Seismic Swarm Analysis: Flores Sea, December 2021

A seismic swarm designated S20211214.1 occurred in the Flores Sea from 03:41 on 14 December 2021 to 18:58 on 16 December 2021. Over 63 hours and 16 minutes, 53 earthquakes were recorded, providing insight into clustered seismic activity in this tectonically dynamic region of Indonesia.

The sequence began with a magnitude 4.5 event at 9 km depth, followed closely by a 4.3 event at 10 km. Early activity featured predominantly shallow events between 9 and 30 km, with magnitudes ranging from 2.6 to 4.3. Several deeper shocks reached 62–103 km, including a 3.7 at 70 km and a 2.9 at 103 km. Activity tapered on 15 and 16 December, concluding with a 3.1 event at 29 km. Magnitudes stayed below 5.0 throughout, consistent with swarm characteristics where energy releases gradually without a dominant mainshock.

Swarm events like this reflect fluid migration or stress adjustments along faults rather than typical foreshock-mainshock-aftershock patterns. Depths indicate involvement of both crustal and upper mantle structures, common where the Indo-Australian plate interacts with the Eurasian plate. The Flores Sea lies along the Flores Thrust, a back-arc thrust system linked to subduction processes. Historical records note frequent moderate seismicity here, influenced by volcanic arcs and regional compression.

According to SeismoSight internal classification, this represents the sole swarm in the Flores Sea since 1 January 2000. The previous documented swarm also occurred in 2021, underscoring the rarity of such clustered sequences in the area’s instrumental record.

Geological context reveals the Flores Sea as part of the complex Banda Sea region, where oblique convergence drives thrust faulting and occasional strike-slip motion. The underlying geology includes sedimentary basins overlying oceanic crust, with nearby volcanic islands such as Flores contributing to elevated heat flow that may facilitate swarm triggering through hydrothermal processes.

This 2021 swarm highlights the value of dense monitoring for understanding transient seismic episodes in subduction-related settings. Continued observation aids in distinguishing swarms from more hazardous sequences, supporting regional hazard assessment.

References

SeismoSight internal swarm database (S20211214.1 classification).
USGS Earthquake Catalog (regional tectonics background).
Global CMT Project (Flores Thrust fault parameters).