Not that long ago, on 2 May 2015, seismographs shuddered to the rhythm of what may have been Michigan’s second largest ever (well, second largest ever recorded) earthquake.
The immediate response of the news and social media was to wonder whether fracking was the culprit for this M4.2 tremblor. It took them a few hours to conclude that it wasn’t.
Here endeth the seismic story of Michigan.
Tectonic Setting: Stable Continental Interiors
Of course this is an oversimplification. Of course the Earth rattles more than just a little from time to time. But the truth is that unlike some of its fellow states which shake like jello at the slightest provocation, the Great Lake State is pretty stable overall — something for which its residents may well be deeply grateful.
Stable continental interiors are just what the name says — places far from the tectonically-difficult plate margins where earthquakes, often large and sometimes damaging, are a way of life. As the USGS observes: “Most of the enormous region from the Rockies to the Atlantic can go years without an earthquake large enough to be felt, and several U.S.states have never reported a damaging earthquake.”
Michigan is one of them.
Why is that? Well, in the absence of any tectonics to discuss, we are left with geology. Michigan (again this is a simplification because the highways and byways of geology are sadly of interest mainly to geologists) much of the state is composed of thick layers of rock largely undisturbed by major continent-ravaging events.
There are local faults, of course, and these are the sources, in all likelihood, of the little earthquake activity on record. But, as the attached cross-section of the lower part of the state shows, the thick layers of rock are largely uniform in nature and are therefore unlikely to fracture on a large scale.
Historic Michigan Earthquakes
A measure of the challenge presented to anyone trying to document earthquakes in Michigan is presented by a search of the United States Geological Survey’s earthquake archive. Interrogation of the data for the past century (yes, century) produced just six events with a recorded magnitude in excess of M1.0 — and two of those were mining explosions.
Don’t despair just yet. Although seismic recording as we know it barely dates back beyond the early twentieth century, there are written records of tremors having been felt across the state. The USGS in its (necessarily brief) earthquake history of Michigan details what seems like every single one of them, plus a number of others elsewhere.
For the record, the largest dates back to 1947, where an M4.6 earthquake occurred close to where the most recent M4.2 struck. It “damaged chimneys and cracked plaster over a large area of south-central Michigan,” according to the USGS, which goes on to note that: “Reports of damage to chimneys and some instances of cracked or fallen plaster, broken windows, and merchandise thrown from store shelves were common over the epicentral area.”
And those mining events can’t be ignored. The map of the archive goes back just a century but the earthquake history includes three noteworthy explosions in the mining area of north Michigan, although these are not, of course, technically earthquakes.
Noisy Neighbours: Keep Quiet Down There!
Large shocks can be felt across large areas and don’t stop at state boundaries. It’s likely (though we can’t say for certain) that most earthquake damage in Michigan results from earthquakes which occurred elsewhere. In 1811 and 1812, for example, a destructive series of major tremors struck New Madrid in Missouri and the USGS recorded that: “As many as nine tremors from the New Madrid earthquake series were reported felt distinctly at Detroit.”
Two Charlestons, in NC and in MO – each experienced earthquakes which were felt in parts of Michigan, in 1886 and 1895 respectively. And those noisy Canucks aren’t too thoughtful either, as large tremors up in Quebec and the St Lawrence valley have created minor disturbances, too.
Sleep Soundly, Michigan
It’s worth remembering that very large earthquakes — such as those in New Madrid in the early nineteenth century — can have repercussions hundreds of miles form their epicentres and illustrate the point that nowhere can really be considered invulnerable to earthquakes. But all in all, if you have nightmares about earthquakes, you might want to try Michigan. It’s about as safe a place as you’ll find.