Seismic Swarm in the Macquarie Island Region: Analysis of the July 2025 Event

A seismic swarm designated PS20250728.1 occurred in the Macquarie Island region, beginning at 22:10 on 28 July 2025 and concluding at 07:09 on 30 July 2025. Over 32 hours and 58 minutes, ten earthquakes were recorded. The sequence included a magnitude 7.0 event at 31 km depth, followed by nine additional shocks ranging from magnitude 5.0 to 5.8, with focal depths between 10 km and 34 km.

The events clustered tightly around a central point, with the magnitude 7.0 mainshock located approximately 28 km from the swarm centroid. Subsequent activity showed a typical aftershock decay pattern, with magnitudes decreasing rapidly after the initial rupture. Depths remained predominantly shallow to intermediate, consistent with regional crustal and upper-mantle faulting.

The Macquarie Island region lies along the Macquarie Ridge Complex, a major tectonic boundary separating the Australian and Pacific plates. This complex accommodates oblique convergence and strike-slip motion, producing one of the most active seismic zones in the Southern Ocean. The ridge features a series of en-échelon transform faults and short spreading segments that have evolved over the past 30 million years through progressive changes in plate motion.

Seismicity in the area is driven by the relative plate velocity of approximately 30–40 mm per year. Historical records document repeated large earthquakes along the ridge, reflecting both locked and creeping fault segments. The 2025 swarm aligns with this established pattern of clustered activity along the central portion of the ridge.

Geological mapping of Macquarie Island itself reveals uplifted oceanic crust and mantle rocks, including gabbros, peridotites, and basalts that provide direct evidence of the ridge’s formation. The island’s position on the plate boundary explains its frequent exposure to strong ground shaking, although its remote location limits direct human impact.

The July 2025 sequence represents the strongest event recorded in the region since 2000. Its occurrence within an already active transform system underscores the persistent strain accumulation along the Australian-Pacific boundary. Continued monitoring of aftershock rates and any migration of activity will help refine assessments of potential future large ruptures in this remote but tectonically dynamic setting.

References

SeismoSight internal swarm classification PS20250728.1
USGS Earthquake Catalog (regional plate boundary data)
Geological Society of Australia, Macquarie Island tectonic studies