Magma intrusion reshaped plate motion during Antarctica’s largest recent earthquake swarm

One of the most intense seismic sequences ever recorded near Antarctica was not mainly tectonic. Instead, precise satellite measurements reveal that deep magma pressurization and dike intrusion forced plates apart, accelerated rifting, and rotated an entire island within a single year.

The most striking result of the study is the scale and speed of deformation. King George Island, part of the South Shetland Microplate, abruptly changed how it moved. Its ground velocity increased by roughly a factor of ten, and its direction of motion rotated by about 90 degrees. Within a single year, the island shifted by 12.8–16.7 cm (5.0–6.6 inches), an exceptional displacement for continental crust.

This dramatic change coincided with more than 30 000 earthquakes detected near the South Shetland Islands since August 2020. On several days, seismic networks recorded more than 1 000 events within 24 hours. The earthquakes clustered along the Bransfield Strait, a narrow back-arc basin separating the Antarctic Peninsula from the South Shetland Islands.

Although two very large earthquakes, M7.5–M8.1, struck near the South Sandwich Trench in August 2021, detailed analysis shows that the long-term deformation observed around the Bransfield Strait cannot be explained by earthquake slip alone. High-frequency Global Navigation Satellite System data revealed no significant abrupt ground motion during even the strongest events. Cumulative seismic slip accounts for only about 1–4% of the observed surface displacement.

Instead, the deformation unfolded gradually over months. Before mid-2020, GNSS stations on King George Island moved steadily toward the northeast. After August 2020, northward velocities surged, while east–west motion reversed direction entirely. The island began moving toward the northwest, perpendicular to the Bransfield Strait, indicating active crustal stretching rather than simple plate sliding.

This shift had immediate consequences for the rift itself. Before the swarm, the Bransfield Strait widened, but during the peak of seismic and volcanic activity, the extension rate really jumped. Between 2016 and 2023, the strait widened by about 11.2 cm (4.4 inches), with roughly 7 cm (2.8 inches) of that occurring within just two years. This rapid opening exceeds what long-term tectonic forces alone can sustain.

Vertical motion provided another critical clue. One GNSS station in southeastern King George Island recorded continuous uplift of about 10 cm (4 inches) between August 2020 and August 2022, with no sign of reversal. Stations farther from the strait showed little or no vertical change, indicating a localized pressure source beneath the island. Even earlier, in May and June 2019, all three island stations recorded short-lived surface bulges of 1–2 cm (0.4–0.8 inches) followed by rapid subsidence, a classic signature of magma chamber pressurization and withdrawal.

Sea-level observations further supported the interpretation. Tide gauges along the Antarctic Peninsula recorded non-tidal sea-level fluctuations of about 0.3–0.4 m (1.0–1.3 feet) several hours after the August 12, 2021, earthquake. Modeling of tsunami propagation speeds, around 500–700 km/h (310–435 miles/h), matched the observed arrival times. No comparable signal followed smaller nearby earthquakes, confirming that only the largest events coupled with significant submarine deformation produced measurable sea-level anomalies.

The deformation zone overlaps the Bransfield Strait rift system, which includes the submarine Orca Volcano. This volcanic setting explains the observed pattern of slow inflation, rapid horizontal acceleration, sustained uplift, and dense earthquake swarms. To quantify the process, the researchers applied a combined deformation model that links deep magma pressurization to shallow crustal intrusion.

The best-fitting model places a pressurized magma source at a depth of about 19 km (12 miles) beneath the strait, with a volume increase of approximately 0.62 km³ (0.15 miles³). Above it, a near-vertical dike propagated upward, about 18 km (11 miles) long and 12 km (7 miles) wide, opening by roughly 2.87 m (9.4 feet).

Taken together, the observations define a clear sequence. In the years before the swarm, magma accumulated at depth with little surface expression. During the swarm, rapid dike intrusion forced the crust apart, triggered intense seismicity, rotated King George Island, and accelerated rifting of the Bransfield Strait. After seismic activity declined, horizontal motion slowed, but vertical uplift persisted, indicating continued magmatic pressure beneath the crust.

The study demonstrates that earthquake swarms at complex plate boundaries can be driven primarily by volcanic processes, even in regions where major tectonic earthquakes occur nearby. It also shows the power of long-term geodetic monitoring. Without years of continuous GNSS data, the magmatic origin of this Antarctic seismic crisis would likely have remained unresolved.

As monitoring networks expand across polar regions, integrating geodesy, seismology, and ocean observations will be essential for understanding and assessing volcanic, seismic, and tsunami hazards in some of the most remote and rapidly evolving environments on Earth.

References:

¹ Geodetic evidence of the volcanic magma origin of the earthquake swarm in the Scotia-Antarctica Plate Boundary – Liangyu Chen et al. – Geophysical Journal International – December 13, 2025 – https://doi.org/10.1093/gji/ggaf500 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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