April 30, 2011
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.