Mw 7.5 South Sandwich Isl

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  • #1
davenn
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another day, another big quake

Mw7.5 South Sandwich Isl, South Atlantic

M 7.5 - South Sandwich Islands region​

  • 2021-08-12 18:32:54 (UTC)
  • 57.596°S 25.187°W
  • 63.3 km depth
210812  UT Mw7.5 South Sandwich Isl.gif
 

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  • #2
berkeman
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You need to turn off your seismo Dave! That's the only way to stop all of these earthquakes!
 
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  • #3
Astronuc
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There's a lot of seismic activity in the region. About 30 aftershocks from 4.8 to 6.2 Mag, and one 4.9 mag earthquake near the South Georgia Island.

https://en.wikipedia.org/wiki/South_Georgia_and_the_South_Sandwich_Islands
https://earthquake.usgs.gov/earthquakes/eventpage/us6000f4ly/executive

From USGS us6000f4ly/executive:
Over the previous century, seven other M7+ earthquakes occurred within 250 km of the August 2021 earthquake. The largest of these earthquakes was an M7.8 in May 1964. M7+ earthquakes in the South Sandwich Island region historically have occur at intermediate-depths (broadly defined as 70-300 km depth). The largest earthquake in the region was an M8.1 earthquake on June 27, 1929 and was located ~360 km northwest of the of August 2021 earthquake along the northern boundary of the Scotia plate with the South America plate. None of these earthquakes are known to have caused fatalities, likely because of their remote location far from population centers that might be vulnerable to earthquake shaking, as well as their intermediate depths which are not normally conducive to the generation of tsunamis.
 

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  • South_Georgia,South_Sandwich_Islands_2021-08-12_Earthquakes.png
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  • #4
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I had no idea how far the Sandwich Islands are from the South Sandwich Islands. (My geography stinks) 8500 miles.
 
  • #5
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M 6.3 - South Sandwich Islands region​

https://earthquake.usgs.gov/earthquakes/eventpage/us6000f5h2/executive
  • 2021-08-13 10:34:29 (UTC)
  • 60.491°S 27.177°W
  • 10.0 km depth
This one is relatively shallow, and it's near the intersection of the Antarctic, Scotia (and Sandwich) and South American plates.
https://en.wikipedia.org/wiki/Scotia_Plate
https://en.wikipedia.org/wiki/Antarctic_Plate
https://en.wikipedia.org/wiki/South_American_Plate

Another 30 aftershocks including the 6.3 Mag since my previous post.

All aftershocks between 7.5 and 6.3 Mag in format: Mag, Date/Time, Depth

5.1 2021-08-13 03:11:18 (UTC-07:00) 10.0 km
5.0 2021-08-13 02:01:24 (UTC-07:00) 35.0 km
5.1 2021-08-13 01:46:29 (UTC-07:00) 53.5 km
5.2 2021-08-13 01:33:13 (UTC-07:00) 10.0 km
5.0 2021-08-13 01:25:00 (UTC-07:00) 54.8 km
5.0 2021-08-13 00:13:56 (UTC-07:00) 37.1 km
5.2 2021-08-13 00:08:37 (UTC-07:00) 10.0 km
5.2 2021-08-13 00:04:34 (UTC-07:00) 10.0 km
5.0 2021-08-12 23:41:46 (UTC-07:00) 66.8 km
5.0 2021-08-12 23:34:34 (UTC-07:00) 10.0 km
5.5 2021-08-12 23:14:11 (UTC-07:00) 10.0 km
5.1 2021-08-12 22:44:04 (UTC-07:00) 45.3 km
5.0 2021-08-12 22:41:24 (UTC-07:00) 45.8 km
5.5 2021-08-12 22:24:58 (UTC-07:00) 31.6 km
5.3 2021-08-12 22:24:04 (UTC-07:00) 58.7 km
5.3 2021-08-12 21:53:09 (UTC-07:00) 10.0 km
5.1 2021-08-12 21:03:58 (UTC-07:00) 10.0 km
5.1 2021-08-12 20:53:21 (UTC-07:00) 35.0 km
5.7 2021-08-12 20:43:19 (UTC-07:00) 66.2 km
5.1 2021-08-12 20:38:23 (UTC-07:00) 10.0 km
4.6 2021-08-12 19:57:27 (UTC-07:00) 10.0 km
5.2 2021-08-12 19:33:40 (UTC-07:00) 10.0 km
4.9 2021-08-12 19:29:33 (UTC-07:00) 25.4 km
5.3 2021-08-12 18:47:37 (UTC-07:00) 10.0 km
4.9 2021-08-12 18:39:15 (UTC-07:00) 46.1 km
5.0 2021-08-12 18:26:41 (UTC-07:00) 31.1 km
4.6 2021-08-12 18:08:59 (UTC-07:00) 10.0 km
5.1 2021-08-12 17:46:19 (UTC-07:00) 10.0 km
5.0 2021-08-12 17:42:25 (UTC-07:00) 10.0 km
5.1 2021-08-12 17:04:15 (UTC-07:00) 10.0 km
5.4 2021-08-12 16:50:11 (UTC-07:00) 10.0 km
5.0 2021-08-12 16:32:56 (UTC-07:00) 19.9 km
5.4 2021-08-12 16:24:27 (UTC-07:00) 51.0 km
5.2 2021-08-12 16:23:45 (UTC-07:00) 52.9 km
4.9 2021-08-12 16:20:03 (UTC-07:00) 35.0 km
5.0 2021-08-12 16:08:13 (UTC-07:00) 35.0 km
4.8 2021-08-12 16:07:41 (UTC-07:00) 35.0 km
4.9 2021-08-12 15:40:01 (UTC-07:00) 35.0 km
4.9 2021-08-12 15:19:11 (UTC-07:00) 35.0 km
5.1 2021-08-12 15:13:10 (UTC-07:00) 35.0 km
5.5 2021-08-12 15:08:35 (UTC-07:00) 35.0 km
5.1 2021-08-12 14:56:55 (UTC-07:00) 10.0 km
5.1 2021-08-12 14:48:23 (UTC-07:00) 60.7 km
4.9 2021-08-12 14:25:14 (UTC-07:00) 35.0 km
5.2 2021-08-12 14:19:55 (UTC-07:00) 35.0 km
5.6 2021-08-12 14:07:01 (UTC-07:00) 69.3 km
5.2 2021-08-12 13:49:54 (UTC-07:00) 51.9 km
5.4 2021-08-12 13:38:07 (UTC-07:00) 69.6 km
5.3 2021-08-12 13:36:23 (UTC-07:00) 65.2 km
5.0 2021-08-12 13:28:40 (UTC-07:00) 60.1 km
5.4 2021-08-12 13:15:17 (UTC-07:00) 35.0 km
5.0 2021-08-12 13:14:45 (UTC-07:00) 35.0 km
5.8 2021-08-12 13:06:37 (UTC-07:00) 76.4 km
4.9 2021-08-12 13:05:00 (UTC-07:00) 10.0 km
5.5 2021-08-12 12:58:58 (UTC-07:00) 10.0 km
6.2 2021-08-12 12:44:55 (UTC-07:00) 61.9 km
6.0 2021-08-12 12:43:49 (UTC-07:00) 67.7 km
5.1 2021-08-12 12:39:22 (UTC-07:00) 10.0 km
5.7 2021-08-12 12:29:14 (UTC-07:00) 48.0 km
5.9 2021-08-12 12:07:52 (UTC-07:00) 18.9 km
5.9 2021-08-12 11:49:54 (UTC-07:00) 47.3 km

And this just in

M 5.8 - South Sandwich Islands region​

https://earthquake.usgs.gov/earthquakes/eventpage/us6000f5i9/executive
  • 2021-08-13 11:45:39 (UTC)
  • 57.285°S 25.558°W
  • 43.8 km depth
 
  • #6
davenn
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Well, very interesting developments for this sequence
The USGS have determined there were 2 separate quakes just under 3 minutes apart ........

M 8.1 - South Sandwich Islands region​

  • 2021-08-12 18:35:20 (UTC)
  • 58.416°S 25.321°W
  • 48.3 km depth
Seismic observations suggest this earthquake is part of a complex seismic sequence. Our current interpretation is that this earthquake is the mainshock to a M7.5 that occurred ~170s earlier. The location, depth, mechanism, and magnitude of this earthquake is preliminary and less well constrained than typical events of this size due to interference from the preceding M7.5 foreshock. Research is being conducted on this sequence to better understand the faulting geometry and details of rupture. This analysis will take time and our understanding will likely evolve. This page will be updated as we learn more about these interesting events.
https://earthquake.usgs.gov/earthquakes/eventpage/us6000f53e/executive

This does help to explain the VERY active aftershock sequence. And it also explains why I got around 30 mins or so of
wall to wall signal from what was initially listed as a single Mw7.5 event.
 
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  • #7
Astronuc
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Comparing the 7.5 location and depth: 57.596°S 25.187°W, 63.3 km depth
with the Mag 8.1 location and depth: 58.416°S 25.321°W, 48.3 km depth

Distance
56.963 statute mi​
49.499 naut mi​
91.673 km​
ellipsoidal earth
Distance calculator: https://stevemorse.org/nearest/distance.php

The latter quake was not quite on top of the other, and it was shallower.
 
  • #8
davenn
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The latter quake was not quite on top of the other, and it was shallower.

Yes, about 100 - 120 km south of the M7.5
 
  • #9
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I had no idea how far the Sandwich Islands are from the South Sandwich Islands. (My geography stinks) 8500 miles.
In my opinion, that's more history than geography -- Sandwich was a patron of Captain Cook, who named an achipelago of islands that he had discovered after him -- they later became called the Hawaiian Islands -- the Brits also named the (now uninhabited) South Sandwich Islands after him -- I think that neither set of islands, nor the familiar food assemblage, should ever have been named after him.
 
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  • #11
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  • #12
davenn
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M 6.9 - 136 km SE of Perryville, Alaska

Yes, recorded that one reasonably well

210814  UT Mw6.9 Alaskan ashk1.gif


but it hadnt event tailed off properly when the 7.2 hit and then the 7.2 totally overrode the 6.9 in amplitude .....

210814  UT Mw6.9 Alaskan ashk 7.2 Haiti.gif



Dave
 
  • #13
Astronuc
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  • #14
davenn
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Still strong activity in the area

yup and a 6.9 on the 16th ... recorded that one quite well
 
  • #16
Astronuc
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As of Friday, ongoing aftershocks in the 4.7-5.2 Mag range

5.2 2021-08-20 08:56:51 (UTC-07:00) 12.7 km
5.1 2021-08-20 08:07:47 (UTC-07:00) 56.8 km
4.8 2021-08-20 04:16:16 (UTC-07:00) 21.7 km
5.2 2021-08-19 20:41:22 (UTC-07:00) 80.9 km
5.1 2021-08-19 20:21:33 (UTC-07:00) 35.0 km
5.1 2021-08-19 20:13:51 (UTC-07:00) 10.0 km
4.7 2021-08-19 19:42:30 (UTC-07:00) 28.6 km
5.0 2021-08-19 18:09:51 (UTC-07:00) 10.0 km
5.1 2021-08-19 14:26:32 (UTC-07:00) 10.0 km

And the magnitudes don't appear to be diminishing in intensity after one week, although they seem to diminish in frequency.
 
  • #18
davenn
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Another couple of large aftershocks from the South Sandwich Islands

M 6.8 - South Sandwich Islands region​

  • 2021-08-22 00:45:08 (UTC)
  • 60.122°S 24.344°W
  • 11.0 km depth

M 7.1 - South Sandwich Islands region​

  • 2021-08-22 21:33:20 (UTC)
  • 60.291°S 24.895°W
  • 14.0 km depth

Mw6.8 and Mw7.1

2108122  UT M6.8 and 7.1 ashks South Sandwich Isl2.gif
 
  • #19
Astronuc
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Another couple of large aftershocks from the South Sandwich Islands
I was wondering if we might see another in the 7 Mag range, and 6.8 was close.

I was looking at events in the region. I grabbed a couple of screenshots of the earthquakes over 7 days ending yesterday. The 7.1 and 6.8 are at the lower right of the group, near the triple intersection.
 

Attachments

  • SouthSandwichIslands_2021-08-23 Latest Earthquakes(last7days).png
    SouthSandwichIslands_2021-08-23 Latest Earthquakes(last7days).png
    22.9 KB · Views: 14
  • SouthSandwichIslands_Region_2021-08-23 Earthquakes(last7days).png
    SouthSandwichIslands_Region_2021-08-23 Earthquakes(last7days).png
    18.1 KB · Views: 18
Last edited:
  • #20
Astronuc
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Still activity in the South Sandwich Islands, and some isolated tremors along the Antarctic Plate boundary
Code:
5.0  South Sandwich Islands region  2021-08-29 07:53:24 (UTC-07:00)  48.3 km
4.7  Southwest Indian Ridge         2021-08-29 07:13:12 (UTC-07:00)  10.0 km
4.9  South Sandwich Islands region  2021-08-29 06:04:04 (UTC-07:00)  41.4 km
5.3  southwest of Africa            2021-08-28 21:01:18 (UTC-07:00)  10.0 km
4.9  South Shetland Islands         2021-08-28 16:08:55 (UTC-07:00)  14.9 km
5.1  South Sandwich Islands region  2021-08-28 13:10:11 (UTC-07:00)  50.6 km
 

Attachments

  • Earthquakes_08302021_South Sandwich Islands and Antarctic Plate(last7days).png
    Earthquakes_08302021_South Sandwich Islands and Antarctic Plate(last7days).png
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