A very strong M6.9 earthquake hit South Sandwich Islands region at 11:10 UTC on August 16, 2021, at a depth of 14 km (8.7 miles). This is the strongest of 127 aftershocks registered by the USGS after M7.5 and M8.1 on August 12.
While this intense earthquake sequence is still being analyzed, the current understanding is that M7.51 was a foreshock to M8.12 which took place about 170 seconds later.
The M8.1 earthquake occurred as the result of reverse faulting at a depth of approximately 48 km (30 miles) in the Scotia subduction zone.
The focal mechanism solution indicates slip occurred on either a steeply dipping fault dipping to the northwest, or on a shallowly dipping plane dipping to the southeast, according to the USGS.3
"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," the USGS said.
In the 24 hours following the M8.1 mainshock, the USGS located 61 aftershocks of M4.5 or greater.
The aftershock sequence during this time frame includes three aftershocks larger than M6 (M6.0, M6.2, and M6.3).
The aftershocks span a trench-parallel distance of about 470 km (290 miles), stretching from the M7.5 foreshock southward to the triple junction between the South America, South Sandwich, and Antarctica plates.
Preliminary locations for a subset of the aftershocks are located east of the trench indicating the potential presence of outer rise earthquakes, which occur as a result of downward flexure of the subducting slab.
From August 12 at 18:35 UTC (M8.1) to 11:52 UTC on August 16, the USGS registered a total of 126 aftershocks – 32 M4 – 5, 90 M5 – 6, and 4 M6 – 7.
Earthquakes in South Sandwich Islands region from August 12 – 16, 2021. Source: USGS. Credit: TW/SAM, Google
1 Very strong M7.5 earthquake hits South Sandwich Islands region – The Watchers
2 Complex earthquake: USGS says M7.5 in South Sandwich Islands was foreshock of M8.1 – The Watchers
3 M8.1 earthquake South Sandwich Islands region on August 12, 2021 – USGS
Featured image credit: TW/SAM, Google
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