Shaking Kansas…

Kansas the other day experienced its largest earthquake in over a century (and possibly ever), a M4.8 quake in southeastern Kansas.  The quake produced significant damage in Milan, Kansas. This would seem to be a continuation of the trend of injection-induced earthquakes in the region.

Elevation of Precambrian basement, from McBee (2003), "Nemaha Strike Slip Zone" with M4 and higher earthquakes from past 5 years (some spots have multiple events). Latest even in Kansas shown with a star.

Elevation of Precambrian basement, from McBee (2003), “Nemaha Strike Slip Zone” with M4 and higher earthquakes from past 5 years (some spots have multiple events). Latest even in Kansas shown with a star.

The big question is, is this as big as things will go?  Here’s a map with the large basement cutting faults shown along with the M>= 4 earthquakes from the past 5 years.  You will notice that there is a large high-angle fault (the Nemaha or Humboldt fault zone) extending past Wichita (the W on the map) extending well to the north.  If somehow there is enough stress across that fault that it could be triggered by continued fluid injection, larger earthquakes might be possible (M6+ seem plausible).

Right now indications are a mixed bag.  There are no large earthquakes on the fault itself, which is reassuring.  But the focal mechanism from the latest 4.8 is consistent with slip on a high-angle NNE trending fault like the Nemaha, suggesting that the regional stress field does produce some shear stress on that fault, meaning it conceivable could be triggered. (If there was no shear stress on the fault, no amount of injection would trigger a large earthquake on that structure).

We’ll have to wait and see since nobody in charge seems willing to investigate and shut down wells that might be affecting faults in the area.  The Nemaha fault system is part of the reason there is so much petroleum activity in the area as it is associated with the creation of basins hosting the oil and gas deposits being exploited.

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