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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Be sure to read the whole thread on these most recent ones in SC (the tweeter is a seismologist at one of the big California research centers, BTW):

 

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.
 

bjdeming

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IRIS did another general-info thread about hazards; I'm sharing it, in case anyone is wondering about how vulnerable their homes, work, and infrastructure might be to a quake.

There isn't a whole lot that can be done in terms of seismic retrofitting, etc., unless you're a millionaire, but just knowing what to expect helps, right?

 

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When I was active with the local EM agencies we of course discussed what would be the "worst case scenario" for a natural disaster here in upstate SC, and it was a toss-up between forest fires (usually too wet for those) and an earthquake (here or elsewhere) affecting us. The 1886 Charleston quake was sai8d to be felt as a 4-point-something here. The 1913 Union County SC earthquake was a 5.5 event and used as an example to show what the old buildings in the heart of every city here would do. The old downtown businesses are unreinforced masonry which essentially falls apart in a serious quake, and the old wood-frame houses are unattached to foundations (which are usually brick piers). There was a slightly weaker one where I am now in 1924 but I haven't seen any damage reports or pics from it. Long after construction they discovered a fault laying right under the Oconee nuclear power station site maybe 30 miles from here. And a few months ago a 2-point-somnething happened in my neighborhood.

Being in construction I've been interested in what is done for seismic protection and mitigation, and some of the retrofits are cheapish and effective, such as foundation ties and reinforcement, but going much further can add up fast based on exterior wall cladding, height, and other structural details. It's said to be worth doing the basics as protection against the more likely moderate quakes even though they will do nothing for strong ones. As much as I love old houses and buildings and history along with the great craftsmanship in many of them, they aren't up-to-snuff compared to modern builds in almost any kind of natural disaster. My home would likely become a pile of rubble in a 4.0 or high-end EF-2, both unlikely but have happened here before so it's something to think about for sure!

Phil
 

bjdeming

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IRIS is focussing on the South lately. :cool:

Here's an animation they tweeted about the New Madrid Seismic Zone -- the most talked about zone until recently but not the only one in this part of the country.


If you don't have a feel for geologic time, that looks a little scary, so I'm also including a big-picture look at what this continental edge was doing back then.

It isn't some doom-laden disaster waiting to happen down there; it was just the result of natural plate tectonic movements: the tweet goes back 500 million years, the video 200 million years.


Farther east, in what are now seismic zones in East Tennessee, the Elgin area, Charleston, and probably other places saw similar activity even earlier in the Precambrian; I believe a whole ocean (Iapetus) opened and closed along what's now that eastern edge of North America before the Atlantic was even a gleam in the Triassic's eye. And then there were all the mountain-building collisions.

I'm only a layperson, but I get the impression those rocks down there bear lots of scars from well over a billion years' worth of continental bumper cars. Some of the rocks are denser than others, so they shift a little -- even today.

At least with the earlier coverage of the NMSZ, and now the Elgin swarm making headlines, there looks to be more interest and therefore better chances to thoroughly map what's down there well enough to maybe get a better idea of which specific places on the surface might experience strong shaking.

And that could lead to more mitigation work. It's kind of mind-paralyzing to look at a whole multi-state region as a possible disaster site. But if you can specify a county or some other small area -- it's a little like the severe weather polygons: people can get an idea of what trouble might come to them and act on that.

Very ambitious, but if we can ID weather on an exoplanet, we can make detailed basement maps anywhere east of the Mississippi (there are other zones, like up near Montreal).
 

Sawmaster

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TBH, only the New Madrid worries me as to earthquakes, though Summerville/Charleston would likely have a worse direct effect on me here in upstate SC. The biggest difference between these would be that New Madrid could long and severely disrupt the national economy which sustains me, while equal but more localized damage would get a faster and more effective response from a mostly-intact nation.

"The State" which is Columbia SC's leading newspaper published This Article about Hydroseismicity possibly being a trigger for the recent quake swarm here. Personally I'm not sure it has a great effect, but our planet is influenced by everything which happens on it in some way, so there may be some validity to the concept.

Phil
 

bjdeming

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Okay, this IRIS tweet uses an example from the 1906 San Francisco quake, but liquefaction can be expected anywhere the soil quality and water table conditions are right:


The town I lived in before moving to Corvallis is out in the valley and with groundwater almost at the surface -- not surprisingly, the state puts them among areas at high risk for liquefaction; probably a number of southern places are vulnerable, too.

Can't do anything about it, but it's just one more thing to expect to see in a quake (and to be thankful for, if it doesn't happen).

Here's some video from Oregon State showing liquefaction (and tsunami) damage in a port town in 2011. That quake was 1-2 orders of magnitude bigger than the 19th-century New Madrid big ones, and at least 2 orders of magnitude bigger than Charleston 1886, but still...there were sand boils back then, too, in the New Madrid events.

 
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