Seismic Swarm North of Macquarie Island, December 2004

The seismic swarm designated PS20041223.1 occurred north of Macquarie Island in the Southern Ocean. It began at 14:59 on 23 December 2004 and concluded at 05:31 on 24 December 2004, spanning 14 hours and 32 minutes. During this interval, six earthquakes were recorded, all at a focal depth of 10 km.

The sequence opened with a magnitude 8.1 mainshock at 14:59:04 on 23 December. Subsequent events included a magnitude 5.0 at 15:16:32, a magnitude 5.3 at 15:48:21, a magnitude 5.0 at 19:05:35, a magnitude 5.6 at 19:50:05, and a final magnitude 5.4 at 05:31:48 on 24 December. This activity aligns closely with the epicenter of the 2004 Tasman Sea Earthquake, located only 4 km from the swarm centroid.

Macquarie Island and its surrounding seafloor form part of the Macquarie Ridge Complex, a tectonically active transform boundary separating the Australian and Pacific plates. The ridge extends southward from New Zealand and accommodates oblique convergence through a combination of strike-slip and thrust faulting. The island itself represents an uplifted fragment of oceanic crust and upper mantle, exposing rocks formed at a spreading center approximately 10–12 million years ago before being emplaced along the plate boundary.

Seismicity in this region reflects ongoing transpressional deformation. Historical records document recurrent moderate-to-large earthquakes along the Macquarie Ridge, with the plate-boundary geometry favoring right-lateral strike-slip motion punctuated by occasional thrust events. The December 2004 swarm occurred within this framework, illustrating how a major strike-slip rupture can trigger a brief cluster of aftershocks at shallow crustal depths.

Geological studies of the Macquarie Ridge emphasize its role in accommodating roughly 3–4 cm per year of relative plate motion. Bathymetric mapping reveals a narrow, linear ridge crest flanked by fracture zones that segment the boundary into discrete seismic segments. The 2004 sequence is consistent with rupture along one such segment north of the island, where the fault system trends approximately north-northeast.

Post-event analysis of regional tectonics indicates that the Macquarie Ridge remains capable of producing infrequent great earthquakes. The 2004 mainshock released strain accumulated over decades, temporarily elevating aftershock rates before activity returned to background levels. Continued monitoring by regional networks contributes to refined seismic-hazard assessments for this remote but strategically important plate-boundary zone.

References

• SeismoSight internal swarm classification PS20041223.1
• Geological Survey of New South Wales, Macquarie Ridge tectonic summaries
• Australian Earthquake Engineering Society, regional seismicity reports (2004–2023 updates)