Seismic Swarm S20130721.1: The July 2013 Earthquake Sequence near Blenheim

The seismic swarm designated S20130721.1 occurred approximately 29 km east-southeast of Blenheim in New Zealand’s Marlborough region. It began at 19:04 on 20 July 2013 and concluded at 02:03 on 23 July 2013, producing 46 earthquakes within a span of 54 hours and 58 minutes. The events clustered along the tectonically active boundary where the Pacific plate interacts with the Australian plate.

This portion of the South Island lies within the Marlborough Fault System, a network of strike-slip faults that transfer slip from the Alpine Fault to the Hikurangi subduction zone. The swarm’s epicenters align with structures such as the Awatere and Clarence faults, which exhibit right-lateral motion and accommodate oblique convergence at rates of 30–40 mm per year. Focal depths ranged from 5 km to 22 km, consistent with the brittle upper crust in this continental transform setting.

The sequence opened with a magnitude 4.2 event at 19:04 on 20 July, followed rapidly by a magnitude 5.9 shock at 19:17. Subsequent events included multiple shocks of magnitude 4.0–4.6 through the night. The largest earthquake, magnitude 6.5, occurred at 05:09 on 21 July at 17 km depth. Aftershocks continued at a high rate, with notable events of magnitude 5.3 and 5.0 later that day and on 22 July. All listed events remained above magnitude 4.0, illustrating sustained moment release without a single dominant mainshock-aftershock decay pattern typical of isolated events.

Geological records document recurrent large earthquakes in the Marlborough district. The 1848 earthquake of estimated magnitude 7.5 ruptured the Awatere Fault and produced widespread surface deformation. Paleoseismic studies reveal recurrence intervals of several hundred to a few thousand years on the major strands of the fault system. The 2013 swarm released strain within this framework but did not generate surface rupture, remaining confined to moderate magnitudes.

Modern monitoring by New Zealand’s GeoNet network provided dense seismic and geodetic coverage. Continuous GPS stations recorded minor static offsets consistent with the cumulative slip from the sequence. No significant tsunami threat arose because of the inland location and moderate source depths.

The swarm underscores the distributed nature of deformation across the Marlborough Fault System. Clusters of moderate events can occur without immediate progression to larger ruptures, yet they highlight zones of elevated stress that warrant continued surveillance. Updated national seismic hazard models incorporate such sequences to refine probabilistic forecasts for the region.

References
GNS Science GeoNet earthquake catalogue (2013 events).
Stirling, M. W., et al. (2012). National seismic hazard model for New Zealand. Bulletin of the Seismological Society of America.
Benson, A., et al. (2001). Paleoseismicity of the Awatere Fault, New Zealand. New Zealand Journal of Geology and Geophysics.