THE MENOMINEE CRACK AND CLINTONVILLE BOOMS: SEISMIC EVENTS IN MICHIGAN'S UPPER PENINSULA AND NORTHEASTERN WISCONSIN

In spite of its reputation as an aseismic area, there were intriguing occurrences of seismic events in Upper Michigan and northern Wisconsin in 2010 and 2012. Although these two occurrences were only about 100 km apart, there is no evidence indicating that they are related or even due to a similar mechanism.

The Menominee Crack: On October 4, 2010, a loud noise and shaking were observed in an area north of Menominee. This was associated with the appearance of 110m long crack at the crest of a ridge, 1.5m in height and up to 9m wide. The crack is apparently a surficial feature, resulting from stretching of the uppermost soil and clay layers to accommodate the creation of a ridge at some shallow depth. A reasonable model (first proposed by Dr. Norm Sleep) is that the limestone underneath the clay experienced a “pop-up” due to high lateral stresses. Usually, pop-ups occur as a result of recent unloading, as at the base of a quarry or immediately following glacial retreat; neither of these describes this site, so the cause remains speculative.

Clintonville Booms: Residents of Clintonville began hearing infrequent deep, rumbling sounds on March 18, 2012; the booms were sometimes accompanied by felt shaking. Following a relatively large event on March 20 that was clearly a M1.5 earthquake, four seismometers and eight sound sensors were deployed within and around Clintonville to improve the locations of subsequent events. Two events were located beneath the southeastern part of Clintonville at less than 1 km below the surface within the granitic basement. These events were close enough to the surface so that seismic energy of sufficiently high frequency coupled to the atmosphere and propagated as sound. The Clintonville booms were indeed due to earthquakes, most of which were too small to record.