In a remote area, a combination of geophysical methods identifies the movement of magma under the sea floor as the cause.
Even off the coast of Antarctica, volcanoes can be found. The sequence of more than 85,000 earthquakes in 2020 was recorded at the long-standing inactive deep-sea volcano Orca, a swarm earthquake that has reached previously unobserved proportions in this region. The fact that such events can be studied and described in remarkable detail even in such remote, and therefore poorly-tooled, areas is now shown by an international team study published in the journal. Earth and Environment Communications.
Researchers from Germany, Italy, Poland and the United States took part in the study, which was led by Simon Siska of the German Research Center for Geosciences (GFZ) Potsdam. They were able to combine seismic, geodetic, and remote sensing techniques to determine how the rapid transfer of magma from the Earth’s mantle near the crustal-mantle boundary almost to the surface caused a swarm earthquake.
Orca volcano between the tip of South America and Antarctica
Swarm earthquakes mainly occur in volcanically active areas. Therefore, it is suspected that the movement of fluids in the Earth’s crust is the cause. Orca Marine is a large undersea shield volcano that rises about 900 meters above the sea floor and has a base diameter of about 11 kilometers. It is located in the Bransfield Strait, an oceanic channel between the Antarctic Peninsula and the South Shetland Islands, southwest of the southern tip of Argentina.
In the past, earthquakes in this region were moderate. However, in August 2020, an intense seismic swarm began there, with more than 85,000 earthquakes within half a year. It represents the largest seismic disturbance ever recorded there,” reports Simone Cesca, a scientist in Section 2.1 of Seismic and Volcanic Physics of the GFZ and lead author of the now-published study. At the same time as the swarm, a lateral ground displacement of more than ten centimeters was recorded. And a small lift of about one centimeter on neighboring King George Island.
Research challenges in a remote area
Siska studied these events with colleagues from the National Institute of Oceanography and Applied Geophysics – OGS and the University of Bologna (Italy), the Polish Academy of Sciences, Leibniz University in Hanover, the German Aerospace Center (DLR) and the University of Potsdam. The challenge was that there were few conventional seismographs in the remote area, ie only two seismic stations and two GNSS stations (ground stations in JLopal nFlight sSatellites system, which measures the displacement of the Earth). In order to reconstruct the chronology of the disturbances and their evolution and determine their cause, the team thus analyzed data from distant seismic stations and data from the InSAR satellites, which use radar interferometry to measure ground displacement. An important step was to model the events with a number of geophysical methods in order to correctly interpret the data.
Reconstruction of seismic events
The researchers dated the onset of the disturbances to August 10, 2020 and expanded the original global earthquake catalog, containing just 128 earthquakes, to more than 85,000 events. The swarm peaked with two large earthquakes on October 2 (Mw 5.9) and November 6 (MW 6.0) 2020 before subsiding. By February 2021, seismic activity had decreased significantly.
Scientists have identified magma penetration, and the migration of a larger volume of magma, as the main cause of the swarm earthquake, because seismic processes alone cannot explain the strong surface deformation observed on King George Island. The presence of volumetric magma intrusion can be independently confirmed on the basis of geodetic data.
Beginning at their origin, the earthquakes migrated first upward and then laterally: deep cluster earthquakes are interpreted as a response to vertical magma spread from a reservoir in the upper mantle or at the crustal-mantle boundary, while surface crustal earthquakes extend to the northeast and run on top of a growing magma dam. Laterally, which reaches a length of about 20 kilometers.
The earthquakes decreased abruptly by mid-November, after about three months of continuous activity, coinciding with the occurrence of the largest in the series, with a magnitude of 6.0 MW. The end of the swarm could be explained by the pressure loss in the magma dam, accompanying a large fault slip, and could indicate the timing of the seafloor eruption which, however, could not be confirmed by other data.
By modeling GNSS and InSAR data, the scientists estimated that the Bransfield intrusion volume of magma ranged between 0.26-0.56 cubic kilometres. This also makes this episode the largest rocky outburst ever observed in Antarctica.
Simon Cesca concludes: “Our study represents a successful new investigation of seismic volcanic disturbances in a remote location on Earth, where the combined application of seismic, geodesy and remote sensing techniques is being used to understand earthquake processes and magma transport in poorly tooled areas. This is one of the few cases where we can use Geophysical instruments to monitor magma penetration from the upper mantle or crustal mantle boundary to the shallow crust—a rapid transition of magma from the mantle to nearly the surface taking only a few days.”
Reference: “Massive earthquake swarm driven by molten intrusion in Bransfield Strait, Antarctica” By Simon Cesca, Monica Sugan, Okasz Rodzinski, Sanaz Fagidian, Peter Nimes, Simon Blanc, Jessa Petersen, Zigo Deng, Eleonora Rivalta, Alessandro Burke-Van Placencia Linares, Sebastian Hyman and Tursten Dahme, 11 Apr 2022, Available here. Earth and Environment Communications.
DOI: 10.1038 / s43247-022-00418-5