2021 South Sandwich Islands earthquakes
megathrust or Intraplate | |
Areas affected | South Georgia and the South Sandwich Islands |
---|---|
Max. intensity | MMI VII (Very strong) |
Tsunami | Up to 2.46 ft (0.75 m) at South Georgia Island |
Aftershocks | 1,968 Mw 4.0+, up to 7.1 Mw[3] |
Casualties | None |
The 2021 South Sandwich Islands earthquakes were a pair of powerful earthquakes, followed by many strong aftershocks which struck along the South Sandwich Trench in August 2021. The quakes measured 7.5 and 8.1 on the moment magnitude scale, according to the United States Geological Survey.[4][5] The mainshock is tied with another event in 1929 as the largest earthquake ever recorded in the South Atlantic region,[6] and is tied with the 2021 Kermadec Islands earthquake as the second largest earthquake of 2021.
Tectonic setting
These earthquakes occurred in the Scotia
The 7.5 Mw foreshock was located at a depth of 47 km, placing it within the lower half of the lithosphere of the South American plate, and it is likely to have been caused by compressional forces due to the downward bending of the plate.[7] The mainshock occurred about 3 minutes later at a depth of approximately 56 km and 90 km to the south. Due to seismic interference from the earlier quake, the mechanism, faulting geometry, and rupture details of the mainshock are not yet fully understood.[8] The focal mechanism solution indicates slip occurred on either a steep fault dipping to the northwest or a shallow plane dipping to the southeast within the lithosphere of the subducting plate.[8]
Over the previous century, eight other M 7+ earthquakes, including the M 7.5 foreshock, occurred within 250 km of the August 2021 M 8.1 earthquake. The largest of these previous earthquakes was a M 7.8 earthquake in May 1964.[9] M 7+ earthquakes in the South Sandwich Island region have historically occurred at intermediate depths (broadly defined as 70–300 km depth). However, the largest earthquake in the region was a shallow (10 km deep) M 8.1 earthquake on June 27 1929 that was located approximately 450 km northwest of the epicentre of the August 2021 M 8.1 earthquake along the northern boundary of the two plates.[10] None of these earthquakes are known to have caused fatalities due to their remote location.
Earthquakes
The first earthquake, measured at magnitude 7.5, struck at 16:32 PM local time, but did not cause any damage or casualties. The mainshock, which was the much stronger magnitude 8.1, occurred 3 minutes after the foreshock. A
Although the two earthquakes were considered intraslab events occurring within the South American Plate, rather than at the boundary of the two plates, the aftershock distribution suggested a rupture of the plate boundary megathrust as well. Aftershocks, some larger than magnitude 6.0, occurred along a 470-km-long segment of the subduction zone,
Based on the presence of numerous aftershocks on the subduction zone and outer-rise, the earthquake likely ruptured the megathrust all the way to the trench. The aftershock distribution also suggest a large rupture area for the main event.[12]
Characteristics
The event was rather unusual due to its duration of 260 seconds; much longer than usual for its moment magnitude. An initial event measuring 7.5 Mw occurred at a depth of 47 km, lasting 50 seconds. The 7.5 Mw foreshock consisted of two smaller sub-events that initiated along the deeper portion of a thrust fault, but ruptured up-dip.[13]
Shortly afterwards, a 8.16 Mw quake ruptured the shallow subduction zone, with a rupture duration of 180 seconds. The first sub-event ruptured 180 km along the subduction zone interface. Two other sub-events measuring 7.6 Mw and 7.7 Mw occurred 250 km and 300 km south. The 7.6 Mw sub-event involved breaking the shallow subduction zone, displaying characteristics of a
Other events
The South Sandwich Islands mainshock came just 2 weeks after a
Aftershocks
Date | Mw | MMI | Source |
---|---|---|---|
2021-08-12 | 7.5 | VI | [7] |
2021-08-12 | 8.1 | VII | [5] |
2021-08-12 | 6.7 | I | [19] |
2021-08-12 | 6.1 | IV | [20] |
2021-08-12 | 6.0 | IV | [21] |
2021-08-13 | 6.1 | IV | [22] |
2021-08-16 | 6.9 | IV | [23] |
2021-08-17 | 6.1 | IV | [24] |
2021-08-18 | 6.0 | IV | [25] |
2021-08-22 | 6.6 | IV | [26] |
2021-08-22 | 7.1 | IV | [27] |
2021-10-04 | 6.2 | IV | [28] |
2021-11-01 | 6.0 | IV | [29] |
2021-12-03 | 6.0 | III | [30] |
2021-12-16 | 6.0 | III | [31] |
2021-12-20 | 6.0 | III | [32] |
2022-01-25 | 6.0 | III | [33] |
2022-03-06 | 6.0 | IV | [34] |
In the 24 hours following the mainshock, 61 aftershocks of M 4.5 or larger were detected, including three aftershocks greater than M 6. These aftershocks spanned a distance of approximately 470 km along the
By August 24, there were approximately 500 aftershocks between magnitudes 4.5 and 5.1,
In total there were 1,968 aftershocks as of 11 December 2022.[3]
Tsunami
A tsunami with a maximum run-up height of around 75 cm (2.46 ft) was measured at
See also
References
- ^ a b c "Global CMT Catalog". Global Centroid Moment Tensor. Retrieved 10 September 2021.
- ^ "GEOFON Program". GFZ. Retrieved 10 September 2021.
- ^ a b "USGS earthquake catalog". United States Geological Survey.
- ^ "M 7.5 - South Sandwich Islands region". United States Geological Survey. Retrieved 12 August 2021.
- ^ a b "M 8.1 - South Sandwich Islands region". United States Geological Survey. Retrieved 12 August 2021.
- ^ "M 8.1 - South Sandwich Islands Region". United States Geological Survey. 27 June 1929.
- ^ a b c d "M 7.5 - South Sandwich Islands region". earthquake.usgs.gov. Retrieved 30 August 2021.
- ^ a b c d e "M 8.1 - South Sandwich Islands region". earthquake.usgs.gov. Retrieved 30 August 2021.
- ^ "M 7.8 - South Sandwich Islands region". United States Geological Survey. Retrieved 13 August 2021.
- ^ "M 8.1 - South Sandwich Islands region (1929)". United States Geological Survey. Retrieved 13 August 2021.
- ^ "Earthquake of magnitude 7.5 strikes South Sandwich Islands region". Reuters. 12 August 2021. Retrieved 13 August 2021.
- ^ S2CID 244092196. Retrieved 10 September 2021.
- ^ S2CID 244736464.
- ^ "M 8.2 - 91 km ESE of Perryville, Alaska". 29 July 2021.
- ^ "M 8.1 - Kermadec Islands region". USGS-ANSS. Retrieved 4 March 2021.
- ^ "USGS Earthquake Hazards Program: FAQ". Earthquake.usgs.gov. 10 December 2012. Archived from the original on 20 November 2014. Retrieved 24 December 2012.
- ^ "Significant Earthquakes - 2007". United States Geological Survey. Retrieved 13 August 2021.
- ^ "M8+ in 2007". United States Geological Survey. Retrieved 13 August 2021.
- ^ "M 6.7 - South Sandwich Islands region". United States Geological Survey. Retrieved 12 August 2021.
- ^ "M 6.1 - South Sandwich Islands region". United States Geological Survey. Retrieved 12 August 2021.
- ^ "M 6.0 - South Sandwich Islands region". United States Geological Survey. Retrieved 12 August 2021.
- ^ "M 6.1 - South Sandwich Islands region". United States Geological Survey. Retrieved 13 August 2021.
- ^ "M 6.9 - South Sandwich Islands region". United States Geological Survey. Retrieved 16 August 2021.
- ^ "M 6.1 - South Sandwich Islands region". United States Geological Survey. Retrieved 17 August 2021.
- ^ "M 6.0 - South Sandwich Islands region". United States Geological Survey. Retrieved 7 September 2021.
- ^ "M 6.6 - South Sandwich Islands region". United States Geological Survey. Retrieved 22 August 2021.
- ^ "M 7.1 - South Sandwich Islands region". United States Geological Survey. Retrieved 22 August 2021.
- ^ "M 6.2 - South Sandwich Islands region". United States Geological Survey.
- ^ "6.0 - South Sandwich Islands region". United States Geological Survey.
- ^ "M 6.0 - South Sandwich Islands region". earthquake.usgs.gov. United States Geological Survey.
- ^ "M 6.0 - South Sandwich Islands region". earthquake.usgs.gov.
- ^ "M 6.0 - South Sandwich Islands region". earthquake.usgs.gov. United States Geological Survey.
- ^ "M 6.0 - South Sandwich Islands region". United States Geological Survey. 25 January 2022. Archived from the original on 25 January 2022. Retrieved 25 January 2022.
- ^ "M 6.0 - South Sandwich Islands region". earthquake.usgs.gov. Archived from the original on 6 March 2022.
- ^ "USGS earthquake catalog". United States Geological Survey. Retrieved 6 November 2021.
- ^ "USGS earthquake catalog". United States Geological Survey. Retrieved 6 November 2021.
- ^ "USGS earthquake catalog". United States Geological Survey. Retrieved 6 November 2021.
- ^ "M 7.1 - South Sandwich Islands region". United States Geological Survey. Retrieved 22 August 2021.
- ^ "South Sandwich Islands Tsunami, August 12, 2021". nctr.pmel.noaa.gov. Retrieved 28 September 2021.
External links
- The International Seismological Centre has a bibliography and/or authoritative data for this event.
- The International Seismological Centre has a bibliography and/or authoritative data for this event.