Seismic Swarm PS20060331.1 in the Kermadec Islands Region

The Kermadec Islands region forms part of the Kermadec-Tonga subduction zone, where the Pacific Plate converges with and subducts beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces frequent seismic activity, including both isolated large-magnitude events and episodic earthquake swarms. The islands themselves are volcanic, resulting from the subduction process, and the broader area lies within the Pacific Ring of Fire.

Swarm PS20060331.1 began at 13:21 on 31 March 2006 and concluded at 22:07 on 1 April 2006. Over this 32-hour, 46-minute period, 23 earthquakes were recorded. The sequence initiated with a magnitude 6.5 event at 13 km depth, followed by numerous events of magnitude 5.0–5.7 clustered at shallow to intermediate depths.

Key events unfolded as follows:

• 31 March 2006 13:21:00, M 6.5, 13 km
• 31 March 2006 14:05:39, M 5.2, 2 km
• 31 March 2006 14:05:46, M 5.1, 40 km
• 31 March 2006 15:43:53, M 5.5, 10 km
• 31 March 2006 16:11:54, M 5.1, 10 km
• 31 March 2006 16:12:00, M 5.1, 53 km
• 31 March 2006 20:18:49, M 5.2, 33 km
• 31 March 2006 20:38:31, M 5.2, 10 km
• 31 March 2006 20:38:36, M 5.4, 33 km
• 1 April 2006 03:40:45, M 5.1, 10 km
• 1 April 2006 05:15:29, M 5.7, 10 km
• 1 April 2006 05:49:15, M 5.1, 10 km
• 1 April 2006 06:25:43, M 5.3, 39 km
• 1 April 2006 08:19:08, M 5.1, 10 km
• 1 April 2006 08:19:15, M 5.1, 75 km
• 1 April 2006 08:51:37, M 5.1, 55 km
• 1 April 2006 10:32:24, M 5.4, 10 km
• 1 April 2006 11:29:55, M 5.5, 10 km
• 1 April 2006 13:41:24, M 5.3, 10 km
• 1 April 2006 13:43:31, M 5.5, 16 km
• 1 April 2006 17:55:55, M 5.6, 10 km
• 1 April 2006 20:02:13, M 5.3, 14 km
• 1 April 2006 22:07:55, M 5.0, 1 km

This swarm represents one of only three documented since 1 January 2000, with earlier episodes occurring in 2003 (two swarms) and 2005 (one swarm). Such sequences typically reflect distributed stress release along the subduction interface without a single dominant mainshock-aftershock pattern.

The region has also experienced several strong independent earthquakes since 2000, including an M 8.1 event on 4 March 2021 located 32 km from the swarm center, an M 7.4 event the same day 86 km distant, an M 7.1 on 24 April 2023 at 89 km, an M 7.0 on 16 March 2023 at 86 km, an M 7.6 on 6 July 2011 at 62 km, and an M 7.0 on 29 September 2008 at 71 km. These events underscore the persistent seismic hazard along the plate boundary.

SeismoSight internal classification identifies swarm PS20060331.1 as a distinct cluster within this tectonically active margin.

References
SeismoSight internal swarm database (PS20060331.1 parameters and historical swarm counts).
USGS Earthquake Catalog (strong event locations and magnitudes since 2000).