Seismic Swarm PS20230124.1: Analysis of Recent Activity Northwest of Tobelo, Indonesia

A seismic swarm designated PS20230124.1 occurred approximately 169 km northwest of Tobelo in Indonesia's North Maluku province. The sequence began at 17:06 on 23 January 2023 and concluded at 20:17 on 24 January 2023, spanning 27 hours and 11 minutes during which seven earthquakes were recorded.

The events displayed moderate magnitudes between 5.0 and 5.8, with focal depths ranging from 9 km to 35 km. The initial shock measured magnitude 5.0 at 30 km depth, followed shortly by a magnitude 5.5 event at 9 km depth. Subsequent activity included a magnitude 5.1 at 18 km, a magnitude 5.8 at 35 km, a magnitude 5.4 at 35 km, another magnitude 5.0 at 35 km, and a final magnitude 5.3 at 35 km. Depths clustered predominantly around 35 km after the earliest events, suggesting activity along a consistent structural level within the crust.

This swarm fits into a pattern of recurrent seismic swarms in the region since 2000, with 13 such episodes documented. Prior occurrences took place in 2000, 2001, 2009 (three swarms), 2014 (three swarms), 2019 (three swarms), 2022, and 2023. These clusters typically involve multiple events of similar magnitude without a dominant mainshock, often linked to localized stress adjustments in tectonically complex zones.

Just days before the swarm, a magnitude 7.0 earthquake struck 156 km northwest of Tobelo on 18 January 2023, approximately 37 km from the swarm's center. Such proximity indicates possible stress transfer or aftershock influence within the broader fault network.

The Halmahera region lies within the tectonically active Molucca Sea Collision Zone, where the Philippine Sea Plate interacts with the Eurasian Plate through subduction and arc-continent collision processes. This setting produces frequent intermediate-depth seismicity along the Halmahera Arc and associated thrust faults. Historical records confirm elevated seismic hazard, with the area experiencing repeated moderate-to-large events driven by ongoing plate convergence at rates of several centimeters per year.

Seismic swarms in subduction-related environments like this often reflect fluid migration, magma movement, or aseismic slip that redistributes stress without producing a single large rupture. Monitoring such sequences provides valuable data on fault behavior and helps refine regional hazard assessments.

References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geological Survey of Indonesia (puskim.pu.go.id)