According to the USGS at approximately 4 am central time (5 am eastern time) a small, 3.8 magnitude earthquake occurred 10 miles west of Elgin, IL and 40 miles WNW of downtown Chicago. The earthquake has been identified by the USGS as a shallow (5 km depth) strike-slip earthquake, strike-slip motion means that the rocks moved side-to-side with little vertical movement. According to the “Did You Feel It” maps of the USGS, the quake was felt over a very wide area with a number of responses from Madison and Middleton, WI 150 miles to the north. Unlike the edges of the continents, where we are in Charleston, the rocks that underlie this region are relatively cohesive and thus ring like a bell when an earthquake occurs, transmitting the seismic waves much further than the broken up rocks.
However, like Charleston, Chicago and the surrounding regions are located in the middle of a lithospheric plate, far from any plate boundaries. Plate boundaries such as California and Haiti, where the plates slide past each-other, are much more prone to earthquakes because the motion of the plates generates the earthquakes. Intraplate earthquakes, as we have here and in Illinois, are the result of stresses in the crust being released along pre-existing faults.
The region in Illinois that felt the earthquake today does not have a long history of seismic activity, though, it lies north of the relatively active New Madrid Seismic Zone. Thus, this serves as a reminder that places in South Carolina that may not have felt an earthquake in the past, could be affected by an earthquake in the future. It also reminds us that while South Carolina is underlain by rocks that are more broken up than the region under Illinois, large earthquakes have the potential to affect large portions of the state.
