The South Shetland Islands and the northern part of the Antarctic Peninsula have the highest density of stations in Antarctica. But they also lie in one of the most seismically and volcanically active areas of the southern continent, thanks to the ever-widening Bransfield Strait. The series of earthquakes that shook the region between August 2020 and February 2021 was unusual even for science. An international team has now discovered the reason for the swarm quakes but had to resort to new methods of analysis in order to do so.
Massive magma intrusions from the Earth’s mantle, which first quickly pushed into the crustal region and then spread horizontally in a northeast-southwest direction like a dam, were responsible for more than 85,000 earthquakes in six months, the research team concluded in its just-published study. Two types of earthquakes were recorded: deeper earthquakes caused by the vertical spreading of magma and crustal earthquakes triggered in the earth’s crust itself by the formation of the dam. This magma dam had a length of about 20 kilometers at the end and extended northeast of an inactive deep-sea shield volcano, which the researchers call “Orca”, along the edge to the shelf area of King George Island.
The research team, led by Dr Simone Cesca of Germany’s GeoForschungszentrum (GFZ) Potsdam, also discovered in analysing the nearly 85,000 quakes that the ground on King George Island, the largest of the South Shetlands, was shifted nearly 11 centimeters to the side and one centimeter upward by the quakes. “In the past, seismicity in this region was moderate. However, in August 2020, an intense seismic swarm began there, with more than 85,000 earthquakes within half a year,” explains Dr Cesca, who is also the lead author of the study. “It represents the largest seismic unrest ever recorded there.”
Although the region is seismically active, only two stations in the area are equipped with appropriate recording equipment, including Argentina’s Carlini Station. To investigate the causes of the series of earthquakes, the team had to access data from more distant seismic stations, from GNSS (Global Navigation Satellite System) stations, and from satellite radar data (InSAR), then combine them with several geophysical models. As a result, Dr Cesca and the team of scientists from Italy, Poland and the US were able to trace the starting point of the earthquake series back to 10 August 2020. The team then recorded four phases until the end of activities in February 2021. The first phase was characterised by weak earthquakes that occurred at depths of 10-15 kilometers.
But the second phase began in late August with the first more severe quake, measuring 4.5 on the Richter scale, when magma from the depths pushed faster into the earth’s crust and moved toward the Orca volcano. This also produced the first of the two most severe quakes, both magnitude 5.9, on 2 October.
The third phase, with dam formation, then began on 21 October and culminated on 6 November with magnitude 6 earthquake. From then on, the fourth phase began, which meant the relatively abrupt end of activities, caused by a loss of pressure. In total, the team calculated an amount between 0.26-0.56 cubic kilometers of magma pushed upward, a record in Antarctica.
For research, the swarm quakes are a stroke of luck, while the work of Dr Cesca and her team means a complete success. “Our study represents a new successful investigation of a seismo-volcanic unrest at a remote location on Earth, where the combined application of seismology, geodesy and remote sensing techniques are used to understand earthquake processes and magma transport in poorly instrumented areas,” she said. The results of the work have just been published in the latest issue of the journal Nature Communications Earth & Environment and represents a first. “This is one of the few cases where we can use geophysical tools to observe intrusion of magma from the upper mantle or crust-mantle boundary into the shallow crust — a rapid transfer of magma from the mantle to almost the surface that takes only a few days,” Dr Cesca said. Hopefully, there are no bigger surprises lurking beneath the surface of the Bransfield Strait, as the region not only has the highest number of research stations, but is also a popular tourist destination.
Dr Michael Wenger, PolarJournal
Link to study: Cesca et al (2022) Nature Comm Earth Environm 3 (89) Massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica; https://doi.org/10.1038/s43247-022-00418-5
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