Seismic Swarm PS20230302.1: Analysis of Activity Near Port-Olry, Vanuatu

A seismic swarm designated PS20230302.1 occurred 64 km west-southwest of Port-Olry, Vanuatu, beginning at 18:04 on 2 March 2023 and concluding at 00:54 on 3 March 2023. Over 6 hours and 49 minutes, seven earthquakes were recorded, with magnitudes ranging from 4.7 to 6.5 and focal depths between 10 and 17 km. The sequence opened with the largest event, a magnitude 6.5 shock at 17 km depth, followed rapidly by events of 5.7, 5.2, 5.2, and 4.7 within the first ten minutes. Two additional shocks of 5.1 and 5.3 occurred later that evening and early the next morning, all at shallow depths near 10–12 km.

This tightly clustered activity exemplifies swarm behavior, in which multiple events of comparable size occur without a single dominant mainshock-aftershock pattern. In tectonically active subduction zones such as the New Hebrides arc, swarms often reflect transient stress changes along the plate interface or within the overriding crust. The short duration and limited spatial extent of the sequence suggest localized strain release rather than widespread rupture propagation.

Vanuatu occupies a central segment of the Pacific Ring of Fire, where the Australian plate subducts eastward beneath the Pacific plate at rates exceeding 10 cm per year. This convergent margin produces frequent moderate-to-large earthquakes and hosts several active volcanoes. The region around Port-Olry lies near the northern Vanuatu subduction segment, an area characterized by oblique convergence and complex faulting that accommodates both thrust and strike-slip motion.

Since 2000, ten seismic swarms have been documented in the broader area, occurring in 2000, 2008, 2009, 2010 (two swarms), 2011, 2016 (three swarms), and 2023. These episodes indicate episodic clustering of seismicity superimposed on the steady background rate driven by plate convergence. Four strong earthquakes of magnitude 7.0 or greater have also struck within 75 km of the swarm centroid since 2000: a magnitude 7.0 event 23 km west-northwest of Port-Olry on 8 January 2023, a magnitude 7.1 shock 31 km northeast of Port-Olry on 20 October 2015, another magnitude 7.1 event 16 km northeast of Port-Olry on 9 January 2001, and a magnitude 7.0 earthquake 29 km west-northwest of Luganville on 4 October 2000. The proximity of these large events underscores the persistent seismic hazard along this portion of the subduction zone.

Shallow focal depths recorded during the March 2023 swarm are consistent with rupture within the upper plate or near the plate interface, where frictional properties can favor swarm-like sequences. Continued monitoring by regional and global networks remains essential for distinguishing between purely tectonic swarms and those potentially linked to magmatic processes at depth.

References

USGS Earthquake Catalog (events since 2000)
Global CMT Catalog (moment-tensor solutions)
Pacific Island Seismograph Network reports