A seismic swarm designated PS20120904.2 was recorded approximately 253 km south-southwest of Jimbaran, Bali, Indonesia. The sequence began at 18:23 on 3 September 2012 and concluded at 13:40 on 4 September 2012, spanning 19 hours and 17 minutes. During this interval, five earthquakes were registered, with magnitudes ranging from 4.5 to 6.1 and focal depths between 9 and 14 km.
The initial event at 18:23:05 on 3 September reached magnitude 6.1 at a depth of 14 km. Subsequent shocks included a magnitude 5.0 event at 04:27:13 on 4 September (10 km depth), followed by two magnitude 5.2 earthquakes at 12:07:26 (10 km) and 13:31:04 (14 km). The final event, magnitude 4.5, occurred at 13:40:08 at 9 km depth. These closely spaced occurrences in both time and location characterize the swarm as a cluster of moderate activity rather than a classic mainshock-aftershock sequence.
The swarm’s location places it within the Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of approximately 6–7 cm per year. This tectonic boundary forms the Sunda Trench, a deep oceanic trench extending along the western and southern margins of Indonesia. Subduction-related megathrust earthquakes dominate the regional seismic hazard, with intermediate-depth events also occurring within the subducting slab.
Bali and its southern offshore region have experienced recurrent seismic activity throughout recorded history. Notable events include the 1976 magnitude 6.2 earthquake near Bali and larger regional shocks such as the 2004 magnitude 9.1 Sumatra-Andaman earthquake, which triggered widespread aftershock sequences across the broader subduction interface. The 2012 swarm occurred in an area of ongoing plate convergence where stress accumulation along the megathrust and within the slab can produce clustered moderate-magnitude events.
Focal depths of 9–14 km indicate that the earthquakes occurred near the plate interface or within the upper portion of the overriding plate, consistent with typical seismicity patterns in this segment of the subduction zone. The absence of a single dominant mainshock and the rapid succession of events align with swarm behavior often observed in subduction settings influenced by fluid migration or stress transfer along the slab.
Current monitoring by regional and global seismic networks continues to track activity along the Sunda Trench, providing data that refine understanding of subduction dynamics and improve hazard assessments for southern Indonesia.