Seismic Swarm South of the Kermadec Islands: September 2016 Analysis

A seismic swarm occurred south of the Kermadec Islands from 04:02 on 24 September 2016 to 04:28 on 25 September 2016. Over this 24-hour and 26-minute period, six earthquakes were recorded. All events occurred at a shallow depth of 10 km, with magnitudes ranging from 4.4 to 5.6. The Kermadec region lies along the Kermadec-Tonga subduction zone, where the Pacific Plate subducts westward beneath the Australian Plate at rates exceeding 5 cm per year. This tectonic setting produces frequent shallow and intermediate-depth seismicity as well as arc volcanism. The September 2016 swarm aligns with typical shallow crustal activity driven by plate-boundary stresses in this highly active margin. Event parameters from the swarm are as follows:

• 24 Sep 2016 04:02:15, magnitude 5.6
• 24 Sep 2016 23:27:20, magnitude 5.2
• 24 Sep 2016 23:42:49, magnitude 5.0
• 24 Sep 2016 23:52:03, magnitude 5.2
• 25 Sep 2016 00:22:51, magnitude 4.4
• 25 Sep 2016 04:28:39, magnitude 5.6 Historical records since 2000 indicate six prior swarms in the same area: one in 2001, two in 2004, one in 2005, one in 2008, and one in 2015. Such episodic clustering reflects periodic stress accumulation and release along the subduction interface and overlying crust. Shallow swarms in this setting often result from localized fault slip or fluid migration without leading to larger mainshock-aftershock sequences. The 2016 activity remained moderate in magnitude and short in duration, consistent with background swarm behavior observed along the Kermadec arc.

References

SeismoSight internal swarm classification database.
USGS Earthquake Catalog and regional tectonic summaries.
Global subduction zone studies from the International Seismological Centre.