The 2004 M7.1 Earthquake West of Riverton, New Zealand

On 22 November 2004 at 20:26 UTC, a magnitude 7.1 earthquake struck 255 km west of Riverton on New Zealand’s South Island. The event originated at a shallow depth of 10 km. This earthquake occurred within the tectonically active zone that forms the boundary between the Australian and Pacific plates.

The southwest Pacific margin offshore Southland features the Macquarie Ridge complex, a system of transform faults and transpressional segments. Here the plates interact along a predominantly strike-slip regime with localized compression. The ridge extends southward from the Puysegur Trench, where subduction initiates, transitioning northward into the Alpine Fault system on land. Shallow seismicity is common along these structures because of the relatively thin lithosphere and rapid plate motion of approximately 3–4 cm per year.

Historical records document several large events along the Macquarie Ridge, including the great 1989 M8.2 earthquake and the 2004 M8.1 event farther south near Macquarie Island. The November 2004 M7.1 earthquake fits within this pattern of moderate-to-large strike-slip or oblique-slip ruptures that release accumulated strain along the plate interface. Although the epicenter lay well offshore, the shallow depth allowed strong ground shaking to be felt along the southern coast of the South Island, particularly in Southland and Fiordland.

Since 2000, this event remains the only magnitude 7.1 or greater earthquake recorded within the immediate source region west of Riverton. Regional seismic networks operated by GNS Science and international agencies have improved detection of smaller events, confirming ongoing microseismicity along the same structural trends. These smaller earthquakes help delineate the active fault strands within the ridge complex and refine hazard models for the southern plate boundary.

The 2004 earthquake produced no significant tsunami because its mechanism and depth did not generate substantial vertical seafloor displacement. Post-event analysis by global seismic networks located the hypocenter consistently with the coordinates provided and confirmed a predominantly strike-slip focal mechanism consistent with the regional tectonics.

Geological studies of the Macquarie Ridge indicate that such events contribute to the long-term evolution of the plate boundary, occasionally triggering aftershock sequences that migrate along strike. Continued monitoring supports updated probabilistic seismic hazard assessments for New Zealand’s southern regions.

References

USGS Earthquake Catalog
GNS Science New Zealand National Seismic Hazard Model
Macquarie Ridge tectonic studies (published literature)