In wake of Lake Michigan earthquake, researchers say there may have been more that were never recorded in the past

A 2.9 -magnitude earthquake recorded in Lake Michigan offshore from Chicago's North Shore suburbs last month was only the second to be recorded ever in the lake, but scientists said there may have been others in the past that weren't detected.

The U.S. Geological Survey said the earthquake was recorded about 20 kilometers, or about 12.5 miles, east-northeast of Kenilworth, Wilmette and Evanston, Illinois, at approximately 2:38 p.m. Wednesday, July 8.

There were reports that the quake was felt in multiple north suburbs, including Evanston, Deerfield, Kenilworth, and as far north as Waukegan. According to reports made in the USGS, reports of people feeling the earthquake have come from as far south as Chicago's South Side.

Another 2.9 magnitude earthquake was also reported on Aug. 2, 2024. This earthquake was not near the Chicago area — it was centered offshore from Sturgeon Bay, Wisconsin in Door County, about 245 miles north of Chicago.

Researchers from the DEEPS Seismology Group reported that it is hard to say whether or not similar earthquakes happened in Lake Michigan before those two, particularly when looking back into the 20th century. The country has not always had as many seismometers out to detect earthquakes as today, and the seismometers that were operational weren't always sensitive enough to pick up all the 2.9-magnitude earthquakes in the area around them, researchers said.

Of the 11 earthquakes in the area around Lake Michigan between 1900 and 2011, all but three were of a magnitude of 3.8 or higher. Such higher-magnitude earthquakes are around 10 times less frequent, researchers said.

This may mean that smaller-magnitude earthquakes, like the one offshore from Evanston last month and the one offshore from Door County, Wisconsin, two years ago, may have gone undetected.

There are only three earthquakes during that 111-year timeframe with a magnitude under 3.8 that stand out, researchers said.

A 3.4-magnitude earthquake occurred on March 14, 1905, on near Marinette, Wisconsin, on the west side of the Green Bay body of water. Researchers said this earthquake likely made the U.S. Geological Survey catalog because it was explicitly investigated by researcher Carl Stover and his associates for a survey compiled in 1985.

A 3.0-magnitude earthquake occurred on Sept. 9, 1985, in the west Chicago suburb of Lombard. This was close to the site of another earthquake, magnitude 3.2, that occurred in 2013 and that the DEEPS Seismology Group studied. The group said the 1985 earthquake likely made the USGS catalog because it happened in a densely populated region and many likely felt it.

A 3.5-magnitude earthquake occurred on Sept. 2, 1999, in rural Illinois near the Peru Monocline, a rock fold that stretches through several counties. Researchers believe this may have been a reactivation of faulting in the prehistoric rocks beneath the monocline.

Researchers also noted that the vast majority of earthquakes have happened on the Northern Illinois side of Lake Michigan. The largest was a magnitude-5.1 earthquake in 1909 near Sandwich, Illinois, along a fault zone that extends about 85 miles from Will County to Ogle County.

Nearer to Lake Michigan, some earthquakes have also occurred in the southwest suburbs of Chicago, in 1977, 1997, and 2010, researchers said.

In 2011, the EarthScope USArray project installed sensitive broadband seismometers all around the Lake Michigan region, which allowed for the detection of lower-magnitude earthquakes, down to 2.5. This increased the number of earthquakes that could be recorded tenfold, researchers said.

But before 2011, researchers said, there may well have been a whole lot of earthquakes in Lake Michigan that seismometers just weren't sensitive enough to pick up.

"Given that intraplate earthquake epicenters tend to be scattered across broad areas rather than concentrated along a major fault line, as interplate earthquakes do, it is unlikely, though possible, that the subsurface of Lake Michigan would have been spared till now," researchers wrote.

Researchers also noted that earthquake detection has improved through machine learning techniques, which picked up earthquakes that were recorded by seismometers, but could not be detected by human eyes and the usual algorithms because there was so much interference.

The DEEPS Seismology Group has already used machine learning tools to detect four small-magnitude earthquakes along the Sandwich Fault Line that were not in the USGS catalog.

Hypotheses about the July Lake Michigan earthquake

Researchers studying the context of the recent Lake Michigan earthquake did so by analyzing its seismic waves, which were recorded by several different seismic stations —including Raspberry Shake brand geophones on the Evanston and Chicago campuses of Northwestern University. They identified the P waves — the fastest seismic waves that earthquakes produce — that made it to the recording stations from the hypocenter of the earthquake.

The researchers also used their machine learning programs to search for more earthquake occurrences in the weeks before and after the one in Lake Michigan. One possible aftershock was detected on the morning of July 9.

From this data, researchers hypothesized that the heat wave in Chicago at the time was not the cause of the earthquake, given that the Earth's crust 3 miles underground is already 122 degrees — way above the atmospheric temperatures of the heat wave. Furthermore, temperature fluctuations do not affect the subsurface that deep.

Researchers said glacial isostatic adjustment — the ongoing movement of land on which ice-age glaciers were once found — may or may not have had something to do with the quake. This is the cause of most other mid-continent earthquakes between the Rocky and Appalachian Mountains, researchers said.

Climate change may have been involved too, researchers said, emphasizing that climate change affects not just the atmosphere, but the crust of the Earth indirectly. But there is not a sufficient record on earthquakes in Lake Michigan to draw conclusions, according to researchers.

A likely contributing factor for the earthquake was changing water levels in Lake Michigan. The levels were the highest in decades in 2020 and have dropped by about a meter in the six years since. This would likely change stresses in the crust of the Earth under the lake, and such a rapid change could cause a magnitude-2.9 earthquake, researchers said.

However, similar drops in Lake Michigan water levels have happened about five times over the past century, and while the prior drops may have caused earthquakes in Lake Michigan too, there was nothing around to record them at the time if so, researchers said.

Speaking to CBS News Chicago last month, Northwestern University professor and DEEPS group professor Dr. Suzan Van Der Lee said while earthquakes along fault lines in the Pacific Ocean often prompt tsunami watches or warnings, no such things are a concern along the Lake Michigan shoreline

Dr. Van Der Lee said it's possible there were changes of a centimeter or millimeter along the lakebed, but the earthquake was about 5 kilometers underground, and it's unlikely the bottom of the lake moved, which is what's required for a tsunami.

Van Der Lee also said there could be more opportunities for earthquakes in Lake Michigan, whether in the near or far future.

The video above is from a previous report.