeastward from Gallatin and White Counties in southern Illinois and adjacent Posey County in southern Indiana. The faults extend at least 97 km (60 miles) and span across an area that is about 48 km (30 miles) wide. The faults dip steeply to both the east and west, and displacements on parallel sets of faults have created sets of horsts and grabens in the subsurface beneath the region. Many of the faults have been penetrated by numerous petroleum test wells, but none of the faults are expressed at the surface. Seismic-reflection data show that faults in the Wabash Valley fault system first formed in late Precambrian time when this part of the Earth's crust was being stretched in an approximately east to east-southeast direction. Movement on the faults ceased during most of the Paleozoic Era but the youngest Paleozoic sedimentary rocks, which are Pennsylvanian in age, are vertically offset across faults in the system. These offsets indicate that some faults were reactivated after the Pennsylvanian rocks were deposited. Pennsylvanian rocks in the area are overlain by Quaternary gravels and glacial deposits. There is no clear evidence that these younger deposits are substantially offset across the faults, but there is compelling evidence that earthquakes stronger than the April 18 earthquake have shaken the region in the geologically recent past. Geological field studies in the past 20 years have identified prehistoric liquefaction features along the banks of rivers and creeks indicate at least eight strong earthquakes have occurred in the lower Wabash Valley region in the past 20,000 years, each having an estimated magnitude between about 6.5 to 7.5. The largest of these paleoearthquakes is thought to have occurred about 6,100 years ago and was probably centered about 25 km (15 miles) west of Vincennes, Indiana. The shaking from earthquakes in the magnitude 6.5 to 7.5 range would be 20 to 200 times stronger than the April 18 earthquake.
Earthquakes of the size of the April 18 quake (Mw 5.2) typically produce smaller-magnitude aftershocks in the days following the mainshock. A few of these earthquakes could be large enough to be felt. Typically, earthquakes of this size (Mw 5.2) can cause slight damage within a few tens of miles from the epicenter. The Wabash Valley Seismic zone is located to the north of the more seismically active New Madrid seismic zone, where some of the largest earthquakes in North America occurred in the winter of 1811-1812. The April 18 earthquake is located within the Illinois basin-Ozark dome region, which covers parts of Indiana, Kentucky, Illinois, Missouri, and Arkansas and stretches from Indianapolis and St. Louis to Memphis. Moderately damaging earthquakes have historically occurred at irregular intervals in this region, with a significant earthquake typically occurring every decade or two. The largest historical earthquake in the Illinois basin region was a magnitude 5.4 event in November 1968 that cause damage in southern Illinois. In June 1987, a magnitude 5.2 also struck southern Illinois, and a magnitude 3.9 earthquake occurred in southern Indiana on December 7, 2000. More recently a magnitude 4.6 near Darmstadt, in extreme southwestern Indiana, occurred on June 18, 2002. Typically, smaller-magnitude earthquakes are felt in the area about once or twice a year. 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 because of the properties of the Earth's crust in the 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 the epicenter 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). Because earthquake waves travel efficiently through the Earth in the central and eastern U.S., it is not surprising that this earthquake was felt hundreds of miles away, as far south as Florida.
Preliminary Earthquake Report
U.S. Geological Survey, National Earthquake Information Center
World Data Center for Seismology, Denver