Seismic Swarm Analysis: Flores Sea Event of December 2021
Earthquake swarms represent clusters of seismic events occurring in close temporal and spatial proximity without a dominant mainshock-aftershock sequence. The swarm designated S20211214.2 was recorded in the Flores Sea, Indonesia, beginning at 04:16 on 14 December 2021 and concluding at 02:31 on 17 December 2021. Over this 70-hour period, 38 earthquakes were detected, with the majority exhibiting shallow focal depths near 10 km and magnitudes ranging from 2.7 to 4.9.
The sequence initiated with a 4.4-magnitude event at 10 km depth, followed rapidly by additional shocks of 4.5, 4.2, and 4.4 within the first hour. Activity continued with events predominantly between 3.0 and 4.0 throughout 14 December, including a notable 4.9-magnitude earthquake at 22:01. On 15 December, activity persisted with peaks including a 4.5-magnitude shock at 17:42, while isolated deeper events reached 118 km. The final recorded events on 17 December measured 3.7 and 3.8 at shallow depths. This pattern reflects sustained low-to-moderate energy release consistent with swarm behavior.
The Flores Sea lies within the tectonically complex eastern Indonesian archipelago, where convergence between the Australian and Sunda plates drives regional deformation. Subduction along the Flores Thrust and associated back-arc structures produces frequent seismicity. Historical records indicate major events such as the 1992 magnitude-7.8 Flores earthquake, which generated a destructive tsunami, underscoring the area's capacity for significant tectonic release. Since 2000, only this single swarm has been classified in the region according to internal records.
Such swarms may arise from fluid migration along faults or stress perturbations in the overriding plate. Continued monitoring remains essential given the proximity to populated islands including Flores and Sulawesi.
References
SeismoSight internal swarm classification data (S20211214.2).
USGS Earthquake Catalog for regional tectonics and historical events.
Global CMT catalog for focal mechanism context in the Flores Sea.