Seismic Swarm PS20230109.1: Analysis of Recent Activity Near Port-Olry, Vanuatu

A seismic swarm designated PS20230109.1 was recorded approximately 38 km west of Port-Olry, Vanuatu. The sequence began at 12:32 on 8 January 2023 and concluded at 21:21 on 9 January 2023, spanning 32 hours and 49 minutes. During this period, six earthquakes were registered, with magnitudes ranging from 4.4 to 7.0 and focal depths between 13 km and 35 km.

The largest event reached magnitude 7.0 at a depth of 29 km on 8 January at 12:32:42 UTC. Subsequent events included a magnitude 5.0 at 35 km depth, followed by magnitude 5.2 and 5.0 shocks at shallower depths of 30 km and 13 km. Additional activity comprised a magnitude 4.4 event at 22 km and a final magnitude 5.0 at 29 km depth. This pattern reflects typical swarm behavior, characterized by clustered events without a dominant mainshock-aftershock sequence.

Vanuatu lies along the New Hebrides subduction zone, where the Indo-Australian Plate converges with the Pacific Plate at rates exceeding 10 cm per year. This tectonic setting produces frequent moderate-to-large earthquakes and volcanic activity across the archipelago. The region experiences elevated seismicity due to the complex interaction of subduction, back-arc spreading, and transform faulting.

Historical records indicate nine prior swarms in the area since 2000, occurring in 2000, 2008, 2009, 2010 (two instances), 2011, and 2016 (three instances). Strong earthquakes have also been documented nearby, including events of magnitude 7.0 to 7.3 within 100 km of the swarm center. Notable examples include a magnitude 7.0 shock 23 km west-northwest of Port-Olry in January 2023 and earlier magnitude 7.1 events near the same locality in 2015 and 2001.

Such swarms contribute to ongoing monitoring efforts aimed at understanding stress transfer and potential links to larger ruptures in this highly active margin. Continued observation supports regional hazard assessment in a nation where seismic events pose significant risks to infrastructure and communities.

References

• SeismoSight internal swarm classification data (PS20230109.1 parameters and historical statistics).
• Geological Survey of Vanuatu and regional tectonic summaries on subduction dynamics.