Wednesday, February 10, 2016

Earthquakes in Minnesota

I was co-leading a field trip for a Minnesota Master Naturalist group over the weekend, and at one point we strayed onto a tangent involving faulting and earthquakes in Minnesota. Given we've already covered one of the rarest of the geological rare birds of the state with "Minnesota's dinosaurs", let's add another notch to the rock hammer with the subject of earthquakes in the Land of 10,000 Lakes.

If you've lived in Minnesota any length of time, you may have noticed a decided absence of earthquakes. This state is about as seismically stable as you can get. The USGS page on Minnesota earthquakes is pretty thin. The USGS's simplified earthquake hazard map from 2014, included below, shows that if you want to get away from earth tremors, your best bests are central Texas, peninsular Florida, and a swath of the Upper Midwest including North Dakota, Minnesota, Iowa, Wisconsin, and Michigan. While I can't speak for Texas and Florida, the Upper Midwest has been shielded from direct tectonic activity for more than a billion years. The last time the area was well and truly active was during the spasm of volcanism and faulting that produced the great Midcontinent Rift, a 1.1-billion-year-old record of when North America nearly split. Geologic echoes of this event are preserved in such features as the location of Lake Superior, part of the course of the St. Croix River, and the quirk of structural geology known as the Twin Cities Basin. During the Paleozoic, ancient faults occasionally caused subsidence in the area that would become the Twin Cities (which helps explain why we have the distinct local divisions of the Platteville Formation). The effects of the faults are not obvious in Minneapolis and St. Paul themselves, but if you go down to Lock & Dam 2 at Hastings and try to follow the position of the off-white Praire du Chien Group bluffs along the opposite side of the river, you'll notice that they don't stay at the same elevation. This is because the rocks have been faulted; in fact, there are several faults between the east end of Grey Cloud Island and Hastings, near the eastern margin of the former rift valley. These include the helpfully named Cottage Grove, Afton–Denmark, and Hastings fault zones. The namesake ravine of Cottage Grove Ravine Park is there because drainage found the weakness created by one of these faults.

USGS simplified earthquake hazard map, 2014 edition, taken from here.

There are plenty of other faults in Minnesota; it's just that there's little reason for them to be active. A fault is a way to relieve tectonic stresses. Remove the stress, and the fault just sits there. With the low-stress lifestyle of Minnesota faults, there has been little shaking. As reported by the Minnesota Geological Survey in this handy short document, the recorded earthquake history of Minnesota, going back to 1860, includes 20 temblors with epicenters in the state. None of them exceeded a magnitude of 5. There was an interesting little spurt of 6 between 1979 and 1982, but it seems much less impressive when you discover that that half of them had magnitudes of 1 or less, including one with a magnitude of 0.1 (Rush City, May 14, 1979). From the list, it almost appears as though during this stretch someone had access to more sensitive detectors, because otherwise all of the quakes listed are magnitude 3.0 or greater. The only recorded quake particularly near the Twin Cities was the April 24, 1981 Cottage Grove earthquake, of 3.6 magnitude. This one is of some extra interest to me because I'm from Cottage Grove and happened to have been born around that time. Otherwise, they're fairly evenly distributed across the state, except for the Arrowhead and the southeastern end, where they haven't been recorded, and a strip from the western "knob" of the state to a little north of the center of the state, where they are most abundant (in a relative sense, of course). The most recent was a 3.12 magnitude quake in Brandon back on April 29, 2011. In terms of distribution over time, you can average the population to roughly one 3–4 magnitude earthquake per decade, but of course they don't plan themselves that neatly in practice. For example, there had been 17 years between the 2011 quake and the next most recent, which was only about eight months after *its* most recent predecessor.

Actually, I'm somewhat surprised in hindsight that we don't have more seismic activity. Minnesota was mostly glaciated not too long ago, geologically speaking, and one of the side effects of glaciation is post-glacial rebound. In short, all sorts of interesting things happen after you remove a heavy continental ice sheet. Without that load, the crust springs up (in a manner of speaking), then settles back down, then continues adjusting until it has reached equilibrium. One way this process can be expressed is through earthquakes.

Further reading:

Chandler, V. W. 1994 (revised 2014). Minnesota at a glance: earthquakes in Minnesota. Minnesota Geological Survey, St. Paul, Minnesota.

Mooney, H. M. 1979. Earthquake history of Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 23.

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