Southern earthquakes (1 Viewer)

bjdeming

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With both the Charleston and New Madrid seismic zones in the South, perhaps a thread is useful. (Update: Forgot that there's an Eastern Tennessee Seismic Zone, too.)

South Carolina has been a little active lately, with five since the 20th and thirteen quakes since November, reportedly. Did anyone feel the shaking from any of these?

Sorry, I haven't read up on this seismic zone, but here some information links:
 
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bjdeming

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WLTX met (possibly CGI'd???) discusses the South Carolina quakes (here is the accompanying website article):

 

bjdeming

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Just found this; it was posted very recently and has over 2 million views. Check out the YouTube site, too. :cool:
 

bjdeming

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Am guessing this was associated with the East Tennessee Seismic Zone:


They do rumble sometimes. I was up in the Adirondacks in the early 80s when the waves of an M3-something centered in New Hampshire came through: sounded just like a big truck, but there was no traffic. And trucks never made my cabinet door slowly swing open (it was a fairly flimsy cabin).
 

Sawmaster

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South Carolina AND north Florida. Not damaging.

Been living in upstate SC most of my life and we used to see only a few minor tremors in the State each year, but recently there's been a 'swarm' of them near Elgin mid-state and nobody seems to understand why. A couple months ago we had a 2 point something jolt centered in my neighborhood (Pickens County). Woke me up, sounded like something had hit the window blinds. Neighbors about a mile away are hearing unexplained "boom" sounds at night and a couple have reported feeling jolts too, but USGS hasn't reported anything. Strange.

According to the Geologists this is supposed to be a seismically stable area (or so I was told 15 years ago) but if another severe Charleston/Summerville fault quake happens we might get 4.2 damage. EM people say that is our 'worst case scenario' in the upstate as we've got a lot of old unreinforced masonry buildings. Geologists are saying the Elgin swarm isn't related to the Summerville fault but I do have to wonder!

Phil
 

TH2002

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Been living in upstate SC most of my life and we used to see only a few minor tremors in the State each year, but recently there's been a 'swarm' of them near Elgin mid-state and nobody seems to understand why. A couple months ago we had a 2 point something jolt centered in my neighborhood (Pickens County). Woke me up, sounded like something had hit the window blinds. Neighbors about a mile away are hearing unexplained "boom" sounds at night and a couple have reported feeling jolts too, but USGS hasn't reported anything. Strange.

According to the Geologists this is supposed to be a seismically stable area (or so I was told 15 years ago) but if another severe Charleston/Summerville fault quake happens we might get 4.2 damage. EM people say that is our 'worst case scenario' in the upstate as we've got a lot of old unreinforced masonry buildings. Geologists are saying the Elgin swarm isn't related to the Summerville fault but I do have to wonder!

Phil
I myself wonder if there are some previously undiscovered blind thrust faults in SC. Northridge '94 happened on a previously undiscovered blind thrust fault and the Puente Hills fault (which poses an even greater risk to LA than the well-known SAF) wasn't discovered until 1999.
 

bjdeming

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The videos are pretty good -- don't worry; they have some for people who aren't as slim and bendy as that model.

I was in one of these drills here and managed to sort of fit under a library desk. Felt like a fool -- you do when acting this way in public. The drill gets you over that so it's easier to respond naturally if you ever need to. I probably could use some reinforcement.
 

bjdeming

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And here's the USGS tectonic summary JP pointed out (sorry for the piecemeal posts -- just stopped working a little while ago, and I'm playing catch-up on the news today).

One thing I'll add, though as a layperson: the present southeastern coast of North America's cratonic core down there has experienced lots of things -- continental collisions, ocean formation, and so forth. However, for hundreds of millions of years, it has been what is called a passive margin in plate tectonics: two plates aren't grinding side by side, like in California, nor is there a subduction zone there, as where I live or, closer to you, in Puerto Rico.

It's just hangin' out.

So that's why scientists aren't sure what's going on, I think, with the Elgin swarm. There isn't any obvious explanation for it available yet in plate tectonic theory. (Don't quote me: I am a layperson, but I do read up on this stuff for work and personal interest: just take what I say FWIW and listen to the experts.)

Plate tectonics is a new and rather simplified model for what a very old and complex Earth does. There are still many mysteries. They may figure it out, or most likely, the swarm will just taper off. That's what usually happens.

Anyway, here's the USGS summary:

Tectonic Summary​

This June 29 M3.6 Elgin, South Carolina, earthquake is part of an ongoing sequence in central South Carolina. The sequence started on December 27, 2021, with an M3.3 earthquake near Lugoff, South Carolina. Between December 27, 2021, and June 29, 2022, there have been about 40 earthquakes in this sequence spanning M1.3 to M3.6. Five of the earthquakes were M3.0 or larger and were widely felt. The earthquakes have occurred at shallow depths of 7 km or less. Shaking from earthquakes in the eastern U.S. extends to greater distances from the epicenter than earthquakes of similar magnitudes in the western U. S. due to different geological conditions. These two factors have led to these earthquakes being widely felt. Several of these earthquakes have over 3000 associated “Did You Feel It?” reports of felt shaking, but the shaking reported has not been at intensities that typically lead to damage.
Earthquakes are not uncommon in the vicinity of the 2021-2022 South Carolina sequence, but having a sequence of about 40 earthquakes in such a short time is unusual. Many earthquakes of similar magnitudes have occurred in the eastern U.S., but it is extremely rare for them to be foreshocks to much larger earthquakes. This swarm will continue for an unknown length of time, and if it stops it may resume sometime in the future.

The largest earthquake in the region of this sequence was the M4.8 1913 Union County, SC, earthquake about 90 km to the northwest. Earthquakes have occurred periodically around the Monticello Reservoir ~30 km west of the 2021-2022 sequence since the 1970s, but the current earthquakes do not appear to be related to the reservoir. The 2021-2022 sequence is additionally located ~140 km northwest of the ~M7 1886 Charleston earthquake. The 2021-2022 sequence is not associated with the seismic zone of the 1886 earthquake.

Key Points

- An ongoing earthquake sequence began in central South Carolina near the towns of Elgin and Lugoff on December 27, 2021, with an M3.3 earthquake.

- There have been about 40 earthquakes of M1.3 or larger in the sequence, with the largest being the M3.6 earthquakes on June 29, 2022.

- Several of the earthquakes have been felt, as is common for even small magnitude earthquakes in the eastern United States. None of the earthquakes so far have produced shaking intensities where damage to buildings is expected.

- Small magnitude earthquakes like these are relatively common in South Carolina, although the number of earthquakes in the time span of the current sequence is unusual.

- This earthquake sequence is not related to the region of seismicity associated with the great 1886 Charleston earthquake.

- In October 2021 there was a series of seven earthquakes, some felt, near Jenkinsville, South Carolina and the Monticello Reservoir. The current earthquake sequence does not appear to be related to those earthquakes.

-There are no oil or gas operations in these crystalline Piedmont rocks where these earthquakes occurred, so the earthquakes are not the result of oil and gas production.
 

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