Southeastern Illinois Aftershocks

Earthquakes that occur in an area after a main shock are called aftershocks. Aftershocks are most common immediately after the main shock, and their average number per day decreases rapidly as time passes but may be expected in the area for weeks. Most aftershocks will not be felt but will be recorded by seismographs in the area. Some may approach the magnitude of the original main shock. About 5 to 10% of the time, an aftershock is as large as or larger than the main shock. Large aftershocks may cause additional damage, particularly to structures already weakened by the main shock.

So far the largest aftershocks recorded by the permanent seismic network in the earthquake area have been the 4.6 magnitude event on Friday morning at 10:14 a.m. and a 4.0 magnitude event on Monday morning at 12:38 a.m. As of Monday, April 28, 2008, there have been a total of about 29 aftershocks recorded by the permanent network that has seismographs at Olney and Mt. Carmel. There will be many more earthquake aftershocks recorded by the portable seismographs installed by Indiana University and the Center for Earthquake Research and Information from the University of Memphis. These instruments are located around the epicenter and will be able to record very small earthquakes and will be able to accurately determine their location in three dimensions. These data are being recorded on site and will be collected in the next few weeks. Analysis of these data will take several more weeks or months.

Aftershock graphs

Here are the aftershocks plotted in time to show how they decrease in frequency. (Click to enlarge.) One graph shows all the aftershocks and the other expands Friday. Note that these are only the larger aftershocks recorded by the permanent network. We will have smaller events recorded by the portable network in a few weeks. The red dot is the main shock.