Seismic Swarm S20111223.1 Near Christchurch, New Zealand

Seismic swarm S20111223.1 occurred 22 km ENE of Christchurch, New Zealand, beginning at 01:11 on 23 December 2011 and concluding at 07:39 on 24 December 2011. Over this 30-hour 28-minute period, 25 earthquakes were recorded. The events form part of the ongoing aftershock sequence in the Canterbury region following the major 2010–2011 earthquakes.

The swarm exhibited a concentrated series of moderate-magnitude events. The largest reached magnitude 4.8, with multiple shocks of 4.6 and 4.4. Depths ranged primarily between 5 km and 13 km, consistent with shallow crustal faulting typical of the area. Activity peaked in the early morning hours of 23 December, with several events occurring within minutes of one another before gradually declining toward the swarm’s end.

Geologically, the Christchurch area lies within the Canterbury Plains, where deformation is driven by the oblique convergence of the Pacific and Australian plates. The regional tectonic setting features a network of buried strike-slip and reverse faults that accommodate strain transferred from the Alpine Fault to the south. The 2010 Darfield earthquake initiated a prolonged aftershock sequence that reactivated several of these structures, producing frequent seismic swarms through 2011 and into 2012.

Historical records maintained since 2000 indicate only three swarms in the vicinity prior to this event, occurring once in 2010 and twice in 2011. Swarm S20111223.1 therefore represents a continuation of the heightened seismicity that characterized the post-Darfield period rather than an isolated episode.

Analysis of event timing shows clustering within the first 12 hours, followed by lower-frequency activity. Magnitudes remained in the 4.0–4.8 range throughout, with no events exceeding 4.8. This pattern aligns with typical swarm behavior in which stress redistribution on nearby fault segments triggers repeated moderate releases without a single dominant mainshock.

The shallow focal depths observed (mostly 8–12 km) suggest rupture on faults within the upper crust, where sedimentary cover overlies basement rock. Such depths are common in the Canterbury sequence and contribute to the felt intensity of even moderate-magnitude events in the region.

Ongoing monitoring by GNS Science continues to track residual aftershock rates in Canterbury, providing critical data for seismic hazard assessment. Swarm S20111223.1 underscores the protracted nature of post-seismic relaxation in this tectonic environment.

References

GNS Science, Canterbury Earthquake Sequence reports (2010–2012)
USGS Earthquake Catalog, Christchurch region events
New Zealand National Seismic Hazard Model documentation