SeismoSight has registered Swarm S20250930.1 in the Bali Sea, a region of notable tectonic complexity within the Indonesian archipelago. The swarm began at 16:49 on 30 September 2025 and concluded at 22:54 on 1 October 2025, spanning 30 hours and 4 minutes. During this period, 47 earthquakes were recorded, providing valuable data on clustered seismic behavior in a subduction-influenced setting.
The sequence featured a range of magnitudes, with the largest event reaching 6.0 at a depth of 19 km. Subsequent events were predominantly smaller, with magnitudes between 2.5 and 3.9 and focal depths mostly between 10 km and 20 km. Notable events included multiple shocks of 3.4 and 3.1 magnitude clustered in the early hours of 1 October, alongside numerous events near 2.7–2.9. Depths remained relatively shallow throughout, consistent with crustal adjustments along regional fault systems.
This swarm illustrates typical characteristics of seismic swarms, where energy release occurs through numerous events of comparable size rather than a dominant mainshock followed by decaying aftershocks. The temporal distribution shows initial high activity tapering over the first day, with a secondary pulse around midnight on 1 October. Depths averaging 13–15 km suggest involvement of the overriding plate above the subduction interface.
The Bali Sea occupies a critical position in the Sunda Arc, where the Indo-Australian Plate subducts beneath the Eurasian Plate at rates of approximately 6–7 cm per year. This convergence drives frequent seismicity and volcanism across the arc, including nearby active centers on Bali and Lombok. Historical records document recurrent moderate-to-large earthquakes in the vicinity, reflecting long-term strain accumulation and release along the megathrust and associated strike-slip faults.
Tectonic studies indicate that the Bali Sea experiences both interplate and intraplate events, with swarms often linked to fluid migration or stress triggering in fractured crust. The 2025 sequence aligns with patterns observed in prior episodes, underscoring the area's persistent seismic hazard. Monitoring such swarms aids in refining regional hazard models and understanding precursory signals in subduction zones.
Further analysis of waveform data and geodetic measurements could clarify whether this swarm correlates with slow slip events or magmatic processes at depth. Continued observation remains essential given the proximity to populated islands.
References
SeismoSight internal swarm classification S20250930.1
USGS Earthquake Catalog (regional tectonics)
Global CMT Project (focal mechanism data)