It’s not every day Illinois sees an earthquake like the 2.9-magnitude tremor that struck Lake Michigan on Wednesday afternoon. But quakes in the area are more common than some might think, experts say.
So far this decade, the state has seen 44 earthquakes, according to Robert Bauer, a principal scientist of engineering geology with the Illinois State Geological Survey. Wednesday’s marked just the latest.
The mild earthquake was recorded off the shore of the northern suburbs near the Illinois-Wisconsin border. Stretching 3.1 miles deep, the quake took place 13.8 miles northeast of Evanston, per the U.S. Geological Survey.
From year to year, local seismic activity varies greatly, Bauer said. On average, though, the state receives about 10 earthquakes a year. That’s certainly more infrequent than other areas of the country — with Illinois being in what USGS deems a “stable continental region” — but the occasional quake is far from a rarity.
But what caused Wednesday’s quake and others like it? It goes back to the basics of geology.
The Earth has four major layers, and the two outermost make up a thin surface that coats the planet. That surface, though, is in fragments.
The Earth is made up of many pieces. More than that, those pieces — called tectonic plates — are constantly moving. The pieces meet at plate boundaries, where they usually grind up against each other slowly.
Earthquakes, at their most basic level, are a function of friction between those pieces. As for what determines why earthquakes are more frequent in some areas than others, that’s a little more complicated.
Quakes are concentrated at plate boundaries. At boundaries, because the edges of plates are rough, areas can get stuck while the rest of the plate keeps moving, per USGS. An earthquake occurs when an edge is stuck for long enough that the energy that would usually cause the pieces to slide past one another is stored up instead. When that tension snaps, a large amount of energy is released.
But there are also intraplate earthquakes. That’s the case for Illinois, which sits far from the nearest boundary. These tremors are made possible by faults — cracks in the bedrock — deep below the surface, according to Bauer.
Beneath Illinois, there are preexisting faults tied to an earlier geologic era, essentially ancient cracks long buried over time, Bauer explained. Though Illinois isn’t bordered by a boundary, pressure from boundaries to the east and west can stress these faults, inducing earthquakes, Bauer said.
There are two major seismic zones that affect Illinois, according to the Illinois Emergency Management Agency. That includes the Wabash Valley Seismic Zone and the New Madrid Seismic Zone. The Wabash Valley Seismic Zone is located between southeastern Illinois and southeastern Indiana, whereas the New Madrid Seismic Zone is located in the Central Mississippi Valley, covering a wide swath across several states from Illinois to Arkansas.
The bulk of the state’s earthquakes happen in Southern Illinois because those old layers of the Earth are more broken up downstate than they are up north, Bauer said.
Historically, some of the strongest earthquakes the state has seen have come in at just over a 5.0 magnitude, Bauer said. In recent years, Bauer pointed to a 5.2 magnitude earthquake recorded in Southern Illinois near Mount Carmel in 2008. Going back to 1968, there was a 5.3 magnitude quake, also downstate in Hamilton County. And more than a hundred years ago, in the Chicago area, a 5.1 magnitude quake shook Aurora, which caused some damage and led to a few fires, Bauer said.
As far as magnitude goes, however, earthquakes are also felt a bit differently in the Midwest than, say, California, which lies close to a plate boundary. Being in the center of a tectonic plate, an earthquake in Illinois will radiate a lot further than one by a boundary, Bauer said.
Think of it like a bell, Bauer explained. When a bell is cracked, the waves don’t go very far. But with a bell that’s complete, it rings. In other words, an intraplate earthquake goes a long way.
On Wednesday, local North Shore officials reported no damages as a result of the Lake Michigan tremor, nor were there any reports of injured boaters or swimmers.
And while earthquakes may not always be top of mind here in intraplate territory, Bauer encouraged earthquake awareness — safety especially — nonetheless.
The Associated Press contributed.
