Minnesota does experience earthquakes, though they are infrequent and register at low magnitudes compared to events common along tectonic plate boundaries. These ground movements are examples of intraplate seismicity, happening far from the edges of the North American tectonic plate. The state’s location deep within the continental interior affords it a high degree of stability.
Historical Record and Frequency
Minnesota’s recorded seismic history is sparse, with only about 20 confirmed earthquakes having epicenters within the state since 1860. Magnitudes have ranged from minor tremors as low as 0.1 to larger shocks near 5.0. Most seismic activity is minor, with the majority of recorded events falling below magnitude 3.0. The state averages roughly one earthquake of magnitude 3 or 4 per decade. Many low-magnitude quakes are only detected by sensitive instruments and go unfelt by the general population.
Geological Context of Minnesota Quakes
The underlying cause of Minnesota’s occasional quakes is the reactivation of ancient fault lines within the continental crust. Since the state is situated in the middle of the North American Plate, seismic events are not caused by the direct collision or sliding of plates. Instead, modern tectonic stresses, transmitted across the vast plate from distant boundaries, exert pressure on the crust. This compression results from the North American Plate’s slow westward drift away from the Mid-Atlantic Ridge.
The pressure, usually oriented east-west, seeks out zones of weakness in the older crust. Minnesota contains a major zone of weakness: the 1.1-billion-year-old Midcontinent Rift System (MCR). The MCR is a massive, failed continental rift that left behind a deep, buried scar of ancient rocks and deep fault systems. When the accumulated stress exceeds the strength of these pre-existing faults, movement occurs, releasing energy as an earthquake.
Notable Events and Epicenters
The strongest instrumentally recorded earthquake in Minnesota’s history occurred on July 9, 1975, near the town of Morris in Stevens County. This event registered a magnitude of 4.6 and had a maximum Mercalli intensity of VI, meaning it was strong enough to cause minor damage. Reports from the time indicated the shaking rattled dishes and windows, and the tremor caused minor cracking in the foundations of at least two homes near the epicenter.
Before the Morris quake, a significant historical event was the September 1917 earthquake near Staples, which was estimated to be near magnitude 4.8, though modern instruments were not widely available for a precise reading. These larger events illustrate that while rare, quakes can be felt across a wide area, with the 1975 event felt over roughly 82,000 square kilometers, including parts of the Dakotas and Iowa.
Epicenters tend to cluster in areas where ancient structures are present, such as along the western margin of the Midcontinent Rift and the Great Lakes Tectonic Zone. A more recent, notable event near a major population center was the 1981 Cottage Grove earthquake, which registered a magnitude of 3.6. This specific epicenter was located southeast of the Twin Cities, likely associated with a fault zone near the eastern margin of the former rift valley. While a severe, damaging earthquake is considered highly unlikely, the state’s history shows that western and central Minnesota remain the most seismically active areas.