Great Lakes tectonic zone southern Ontario. Several earthquakes have been documented in Minnesota in the last 120 years, with at least six in the GLTZ. The epicenters show a clear relationship to tectonic features of the state; four epicenters lie along the Great Lakes tectonic zone. Depths are estimated at . The best-documented event occurred on July 9, 1975, near Morris, Minnesota, with a magnitude of 4.6, and a felt area of covering parts of four states. Wisconsin has had no earthquakes along the GLTZ, Michigan's Upper Peninsula has had four earthquakes in the vicinity of the GLTZ – Negaunee, Newberry and two in
Great Lakes pass the locks on the Saint Lawrence Seaway because the ships are too wide. Only four bridges are on the Great Lakes other than Lake Ontario because of the cost of building structures high enough for ships to pass under. The Blue Water Bridge is, for example, more than 150 feet high and more than a mile long. The Great Lakes are used to supply drinking water to tens of millions of people in bordering areas. This valuable resource is collectively administered by the state and provincial governments adjacent to the lakes, who have agreed to the Great Lakes Compact