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Massive earthquake swarm in Antarctica driven by magmatic intrusion at Orca volcano

Massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica

New research published in Nature determined that an intense earthquake swarm at the Bransfield Strait, Antarctica in 2020/21 was caused by the rapid transfer of magma from the Earth’s mantle near the crust-mantle boundary to almost the surface.

  • The swarm started in August 2020 with 128 M4+ earthquakes and counted approximately 85 000 before it was over in 2021.
  • The swarm was located close to the Orca submarine volcano, previously considered inactive.
  • There was no evidence of similar volcanic swarms in the past experiments: even if the off-shore volcanism and extensional tectonics could sometimes increase the seismic rate in the back-arc basin, a comparably energetic swarm has never been recorded in the region.

During its early phase, 3 186 earthquakes were reported, based on the analysis of seismic data at a local station. The swarm peaked with two large earthquakes – M5.9 on October 2 and M6.0 on November 6.

The estimated seismic moment released from August 2020 to February 2021 was ~3.88 × 1018 Nm which is equivalent to Mw 6.4.

The seismicity was localized close to the Orca seamount, a seafloor caldera shield volcano of 900 m (2 950 feet) bathymetric height offshore the coast of King George Island, where earthquakes were felt at Antarctic bases.

While the nearby plate boundaries and rift are seismically active and hosted swarm episodes of presumed magmatic origin, earthquakes there are typically moderate.

South Shetland Island map.
South Shetland Island map. a Globe showing the location of the study region (red circle). b Regional map showing the tectonic setting (red circle denotes the location of the swarm). c Map of the Brainsfield Basin, between the Shetland Islands and Antarctic Peninsula. The regional seismicity was moderate prior to the unrest (white circles are earthquakes with magnitudes M5+ from the USGS catalog in the period 1971–2020, focal mechanisms are centroid moment tensor solution from the Global CMT catalog in the period 1976–2020). Red arrows indicate the background horizontal ground velocity relative to Antarctica at GNSS sites showing NW-SE extension in the Bransfield Strait (ellipses show 95% confidence regions). A rectangle denotes the extent of the local map (panel c) and a red circle the average location of the unrest seismicity (based on the USGS seismic catalog). Major regional tectonic features are the subduction of the Phoenix Block (the thick solid line marks the Phoenix-Shetland plate boundary) and the active rift along the Bransfield Basin (thin double black line and single line mark active ridge and transform fault segments, respectively, after the University of Texas Institute for Geophysics, UTIG, database). d Local map showing the bathymetric details and the Orca seamount building next to the region of the unrest (red circle). Panels c and d have been plotted with GeoMapApp (www.geomapapp.org), using the Global Multi-Resolution Topography (GMRT). Credit: Nature, Cesca et al.

The South Shetland Islands – Antarctic Peninsula region hosts the remnant segment of a formerly larger subduction zone, which is today limited to the Phoenix block segment.

The Bransfield Basin, which separates the South Shetland Islands and the Antarctic Peninsula lies in a unique tectonic environment.

Although active subduction occurred during most of the past 200 million years, it slowed dramatically at about 4 million years ago when the Phoenix-Antarctic spreading center was abandoned offshore, leaving a small remnant of the former Phoenix plate incorporated into the Antarctic plate.

Even though geochemical data indicate that unaltered basalt dredged from the Bransfield basin is like midocean ridge basalt, there is no clear evidence for normal seafloor spreading.

South Shetland Island earthquake swarm - Overview of seismological and geodetic observations and results
Seismological results include centroids, double couple components of moment tensors, and distribution of principal axes, based on regional data, and locations from the single-station approach. Geodetic modeling results include the locations of the depleting source and sub-vertical dike and the corresponding fit of GNSS cumulative displacement from 1 August 2020 to 1 February 2021 (horizontal uncertainty ellipses are plotted for 1 standard deviation, while the corresponding vertical uncertainties are 0.2 and 0.3 cm at DAL5 and UYBA, respectively), together with the downsampled spatial distribution of displacements along the line of sight (LOS) from InSAR processing (for the time period of 17 July 2020 to 6 February 2021); due to the different orientation of satellite trajectory and displacement vectors, LOS displacement projections are smaller than absolute displacements recorded by GNSS data. Local stations JUBA (seismic) and DAL5 (GNSS) are located at the Carlini Antarctic base (Argentina) and R4DE2 (seismic) and UYBA (GNSS) at the Artigas Antarctic base (Uruguay). Credit: Nature, Cesca et al.

“We resolve further details of the seismicity distribution: strike-slip events are spatially clustered NE of the Orca volcano, while normal faulting events further extend along a narrow band oriented NE-SW,” the researchers noted.

Centroid depths, which are resolved with an average uncertainty of 2 – 3  km (1.2 – 1.9 miles), indicate that the seismicity was shallow, rarely exceeding 15 km (9.3 miles).

GNSS and InSAR data along King George Island coastline show a transient surface deformation from August 2020 and persisting over months superposed to the background spreading rate, which researchers estimated as 2.1 cm (0.8 inches) per year.

Researchers concluded that the magmatic intrusion must have been fed by a deeper reservoir, which should presumably lay at least at ~20 km (12 miles) below a region located NE off the Orca seamount, where the first seismic sign of unrest appeared.

“The depletion of such reservoir should produce a deformation signal with the opposite sign compared to the dike opening, as recently observed offshore Mayotte, Comoro Islands.”

Summary cartoon of the volcano-tectonic unrest at Bransfield Strait, Antarctica
Summary cartoon of the volcano-tectonic unrest. a Vertical cross-section along profile AB, with strike-slip events (red) marking the magma path (arrows) in different phases (numbers), and normal faulting events (blue) being triggered above the lateral dike intrusions. b Temporal evolution of seismicity along profile AB, based on moment tensor solutions (circles), template matching (only events with threshold at 12 times the MAD and cross correlation values above 0.55 are shown) and single station analysis (grayscale kernel density plot). Horizontal indigo dashed lines mark the starting time of four phases of the unrest that are recognized: weak seismicity unrest marking the location of the depleting reservoir off the Orca seamount, below 15 km depth (phase I), upward and SW migration (phase II), further magma flux and formation of a horizontal dike towards NE (phase III) and seismicity drop, following the largest Mw 6.0 earthquake and dike pressure drop (phase IV). Credit: Nature, Cesca et al.

A seafloor eruption is likely, but not confirmed by sea surface temperature anomalies, the researchers said.

The unrest documents episodic magmatic intrusion in the Bransfield Strait, providing unique insights into active continental rifting.

Reference:

Massive earthquake swarm driven by magmatic intrusion at the Bransfield Strait, Antarctica – Simone Cesca et al. – Nature / Communications Earth & Environment – Published April 11, 2022 – DOI https://doi.org/10.1038/s43247-022-00418-5 – OPEN ACCESS

Featured image credit: Nature, Cesca et al.

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5 Comments

  1. During rapid shift of earth’s magnetic field, earthquakes around the two poles will start taking place and will increase enormously during the final phase of the reversal process. These earthquakes will trigger the eruption of underwater volcanoes. Note that, the densest concentration of volcanoes in the world must be close to the two Polar Regions (there is of course, a physical reason for that, but current Earth science is unaware of it). In fact, a recent study has already found 138 volcanoes in West Antarctica alone, but there’s considerable evidence of higher number of volcanoes under the Antarctic Ice Sheet, some of which are currently active or have been active for some time. https://www.linkedin.com/pulse/massive-increase-earthquakes-close-polar-regions-has-exposed-shrair/

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