U.S. Geological Survey says M7.5 earthquake in South Sandwich Islands was a precursor to M8.1 quake

Complex earthquake: U.S. Geological Survey – Seismic observations show that the M7.5 earthquake that occurred in the South Sandwich Islands on August 12 at 18:32 UTC is part of a complex seismic sequence. Simply put, we are just now learning that there was not one M7.5, but two, 7.5 and 8.1 earthquakes

Seismic observations show that the M7.5 earthquake that occurred in the South Sandwich Islands on August 12 at 18:32 UTC is part of a complex seismic sequence. Simply put, we are just now learning that there was not one M7.5 earthquake, but two, 7.5 and 8.1

“Our current interpretation is that this earthquake is a precursor to the M8.1 earthquake, which occurred about 170 seconds later. Studies of this sequence are underway to better understand fault geometry and rupture details. 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 magnitude because of the interference of the previous M7.5 foreshock, the agency added.

As of 08:52 UTC August 15, at least 126 earthquakes had occurred in the region.

The current understanding of the August 12 M8.1 earthquake in the South Sandwich Islands is that it was the result of a reverse fault at a depth of about 48 km (30 miles) in the Scotia subduction zone, according to a USGS2 tectonic summary report.

A decision on the focal mechanism indicates that the slip occurred either on a steeply inclined fault pointing northwest or on a shallowly inclined plane pointing southeast.

Note that the boundary of the subduction zone slopes westward at this point.

The earthquake has happened within ~3 minutes after M7.5 foreshock, which was located at the depth of 63 km (39 miles) and approximately 90 km (56 miles) to the north.

At the site of the M8.1 earthquake, the South American plate subducts westward beneath the Scotia plate and the South Sandwich microplate (a component of the broader Scotia plate). The rate of subduction 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 are consistent with the fact that the earthquake origin occurred within the subducted South American plate (intraplate earthquake) rather than at the interface between the two plates.

Given the temporal proximity of these two large earthquakes, the fault mechanism for the main M8.1 tremor is difficult to accurately determine due to overlapping seismic waves.

Although earthquakes of this size are usually depicted on maps as points, it is more accurate to describe them as sliding along a larger fault zone. Events like the M8.1 earthquake on August 12, 2021, typically have dimensions (length x width) of 150 km x 75 km (93 x 46 miles).

There have been eight other M7+ earthquakes 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 region 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 M8.1 earthquake in August 2021 along the northern boundary of the Scotia Plate with the South American Plate.

None of these earthquakes are known to have caused human casualties, probably due to their remote location away from populated areas that could be vulnerable to an earthquake.

Within 24 hours after the main strike M8.1 the US Geological Survey registered 61 aftershocks with M4.5 magnitude or higher.

The sequence of aftershocks during this time interval includes three aftershocks with magnitudes greater than M6 (M6.0, M6.2 and M6.3).

These aftershocks extend about 470 km (290 miles) 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 to 76 km (6 to 47 mi), with most events with estimated depths occurring at 50 to 65 km (31 to 40 mi).

The tentative location of some of the aftershocks is to the east of the trough, indicating the possible presence of external uplift earthquakes that occur as a result of downward bending of the subducting plate.

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