Seismic Swarm PS20140313.1: Analysis of Activity South of ‘Ohonua, Tonga

The seismic swarm designated PS20140313.1 occurred approximately 150 km south of ‘Ohonua, Tonga, between 19:25 on 12 March 2014 and 06:09 on 13 March 2014. Over this 10-hour and 43-minute period, six earthquakes were recorded, with magnitudes ranging from 4.8 to 5.6 and focal depths between 2 km and 61 km. This cluster of events highlights the persistent seismic dynamics in the region.

The sequence began with a magnitude 5.6 earthquake at 19:25:41 on 12 March at a depth of 14 km. Subsequent events included a magnitude 5.1 at 20:10:27 (2 km depth), a magnitude 5.0 at 20:10:31 (10 km depth), a magnitude 4.8 at 01:04:18 on 13 March (10 km depth), a magnitude 5.0 at 01:13:45 (61 km depth), and a final magnitude 5.1 at 06:09:22 (10 km depth). These closely spaced occurrences reflect typical swarm behavior, where multiple moderate shocks unfold without a single dominant mainshock.

Tonga lies along the Tonga Trench, one of the most seismically active subduction zones on Earth. Here the Pacific Plate converges with and subducts beneath the Australian Plate at rates exceeding 15 cm per year. This tectonic setting generates frequent earthquakes across a wide range of depths, from shallow crustal events to deeper intraslab activity exceeding 100 km. The swarm's location aligns with the broader pattern of intermediate-depth seismicity associated with the subducting slab.

Historical records since 2000 indicate six prior swarms in the immediate vicinity, occurring in 2001, 2004, 2006, 2009, 2010, and 2011. Each episode involved comparable numbers of moderate-magnitude events clustered over short time windows. A notable larger event, a magnitude 7.6 earthquake on 19 March 2009 located 191 km south of ‘Ohonua and roughly 46 km from the 2014 swarm center, underscores the potential for stronger ruptures within the same tectonic framework.

Such swarms contribute to ongoing monitoring efforts in this high-hazard corridor. The combination of rapid event succession and variable focal depths provides valuable data for understanding stress transfer along the plate interface and within the downgoing slab. Continued observation remains essential given the region's history of both swarm activity and occasional great earthquakes.

References

USGS Earthquake Catalog (historical seismicity data)
Global CMT Project (focal mechanism and depth parameters)
SeismoSight internal swarm classification records