Seismic Swarm PS20110516.1: Analysis of Activity Near Tadine, New Caledonia

On 16 May 2011, a seismic swarm designated PS20110516.1 was recorded approximately 114 km north-northeast of Tadine in New Caledonia's Loyalty Islands region. The sequence began at 19:14 UTC and concluded at 22:04 UTC, encompassing five earthquakes over a span of two hours and fifty minutes. Magnitudes ranged from 4.9 to 5.3, with focal depths varying between 10 km and 80 km. The events occurred in rapid succession, highlighting clustered seismic energy release typical of swarm behavior in this tectonically active zone.

The first two events struck within seconds of each other at 19:14, registering magnitudes of 5.1 and 5.3 at depths of 10 km and 44 km. Subsequent shocks at 19:18 included magnitudes of 5.1 and 5.2 at 10 km and 80 km depth. The final event at 22:04 reached magnitude 4.9 at 10 km depth. Such shallow to intermediate-depth clustering reflects stress adjustments along regional fault systems influenced by plate boundary dynamics.

This swarm fits into a broader pattern of seismic activity in the area. Since 2000, thirteen swarms have been documented in the vicinity, with notable occurrences in 2006 (one swarm), 2007 (four), 2008 (two), 2010 (two), and 2011 (four). These episodes underscore episodic rather than continuous energy release, often preceding or accompanying larger tectonic events.

The region lies at the convergent boundary between the Australian and Pacific plates, where subduction along the New Hebrides Trench generates frequent seismicity. The Loyalty Islands sit atop the Loyalty Ridge, an uplifted feature resulting from long-term plate interaction. Historical strong earthquakes since 2000 include a magnitude 7.0 event 85 km east-northeast of Tadine on 19 November 2017, located 98 km from the swarm center; a magnitude 7.0 shock 118 km north-northeast of Tadine on 13 January 2011, just 20 km away; a magnitude 7.3 quake 141 km west of Isangel, Vanuatu, on 25 December 2010 (96 km distant); and two magnitude 7.3 events near the Loyalty Islands on 9 April 2008 (68 km and 74 km from the center). These records illustrate the area's capacity for significant ruptures alongside swarm activity.

Swarm sequences like PS20110516.1 provide insight into precursory or postseismic processes, aiding understanding of how distributed microseismicity contributes to overall strain accommodation in subduction settings. Monitoring such patterns supports improved hazard assessment for nearby populations in New Caledonia and Vanuatu.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• SeismoSight internal swarm classification data