Wait for it….

OK, another day, another mysterious earthquake in the mid-continent.  This time we are talking a M 4.2 in southern Michigan. We have the anti-fracking crowd immediately inferring that fracking caused it; we have a media report that the USGS said it isn’t from fracking (it isn’t clear where that statement originated).

So first some simple stuff.  The nearest seismometer was 75 km away, and only 13 phases have been used to locate the event so far.  That means that the depth based on travel times is nearly worthless (it is reported as 6 +/- 7 km deep, meaning it is in the upper crust).  The USGS moment tensor solution puts it at 8 km, but no uncertainty is included, so it isn’t clear that the solution is constraining the depth very well; Bob Herrman’s moment tensor solution places the earthquake at 5 km down and seems to indicate it is unlikely to be much shallower than 4 km.  Both solutions indicate it is a strike-slip earthquake.

What you want to know is, are there deep injection wells operating in the area? Are they high volume?  Are they new, or recently increased their disposal rate? (Be careful of early pronouncements; many online databases are not up to date).  GG has seen hints there might be such wells.

While this is a suspicious earthquake, it isn’t that far from the Wabash Valley Seismic Zone, a set of faults in southern Indiana and Illinois that have produced large prehistoric earthquakes. But it isn’t that close, either…

So it shall be interesting to see what the story is…


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