Seismic Swarm PS20090715.1: Geological Context and Event Analysis in Fiordland, New Zealand

The Fiordland region of New Zealand’s South Island lies at a complex tectonic boundary where the Pacific Plate subducts obliquely beneath the Australian Plate along the Puysegur Trench. This setting produces frequent moderate-to-large earthquakes and occasional earthquake swarms. The area features steep, glaciated terrain underlain by Mesozoic basement rocks of the Median Batholith, with active fault systems including the Alpine Fault to the northeast and the Fiordland subduction interface to the southwest. Seismicity is driven by both plate-boundary thrusting and upper-plate strike-slip faulting, resulting in a high rate of background earthquakes.

Swarm PS20090715.1 began at 09:22 on 15 July 2009 and concluded at 16:40 on 17 July 2009, approximately 105 km west-southwest of Te Anau. Over 55 hours and 17 minutes, the sequence registered 15 events. The swarm initiated with a magnitude 7.8 earthquake at 12 km depth, followed within minutes by a magnitude 5.7 event at the same depth. Subsequent activity included multiple magnitude 5.0–5.4 shocks clustered between 5 km and 12 km depth on 15 July, with additional events on 16 and 17 July extending to 30 km depth. The largest aftershock reached magnitude 5.7 at 30 km on 16 July at 22:18.

This sequence represents the sole swarm recorded in the region since 1 January 2000. The preceding swarm occurred in 2003. The mainshock of the 2009 swarm, located 97 km west-southwest of Te Anau and 52 km from the swarm centroid, remains the strongest event in the local catalogue since 2000.

The shallow focal depths of most events (primarily 5–12 km) align with the brittle upper crust above the subduction interface, while the deeper 30 km event indicates activity near the plate boundary itself. Such depth distributions are characteristic of Fiordland seismicity, where both crustal faults and the subduction megathrust contribute to moment release. The rapid succession of magnitude 5+ events following the initial 7.8 mainshock illustrates typical swarm behaviour, in which stress redistribution triggers a concentrated cluster without a clear mainshock–aftershock decay pattern.

References

• GNS Science New Zealand, Fiordland Seismic Monitoring Reports
• USGS Earthquake Catalog, 2009 Dusky Sound Event
• New Zealand National Seismic Hazard Model (2022 update)