A seismic swarm occurred approximately 66 km north-northeast of Ruteng on Flores Island, Indonesia, on 21 February 2022. The sequence began at 12:35 local time and concluded at 18:16, lasting five hours and forty minutes. During this interval, five earthquakes were recorded with magnitudes ranging from 4.5 to 5.7 and focal depths primarily at 10 km, except for the initial event at 20 km.
The events unfolded as follows: the first shock registered magnitude 5.7 at 12:35:58, followed by a magnitude 5.3 event at 13:06:53. Subsequent activity included a magnitude 5.5 quake at 16:22:33, a magnitude 4.5 event at 18:11:03, and a final magnitude 5.2 shock at 18:16:53. Such swarms represent clusters of earthquakes occurring in close temporal and spatial proximity without a dominant mainshock, often linked to fluid migration or minor fault adjustments within active tectonic settings.
Flores Island lies along the Sunda Arc, where the Indo-Australian Plate subducts northward beneath the Eurasian Plate at rates of approximately 7 cm per year. This convergent margin produces frequent moderate seismicity and hosts several active volcanoes, including Ebulobo and Inierie near the swarm epicenter. The island's geology features a mix of volcaniclastics, limestone platforms, and thrust-faulted terrains shaped by ongoing arc-continent collision processes that intensified during the Pliocene.
Historical records indicate that seismic swarms in the region remain infrequent. Since 2000, only two prior swarms have been documented: one in 2008 and another in 2021. These episodes underscore the episodic nature of strain release along secondary faults parallel to the main subduction interface. The 2022 swarm aligns with this pattern, occurring within a zone of elevated background seismicity but without triggering larger events or notable surface deformation.
Regional monitoring by Indonesian authorities continues to emphasize preparedness, given the proximity to populated areas and the potential for cascading volcanic or tsunami hazards in the Flores Sea. Updated tectonic models from regional surveys confirm persistent compression and strike-slip components that facilitate swarm-type sequences at shallow depths.
USGS Earthquake Catalog (earthquake.usgs.gov)
Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) annual reports
Global CMT Project focal mechanism database