Seismic Swarm Analysis: Flores Region, Indonesia, January 2024

A seismic swarm designated S20240125.1 occurred in the Flores region of Indonesia from 12:03 on 25 January 2024 to 08:02 on 26 January 2024. Over 19 hours and 58 minutes, 42 earthquakes were recorded, providing insight into the region's ongoing tectonic activity.

The sequence began with a magnitude 3.9 event at 10 km depth. Within 21 minutes, the largest event of the swarm, a magnitude 5.6 earthquake at 21 km depth, occurred. Subsequent activity consisted primarily of smaller events, with magnitudes between 2.5 and 3.9. Depths remained shallow, clustered between 10 and 31 km, with the majority at or near 10 km. Notable later events included a magnitude 4.1 at 10 km depth that marked the swarm's conclusion. The temporal distribution showed higher frequency in the initial hours, followed by a gradual decline, typical of swarm behavior where no single mainshock dominates.

The Flores region forms part of the Sunda Arc system in eastern Indonesia, where the Australian Plate subducts northward beneath the Eurasian Plate. This convergence drives frequent seismicity and volcanism across the Lesser Sunda Islands. The area experiences both thrust faulting along the subduction interface and strike-slip motion associated with the Flores Backarc Thrust and related structures. Historical records indicate persistent seismic swarms linked to fluid migration and stress transfer within the crust.

Since 2000, only two prior swarms have been documented in the region, occurring in 2008 and 2022. These episodes underscore the intermittent clustering of events rather than isolated large earthquakes. The January 2024 swarm aligns with this pattern, featuring moderate magnitudes and shallow depths that reflect upper-crustal deformation.

Such activity highlights the need for continued monitoring in this densely populated volcanic arc setting. While no damage reports were associated with this swarm, the presence of events up to magnitude 5.6 illustrates the potential for stronger shaking during future episodes.

References

USGS Earthquake Catalog Global CMT Project Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports