Wisconsin is not typically associated with earthquakes, yet the state has a history of experiencing seismic activity. While major, damaging earthquakes are exceedingly rare, the region occasionally registers tremors, some originating within state lines and others from distant centers of activity. The seismic events that occur here are almost always low-magnitude and generally only felt by people located close to the epicenter. The state’s location far from active plate boundaries explains why these events are infrequent and mild compared to those on the West Coast.
Documented Seismic Events
The historical record confirms that Wisconsin has experienced several localized earthquakes, though none have caused widespread destruction. The most significant event to have an epicenter within the state’s proximity occurred on May 6, 1947, just south of Milwaukee in Lake Michigan. This tremor was estimated to be a magnitude 4.2 and was felt across a 3,000-square-mile area of southeastern Wisconsin. Reports from the time described shaking that broke windows, knocked over dishes, and caused office workers to evacuate buildings, even though the damage remained minor overall.
Other historical events, like a 1909 quake estimated at magnitude 5.3 near the Wisconsin-Illinois border, illustrate the region’s potential for localized shaking. Many quakes felt by Wisconsin residents originate in neighboring states, such as Illinois or Michigan. For example, tremors from a 2010 earthquake centered northwest of Chicago were felt across much of southern Wisconsin.
Most minor quakes occur in the magnitude range of 2.0 to 3.5, often only detected by sensitive instruments or by people located directly above the hypocenter. The state also feels the effects of larger seismic zones, including the New Madrid Seismic Zone. Larger events hundreds of miles away can send noticeable shockwaves that travel efficiently through the Midwest’s cold, rigid bedrock.
Geological Triggers for Midwestern Earthquakes
The occurrence of earthquakes in a region far from the edge of a tectonic plate is known as intraplate seismicity. The geological explanation for Wisconsin’s occasional tremors lies deep beneath the surface in the Midcontinent Rift System (MCRS). This immense, 1.1-billion-year-old geological feature is a failed rift where the North American continent began to tear apart but ultimately stopped.
The MCRS runs approximately 1,864 miles across the center of the continent, with one of its two major arms extending southwestward through portions of Wisconsin. This ancient event left behind a massive, buried scar characterized by deep fault lines and thick layers of dense volcanic rock. The rift rocks are mostly covered by younger sedimentary layers, but their presence is detectable through gravity and magnetic surveys.
Though the rift is no longer actively spreading, the fault zones it created represent structural weaknesses in the Earth’s crust. Tectonic stress, which originates from the movement and compression of the larger North American Plate, concentrates along these ancient faults. When the stress exceeds the strength of the rock, the faults slip, resulting in an earthquake.
Current Seismic Risk and Preparedness
The United States Geological Survey (USGS) classifies Wisconsin as having a very low to low seismic hazard. This assessment indicates that the probability of experiencing potentially damaging ground shaking is minimal over a typical 50-year period. While felt earthquakes are possible, the risk of a destructive event is highly improbable, meaning earthquake provisions are often not a primary design factor for standard construction.
Despite the low overall hazard, residents can take simple steps to prepare for unexpected shaking. The basic safety procedure recommended during an earthquake is “Drop, Cover, and Hold On.” This involves dropping to the floor, taking cover under a sturdy piece of furniture like a desk or table, and holding onto it until the shaking stops.
Securing movable items is another practical precaution that can prevent injury or property damage even during a minor tremor. Residents can secure tall, heavy furniture, such as bookshelves and cabinets, by anchoring them to the wall studs. This action helps mitigate the primary hazard in low-risk areas, which is being struck by falling objects rather than structural collapse.