In a remote area, a combination of geophysical methods determines the magma transfer below the seabed as the cause. Volcanoes can even be found off the coast of Antarctica. A sequence of more than 85,000 earthquakes was recorded in 2020 on the deep-water volcano Orca, which was dormant for a long time, a swarm of earthquakes of unprecedented proportions for this region. The fact that such events can be studied and described in remarkable detail even in such remote and therefore poorly equipped areas is now evident from the study of an international team published in the journal Communications Earth and Environment. Researchers from Germany, Italy, Poland and the United States participated in the study, led by Simone Cesca of the German Geosciences Research Center (GFZ) in Potsdam. They were able to combine seismic, geodetic, and remote sensing techniques to determine how rapid magma transfer from the Earth’s mantle near the crust-mantle boundary almost to the surface caused the swarm to shake.
Orca Volcano between the tip of South America and Antarctica
Swarm earthquakes occur mainly in volcanically active areas. The movement of fluids in the Earth’s crust is therefore suspected as the cause. Orca seamount is a large underwater shield volcano with a height of about 900 meters above sea level and a base diameter of about 11 kilometers. It is located in the Bransfield Strait, an ocean canal between the Antarctic Peninsula and the Shetland Islands, southwest of the southern tip of Argentina. Illustration of the seismically active zone off the coast of Antarctica. Credit: Cesca et al. 2022; nature Commun Earth Environ 3, 89 (2022); doi.org/10.1038/s43247-022-00418-5 (CC BY 4.0) “In the past, seismicity in this area was moderate. However, in August 2020 an intense seismic swarm started there, with more than 85,000 earthquakes in half a year. “It represents the largest seismic disturbance ever recorded there,” said Simone Cesca, a GFZ Earthquake and Volcano Physics Section 2.1 scientist and lead author of the now-published study. Simultaneously with the swarm, a lateral displacement of the ground of more than ten centimeters and a slight elevation of about one centimeter was recorded on the neighboring King George Island.
Challenges of research in a remote area
Cesca studied these facts with colleagues from the National Institute of Oceanography and Applied Geophysics – OGS and the University of Bologna (Italy), the Polish Academy of Sciences, the University of Leibniz in Hannover, the German Aerospace Center and the German Aerospace Center. . The challenge was that there are few conventional seismic instruments in the remote area, namely only two seismic and two GNSS stations (Earth Navigation Satellite ground stations that measure ground displacement). To reconstruct the chronology and evolution of the turbulence and determine its cause, the team further analyzed data from more distant seismic stations and data from InSAR satellites, which use radar interferometry to measure ground displacements. An important step was the modeling of events with a number of geophysical methods in order to interpret the data correctly.
Reconstruction of seismic events
The researchers raised the start of the turbulence on August 10, 2020 and expanded the original global seismic list, which contained only 128 earthquakes, to more than 85,000 events. The swarm peaked with two major earthquakes on October 2 (Mw 5.9) and November 6 (Mw 6.0) 2020 before subsiding. By February 2021, seismic activity had dropped significantly. Scientists identify a magma invasion, the migration of larger volumes of magma, as the main cause of the swarm earthquake, because seismic processes alone can not explain the observed strong surface deformation on King George Island. The presence of a volumetric magma penetration can be independently confirmed on the basis of geodetic data. Starting from its origin, seismicity migrated first upwards and then laterally: deeper, grouped earthquakes are interpreted as the response to the vertical propagation of magma from a reservoir to the upper mantle or to the crust-mantle boundary, while the shallower, crustal ones. Earthquakes extend NE-SW are activated at the top of the laterally growing magma embankment, which reaches a length of about 20 km. The seismicity decreased sharply until mid-November, after about three months of continuous activity, in correspondence with the appearance of the largest earthquakes in the series, with a magnitude of 6.0 Mw. The end of the swarm can be explained by the loss of pressure in the magma embankment, which accompanies the sliding of a large fault and could signal the timing of an explosion at the seabed, which, however, could not yet be confirmed by other data. By modeling the GNSS and InSAR data, the scientists estimated that the volume of Bransfield magmatic penetration is in the range of 0.26-0.56 km³. This makes this episode also the largest magmatic turbulence ever observed geophysically in Antarctica.
conclusion
Simone Cesca concludes: “Our study represents a new successful investigation of a seismic volcanic eruption at a remote location on Earth, where the combined application of seismology, geodesy and remote sensing techniques is used to understand seismic processes and transmission. instruments. areas. “This is one of the few cases where we can use geophysical tools to observe magma penetration from the upper mantle or the crust-mantle boundary into the shallow crust – a rapid transfer of magma from the mantle almost to the surface that takes only a few days.” Report: “Mass earthquake caused by magmatic intrusion at the Bransfield Strait, Antarctica” by Simone Cesca, Monica Sugan, Łukasz Rudzinski, Sanaz Vajedian, Peter Niemz, Simon Plank, Gesa Petersen, Zhiguo Deng, Eleonora Rivalta V, Llessan P, Sebastian Heimann and Torsten Dahm, April 11, 2022, Communications Earth & Environment.DOI: 10.1038 / s43247-022-00418-5
