Seismic Swarm PS20180819.1 in the Fiji Region

The seismic swarm PS20180819.1 occurred in the vicinity of the 2018 Fiji Earthquake, centered in a tectonically active zone of the southwestern Pacific. This swarm commenced at 00:19 on 19 August 2018 and concluded at 06:28 on 21 August 2018, spanning 54 hours and 9 minutes. During this interval, 22 earthquakes were recorded, all at depths between 511 km and 643 km, characteristic of deep-focus events within the subducting slab.

The sequence initiated with a magnitude 8.2 mainshock at 600 km depth. Subsequent events included a magnitude 6.3 shock at 575 km depth shortly afterward, followed by multiple events in the 5.0–5.7 range clustered within the first hours. Later activity tapered with isolated magnitude 5.0–5.5 tremors on 19–21 August. All events shared similar focal depths, indicating rupture processes confined to the mantle transition zone.

Geologically, the Fiji region lies along the convergent boundary between the Pacific and Australian plates. The Pacific Plate subducts westward beneath the Fiji Plateau, forming part of the Tonga subduction system. This produces one of Earth’s most active zones for deep earthquakes, as the cold slab descends to depths exceeding 600 km before undergoing phase transitions that can trigger brittle failure. Historical records since 2000 show only two prior swarms in the area: one in 2007 and another in 2016. Strong events include the 2018 magnitude 8.2 Fiji Earthquake, located 7 km from the swarm centroid, and a magnitude 7.1 event in 2004 situated 88 km away.

Such deep swarms reflect episodic stress release along the slab rather than typical shallow crustal faulting. The 2018 sequence, dominated by the initial great event and followed by smaller aftershocks at comparable depths, aligns with known behavior in the Tonga-Fiji subduction zone where intermediate-depth and deep-focus seismicity predominates.

References

SeismoSight internal classification data for swarm PS20180819.1.
USGS Earthquake Catalog for regional tectonics and historical events (2000–2018).
Global CMT Project focal mechanism summaries for deep Pacific events.