Seismic Swarm PS20180906.1: Deep Earthquake Cluster East of Levuka, Fiji

On 6 September 2018, a compact seismic swarm designated PS20180906.1 was recorded approximately 122 km east of Levuka, Fiji. The sequence began at 15:56 UTC and concluded at 16:31 UTC, encompassing nine earthquakes within a 34-minute window. All events occurred at depths between 623 km and 660 km, with magnitudes ranging from 5.0 to 5.7. This activity represents the sole swarm documented in the region since 2000.

The individual events unfolded rapidly. The initial shock registered magnitude 5.7 at 656 km depth, followed two minutes later by a magnitude 5.1 event at 623 km. Subsequent shocks included magnitudes 5.3, 5.1, 5.2, 5.0, 5.1, 5.1, and a final magnitude 5.4, all maintaining depths near 630 km. Such tight temporal and spatial clustering at extreme depths highlights the swarm’s coherent character within the subducting slab.

Fiji lies within a tectonically complex zone where the Pacific Plate subducts westward along the Tonga Trench. Deep-focus earthquakes in this area originate from phase transitions and stress release within the cold, dense slab as it descends through the mantle transition zone. Depths exceeding 600 km place these events near the base of the transition zone, where olivine transforms to spinel structures, facilitating brittle failure even under high confining pressure. The 2018 swarm’s uniformity in depth suggests rupture along a localized segment of the slab interior rather than the plate interface.

Historically, deep seismicity east of Levuka has been recorded by global networks, yet swarm-style clustering remains uncommon. The absence of additional swarms since 2000 underscores the episodic nature of stress accumulation and release at these depths. No significant shallow aftershocks or surface effects were associated with the sequence, consistent with the great focal depths involved.

Ongoing monitoring by regional and global seismic networks continues to refine understanding of slab dynamics beneath the Fiji Plateau. The 2018 event contributes valuable data on the frequency and mechanics of deep earthquake swarms in this subduction setting.

References:
USGS Earthquake Catalog (earthquake.usgs.gov)
Global Centroid Moment Tensor Project (globalcmt.org)
SeismoSight internal swarm classification records