The temblor that shook people awake was the most powerful in Illnois in several decades, "but it's not a damaging earthquake," Robert Nelson said.

The earthquake that shook people awake across the state Friday morning represented the most powerful quake to hit Illinois in the last several decades, but should be the subject more of fascination than fear.


"It’s a big deal for those who experience it, but it’s not a damaging earthquake," Illinois State University geology professor Robert Nelson said. "I hope they enjoyed it."


The epicenter of the earthquake originated underground near West Salem in the southeastern part of the state but was felt as far away as Kansas, upper Michigan and Georgia.


Seismographic instruments in Montana registered immediate readings of the quake, as well. Subterranean waves generated by the earthquake traveled the nation within a matter of minutes and had crossed the globe within two hours.


Nelson said the earthquake appeared to be a tensional rather than compressional event, meaning it occurred as a release of stress as a fault pulled apart rather than pushed together.


It was the strongest quake in southern Illinois since a 5.4 magnitude temblor in November 1968. The U.S. Geological Survey initially rated Friday’s earthquake a 5.4 but later in the day revised that to a 5.2 magnitude. At least three aftershocks followed Friday. The largest was measured at a 4.6 magnitude.


Nelson said earthquakes in the range of Friday’s temblor appear to be near the upper limit of possible earthquakes that occur in faults in the area with any frequency. Earthquakes similar in strength, although usually of a lesser magnitude, can be expected every three to five years.


The quake Friday specifically occurred in the Wabash Valley seismic zone, which is estimated to have produced at least eight prehistoric earthquakes measuring between 6.5 and 7.5 in magnitude over the last 20,000 years.


That zone resides next to a more publicly known and more seismically active area called the New Madrid seismic zone, which generated severe earthquakes in the early 19th century that would, with the density of current populations in the area, cause much more damage if repeated today.


Nelson said major earthquakes from the New Madrid zone occur once every 400 to 500 years and aren’t likely for at least another 200 years.


Matt Buedel can be reached at (309) 686-3154 or mbuedel@pjstar.com.