Complex earthquake: US Geological Survey - Seismic observations show that the M7.5 earthquake that struck the South Sandwich Islands on Aug 12 at 18:32 UTC is part of a complex seismic sequence. Simply put, we are only now learning that there was not one earthquake with a magnitude of 7.5, but two - 7.5 and 8.1
“Our current interpretation is that this earthquake is a precursor to the M8.1 earthquake, which occurred approximately 170 seconds later. Research is underway on this sequence to better understand the geometry of the fault and the details of the fracture. This analysis will take time and our understanding is likely to change, "the USGS said.
The location, depth, mechanism and magnitude of this earthquake are still preliminary and not as well defined as typical events of this level, due to the intervention of the previous foreshock M7.5, the agency added.
As of 08:52 UTC on 15 August, at least 126 earthquakes have occurred in the region.
Current understanding of the M8.1 earthquake on August 12 in the South Sandwich Islands was that it was a reverse fault at a depth of about 48 km (30 mi) in the Scotia subduction zone, according to the USGS2 Tectonic Brief.
The decision on the focal mechanism indicates that the slip occurred either along a steeply inclined fault, directed to the northwest, or along a shallowly inclined plane, directed to the southeast.
Notice that at this point the subduction boundary slopes towards the west.
The earthquake occurred ~ 3 minutes after the M7.5 foreshock, which was at a depth of about 63 km (39 mi) and about 90 km (56 mi) to the north.
At the site of the M8.1 earthquake, the South American Plate subducts westward under the Scotia Plate and the South Sandwich Microplate (a component of the wider Scotia Plate). The subduction rate of the South American Plate relative to the South Sandwich Microplate is ~ 71 mm (2.8 inches) per year.
The depth and mechanism of this event correspond to the fact that the earthquake source arose inside the subducted South American plate (intraplate earthquake), and not at the interface between the two plates.
Given the temporal proximity of these two large earthquakes, the fault mechanism for the main shock M8.1 is difficult to pinpoint due to the superposition of seismic waves.
Although earthquakes of this size are usually depicted as dots on maps, they are more accurately described as sliding over a wider fault zone. Events like the M8.1 earthquake on August 12, 2021 are typically 150 km x 75 km (93 x 46 mi) (length x width).
Eight other M7 + earthquakes occurred within 250 km (155 miles) of the M8.1 earthquake in the previous century, including the M7.5 foreshock.
The strongest of these previous earthquakes was M8.1 in May 1964.
M7 + earthquakes in the South Sandwich Islands have historically occurred at intermediate depths (broadly 70-300 km / 44-185 miles). However, the largest earthquake in the region was the shallow (~ 10 km / 6 mi deep) M8.1 earthquake on June 27, 1929, which occurred about 450 km (280 mi) northwest of the August 2021 M8.1 earthquake along the northern boundary of the Scotian Plate with the South American Plate.
None of these earthquakes are known to have resulted in human casualties, probably due to their remote location away from settlements that could be vulnerable to an earthquake.
Within 24 hours after the main shock, M8.1, the US Geological Survey recorded 61 aftershocks with a force of M4.5 or higher.
The sequence of aftershocks during this period of time includes three aftershocks with a strength greater than M6 (M6.0, M6.2 and M6.3).
These aftershocks extend about 470 km (290 mi) parallel to the fault, from the M7.5 foreshock southward to the triple junction between the South American, South Sandwich and Antarctic plates.
The depth range of these aftershocks is 10 - 76 km (6 - 47 miles), with most events with an estimated depth occurring at depths of 50 - 65 km (31 - 40 miles).
The preliminary location of some of the aftershocks is located east of the depression, which indicates the possible presence of earthquakes of the external uplift, which occur as a result of the downward bending of the subducting plate.