SeismoSight Analysis: Earthquake Swarm PS20111223.1 Near Kaiapoi

On 23 December 2011, SeismoSight recorded swarm PS20111223.1 approximately 15 km east-southeast of Kaiapoi in New Zealand’s Canterbury region. The sequence began at 00:58 and concluded at 02:47 local time, encompassing five events within one hour and forty-nine minutes. Individual events included a magnitude 5.8 earthquake at 9 km depth, followed by a magnitude 5.0 at 2 km, a magnitude 5.4 at 10 km, a magnitude 5.9 at 6 km, and a final magnitude 4.4 at 6 km. These closely spaced occurrences illustrate typical swarm behavior, where multiple moderate shocks unfold without a single dominant mainshock-aftershock pattern.

The Canterbury Plains occupy a tectonically active zone at the boundary between the Pacific and Australian plates. Ongoing oblique convergence produces distributed deformation across the region, expressed through both strike-slip and reverse faulting within the upper crust. Shallow sedimentary basins, filled with Quaternary gravels and silts, overlie Mesozoic basement rocks and amplify ground motion during seismic events. Historical records and modern monitoring confirm that the area experiences episodic clusters of seismicity linked to reactivation of inherited faults.

SeismoSight data indicate that only three swarms have occurred in the region since 1 January 2000. Two took place in 2010 and one, the present sequence, occurred in 2011. Such low frequency underscores the episodic nature of swarm activity amid the broader Canterbury earthquake sequence that began in September 2010. Within that sequence, two notable large events have been documented: a magnitude 7.1 earthquake on 3 September 2010 located 19 km northeast of Methven, approximately 79 km from the swarm center, and the magnitude 7.8 Kaikōura earthquake on 13 November 2016 situated 53 km north-northeast of Amberley, roughly 83 km distant. Both events contributed to regional stress redistribution that likely influenced subsequent smaller clusters.

Geological mapping by GNS Science shows that the swarm epicenters lie near the eastern margin of the Canterbury Plains, where blind thrust faults and strike-slip structures interact with the westward-verging foothills of the Southern Alps. Depths between 2 km and 10 km place the activity within the seismogenic zone above the plate interface. The short duration and moderate magnitudes are consistent with fluid migration or aseismic slip triggering on secondary faults rather than rupture of a major through-going structure.

Continued monitoring remains essential because the same fault network that produced the 2010–2011 sequence and the 2016 Kaikōura event continues to accommodate plate motion. Updated strain-rate models from GNSS networks indicate ongoing accumulation that may generate future clusters in the vicinity of Kaiapoi.

References

• GNS Science. (2023). New Zealand Earthquake Catalogue and Fault Database.
• USGS Earthquake Hazards Program. (2024). Significant Earthquakes Archive.
• Beavan, J., et al. (2012). “New Zealand Earthquakes and the Tectonics of the Pacific-Australian Plate Boundary.” Journal of Geophysical Research.
• SeismoSight internal classification records for swarm PS20111223.1.