Seismic Swarm S20230118.1: Analysis of Activity Northwest of Tobelo, Indonesia

A seismic swarm designated S20230118.1 occurred 152 km northwest of Tobelo in Indonesia's North Halmahera region. The sequence began at 06:19 on 18 January 2023 and concluded at 02:12 on 22 January 2023, spanning 91 hours and 53 minutes. During this period, 63 earthquakes were recorded, with magnitudes ranging from 3.0 to 5.1 and focal depths between 10 km and 63 km.

The swarm exhibited typical characteristics of clustered seismic activity without a single dominant mainshock. Initial events on 18 January included a 5.1 magnitude quake at 35 km depth, followed closely by a 4.8 magnitude event at 10 km. Subsequent activity showed fluctuating magnitudes and depths, with notable events such as a 4.9 magnitude shock at 35 km on the afternoon of 18 January and a 5.0 magnitude quake at 24 km on 20 January. Later stages featured a 5.1 magnitude event at 44 km on 21 January. Depths varied widely, indicating activity across multiple crustal levels, while magnitudes remained predominantly moderate.

Such swarms often reflect localized stress adjustments along fault systems or fluid migration within the crust rather than large-scale tectonic rupture. In this instance, the pattern aligns with historical observations in the area, where similar clusters have occurred without escalating to major destructive events.

The location lies within the tectonically active Molucca Sea Collision Zone, part of the broader Pacific Ring of Fire. Here, interactions between the Philippine Sea Plate and the Eurasian Plate drive frequent seismicity through oblique subduction and strike-slip faulting. North Halmahera experiences elevated earthquake rates due to its position near the Halmahera Arc and associated volcanic systems. Indonesia's overall seismic hazard stems from its position at the convergence of multiple plates, resulting in both shallow crustal events and deeper Wadati-Benioff zone activity.

Historical records indicate four prior swarms in the region since 2000, occurring in 2009, 2014, 2019, and 2022. These episodes underscore recurring episodic seismicity northwest of Tobelo, likely tied to ongoing plate boundary deformation. No significant surface rupture or tsunami generation has been associated with these swarms in available records.

Monitoring of such sequences provides valuable data for understanding regional stress regimes and improving hazard assessment models. Continued observation remains essential given the area's persistent tectonic setting.

References

USGS Earthquake Catalog
Global CMT Catalog
Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) reports