Seismic Swarm PS20081107.1: Geological Context in Vanuatu

Vanuatu lies along the tectonically active New Hebrides subduction zone, where the Australian plate converges with and subducts beneath the Pacific plate at rates exceeding 10 cm per year. This setting produces frequent seismicity, including both isolated large events and episodic swarms. The region has recorded multiple magnitude-7+ earthquakes since 2000, underscoring its elevated hazard level.

On 7 November 2008, seismic swarm PS20081107.1 was recorded 105 km ENE of Port-Olry. The sequence began at 07:19 and concluded at 12:07, encompassing 11 events over 4 hours and 47 minutes. Magnitudes ranged from 5.0 to 6.5, with focal depths between 10 km and 60 km. The two largest shocks (6.4 and 6.5) occurred within seconds of each other at depths of 13 km and 60 km, respectively. Subsequent events clustered at shallow depths near 10 km, with occasional deeper occurrences at 40–60 km. This vertical distribution reflects rupture across multiple segments of the subducting slab and overlying crust.

The swarm represents the only such cluster identified in the local catalog since 1 January 2000; the preceding swarm occurred in 2000. Its timing and spatial extent align with known background seismicity patterns in the northern Vanuatu arc. Comparable activity has been observed near the same trench segment, often linked to stress transfer following larger regional events.

Notable strong earthquakes within 85 km of the swarm centroid since 2000 include an M 7.3 event 48 km ENE of Luganville on 30 March 2026, an M 7.1 shock 31 km NE of Port-Olry on 20 October 2015, an M 7.2 earthquake 56 km E of Luganville on 1 August 2007, and an M 7.1 event 16 km NE of Port-Olry on 9 January 2001. These events illustrate the persistent strain accumulation and release characteristic of the subduction interface.

Swarm sequences such as PS20081107.1 provide insight into short-term fault interactions within the subduction system. The absence of a single dominant mainshock and the rapid succession of moderate-magnitude events suggest distributed failure along pre-existing fractures rather than a classic foreshock–mainshock–aftershock progression. Depth variations further indicate involvement of both the plate interface and intraslab structures.

Continued monitoring of the New Hebrides arc remains essential for refining probabilistic hazard assessments in Vanuatu.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (regional Vanuatu events).