Seismic Swarm Activity in the Banda Sea: Insights from the November 2023 Event

The Banda Sea region in eastern Indonesia represents one of the most tectonically complex areas on Earth, situated at the convergence of the Indo-Australian, Eurasian, and Pacific plates. This setting drives intense seismic activity through multiple subduction zones, including the Banda Arc and associated transforms. The November 2023 swarm, classified internally as PS20231108.1, occurred within this dynamic environment and consisted of 10 earthquakes registered over 28 hours and 21 minutes, from 04:52 on 8 November to 09:14 on 9 November.

The sequence began with a magnitude 6.7 event at 10 km depth, followed closely by a magnitude 7.1 shock at 6 km depth. Subsequent events included several moderate tremors clustered around 10 km depth, with one deeper event at 140 km. The swarm featured two additional magnitude 6.7 and 5.3 events later on 8 November, concluding with a magnitude 5.1 shock. Depths predominantly in the shallow crust suggest activation along upper-plate faults or the megathrust interface, consistent with the region's history of both shallow crustal and intermediate-depth seismicity.

Geologically, the Banda Sea lies within the collision zone where the northern margin of the Australian continent interacts with the Banda Arc. This produces rapid convergence rates exceeding 7 cm per year, fueling frequent large earthquakes and volcanic activity. Historical records since 2000 document only two prior swarms in the area—one in 2008 and another in 2015—indicating that such clustered sequences remain relatively infrequent compared to isolated mainshock-aftershock patterns. A notable magnitude 7.1 event occurred on 2 March 2005 approximately 98 km from the 2023 swarm center, underscoring the persistent potential for strong shaking in the vicinity.

Swarm events like this one often reflect fluid migration or stress triggering along interconnected fault networks rather than a single large rupture. The tight temporal clustering and range of magnitudes observed align with patterns seen in other subduction-related swarms worldwide, where aftershock productivity can be elevated due to the heterogeneous stress field. No significant damage or tsunami reports were associated with the 2023 sequence, likely owing to the offshore location and moderate energy release.

Continued monitoring of the Banda Sea remains essential given its location within the Pacific Ring of Fire. Advances in seismic networks have improved detection of such swarms, aiding in refining regional hazard models that account for both megathrust and intraslab sources.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports