Morganton Quake M2.7 2021-08-17
Residents of Morganton and vicinity got to enjoy a small earthquake at 9:19:28 last Tuesday morning. It wasn’t large, but it was felt. If you’ve never been in an earthquake like that, and wonder how it feels, you can stand by the joints between cement slabs in a parking deck. You’ll feel a bump, a jolt, as a car runs over the jointing. That’s what a small earthquake feels like.
The earthquake near Morganton was 5.6 kilometers down, about three and a half miles. It was also about 6.5 miles from the Brevard Fault, or the Brevard Zone Faults.
The Brevard Fault is one of the major scars of plate tectonics in the Southeast is. It runs from North Wilkesboro through Brevard, to just north of Atlanta and on into Alabama. It’s a major boundary, and in North Carolina it can mark the transition from Piedmont to Blue Ridge. On I-40 headed west, you cross the Brevard Fault at Old Fort, right as you start up the long grade to Black Mountain. If you take Highway 70 from Old Fort to Marion, you follow the trace of the Brevard Fault. In places you can look out the left window of the car and see mountains, and look out the right window and see the lower hills of the Piedmont.
The geologist’s job it to take the surface data and extrapolate to depth. Or you can just cheat and run a seismic survey that will tell you what the major features of the subsurface are. We combined those approaches in our third floor exhibit on North Carolina Geology. The cross section was drawn by NCSU Professor Jim Hibbard, a structural geologist. Professor Hibbard drew it on a 1:1 scale, same scale horizontally as vertically.
The mountains are built by thrust faults, low angle faults where the upper part, the hanging wall, travels long distances as a result of convergent tectonics. Thrust faults are shingled together, piled up into mountain ranges.
The cross section we show in our exhibit shows how the Brevard Fault is a part of a series of thrust faults off a major detachment. They curl up at the end like the runners on a sled and come to daylight. If you plot the hypocenter of the Morganton fault, it’s a bit above the Brevard Fault at depth.
But it’s not exactly there. Here’s where scale is an issue. At this scale, Albemarle Sound is the thinnest of blue lines. At this scale, only the biggest faults show up. The hypocenter may not fall directly one the Brevard Fault underground, but there are lots of similar, smaller, splays off that fault.
This isn’t necessarily the start of something big. The Appalachians in North Carolina are like an old house, and sometimes the floors creak.
Note added on Tuesday 24 August 2021
These old floors just squeaked again.
Today there was another small quake, M2.1, 5 km southeast of Gerton, NC. It was very shallow, only 0.1 km deep. Gerton is slightly west of Bat Cave, and Chimney Rock. Gerton is also 6.86 miles from Fairview, NC, which sits smack dab in the middle of the Brevard Fault Zone. The setting is similar to that of the Morganton quake I discussed above. However, the geologic map of the area shows a small normal fault that parallels Highway ALT 74.
A normal fault is “normal” as in “perpendicular to the surface.” One side is up and the other side is down, and the fault itself is nearly vertical. A thrust fault is over-and-under. The shallow nature and location of the Gerton earthquake suggests that it is the result of small motions on that normal fault.
Source: Robinson, G.R., Lesure, F.G., Marlow, J.I., Foley, N.K., and Clark, S.H., 2004, Bedrock geology and mineral resources of the Knoxville 1 degree X 2 degree quadrangle, Tennessee, North Carolina, and South Carolina (digital version), U.S. Geological Survey, Open-File Report OF-2004-1075. https://ngmdb.usgs.gov/Prodesc/proddesc_63783.htm
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