April 30, 2011
Little Minnesota quake
Posted by Austin Elliott
Early this morning western Minnesota got a little thump from below as a tiny earthquake rattled the kettle-lake-studded rural region around the small town of Alexandria. The epicenter was just north of highway 52, about halfway between Minneapolis and Fargo, and at a “weak” magnitude of 2.5 was only noticed by a smattering of light sleepers at 2:20am. The Minneapolis Star Tribune has a decidedly enthusiastic article reporting the rare occurrence. Although it would have initially seemed like a loud clap of thunder, the thorough rattle or heave of even such a small quake is usually unmistakable as ground motion, although the first guesses of residents far from an active plate boundary were more likely a garbage truck, train, or explosion. There is precedent for this quake: small patches of western Minnesota are subjected to a minor jolt every couple of decades, but the state has one of the lowest number of earthquake occurrences in the country.
Earthquakes in the center of the country–or the North American tectonic plate, rather, since quakes know no political boundaries–while not nearly as common as temblors at the active margins, are not as rare as one might think. In fact, with reference to geologic time they’re happening rather constantly. Have a look at the USGS’s map of the past week’s earthquakes for the whole country:
Little specks show up in all kinds of places that aren’t the grinding west coast boundary between the North American and Pacific plates. In general they’re simply the result of minor stresses built up as the continent is torqued from both sides–sort of like the way a heavy old dining room table might creak slightly if you lifted one end. In the scheme of tectonic forces, these quakes are the manifestation of incredibly minor stresses. Sometimes they turn out to be big ones, but that’s a story for another day.
While there are certainly micro-earthquakes happening all over in response to minor tectonic stresses and adjustments, it’s worthwhile to consider a big stress affecting large parts of the globe, certainly including Minnesota: post-glacial rebound. Although I’m not a regional expert, only a quasi-resident, it seems like the best explanation for Holocene activity on the New Madrid fault zone. This is the first of many papers the Google oracle brought to me:
There are a lot of interesting aspects to this sort of stress, especially its large magnitude and ephemeral nature–the stress as a function of time is likely a pulse or something like a Gaussian shape, depending on the time resolution and how long ice sheets take to melt (hopefully we won’t know this empirically…). This makes recurrence prediction a bit trickier than for more time-invariant stresses like plate velocities.
That’s an excellent point Richard. I’d be interested to look into more competing models of the post-glacial stress field. The paper you linked to does a good job highlighting the combined effects of both elastic rebound and lateral crustal heterogeneity on the strain field, but it would be interesting to see more general models. In this case they’ve appealed to the historically observed seismicity to focus their investigation on the region around the Reelfoot fault, but if crustal heterogeneity is such an important factor, a broader treatment of the continental basement in the midwest might reveal some additional features that concentrate strain. In their model Minnesota’s left stiff and unaffected, but we’ve got our own failed rift up north!