Seismic Swarm in the Flores Region, Indonesia: February-March 2022 Analysis

The Flores region of Indonesia lies within the tectonically active Lesser Sunda Islands arc, where the Indo-Australian plate subducts northward beneath the Sunda plate. This convergence drives frequent seismicity and volcanism along the Flores back-arc thrust zone and associated faults. The area experiences shallow to intermediate-depth earthquakes due to ongoing plate boundary deformation, with historical events underscoring its elevated hazard potential. Updated regional monitoring confirms persistent activity consistent with subduction-related stress accumulation.

A notable earthquake swarm, designated S20220228.1, occurred in this setting from 02:03 on 27 February 2022 until 16:49 on 1 March 2022. Over 62 hours and 46 minutes, 40 events were recorded, with magnitudes ranging from 2.6 to 4.6 and focal depths predominantly at 10 km, occasionally reaching 12–13 km. The sequence began with a 3.9 magnitude event and included multiple events above magnitude 4.0, such as the peak 4.6 magnitude shocks on 27 February at 20:11 and 28 February at 17:10. Activity clustered in two main pulses on 27 February evening and 28 February afternoon, then tapered with a final 3.5 magnitude event on 1 March.

This swarm reflects typical swarm behavior in subduction zones, where fluid migration or aseismic slip can trigger distributed, non-mainshock-aftershock sequences without a single dominant rupture. Depths near 10 km align with upper-crustal faulting in the Flores thrust system. No events exceeded magnitude 5.0, limiting potential for significant damage, though the cumulative energy release warrants attention in a region of dense population and infrastructure.

Historical records since 2000 indicate only two swarms in the Flores region prior to 2022: one in 2008 comprising a single event and the 2022 swarm itself. This low frequency suggests swarms are infrequent relative to isolated tectonic earthquakes, yet they highlight episodic stress release along the arc. The 1992 magnitude 7.8 Flores earthquake, which generated a destructive tsunami, exemplifies the larger events possible in the same tectonic framework, though unrelated to swarm-type activity.

Ongoing monitoring by regional networks continues to track microseismicity, supporting refined hazard models for eastern Indonesia. Such data contribute to understanding plate-boundary dynamics and improving preparedness in this high-seismicity corridor.

References

USGS Earthquake Hazards Program regional summaries for Indonesia.
Global CMT catalog for focal mechanism context in the Flores thrust zone.
SeismoSight internal swarm classification records.