A seismic swarm designated S20220110.2 occurred in Halmahera, Indonesia, beginning at 16:26 on 9 January 2022 and concluding at 07:32 on 11 January 2022. Over this 39-hour period, 31 earthquakes were recorded. The sequence lacked a single dominant mainshock, characteristic of swarm behavior where events occur in clusters driven by localized stress adjustments or fluid migration along faults.
Events commenced with a magnitude 2.9 quake at 10 km depth. Larger shocks followed rapidly, including a magnitude 5.2 at 21:27 and a magnitude 5.4 at 21:59, both at 10 km depth. Subsequent activity included multiple events between magnitudes 2.8 and 3.8 through the evening of 9 January, with depths remaining predominantly shallow at 10–12 km. On 10 January, magnitudes ranged from 2.7 to 3.7 at depths of 10–26 km, showing a gradual decline in frequency and intensity. The swarm closed with a magnitude 4.0 event at 00:43 on 11 January and a final magnitude 2.9 at 07:32, both near 10–11 km depth. Overall, 26 events registered at or below 10 km, with the deepest reaching 39 km.
Halmahera occupies a tectonically complex zone within the Indonesian archipelago, situated at the convergence of the Philippine Sea Plate, the Molucca Sea microplate, and the Sunda Plate. This setting produces active subduction along the Halmahera Arc and associated thrust faults, resulting in frequent shallow to intermediate-depth seismicity. The region features volcanic chains and fault systems that accommodate oblique convergence and plate rotation. Historical records document recurrent moderate earthquakes, with swarms often linked to afterslip or volcanic-tectonic interactions rather than isolated large ruptures.
This January 2022 swarm aligns with the area's established pattern of clustered, low-to-moderate magnitude activity. Depths concentrated near 10 km suggest involvement of upper-crustal structures influenced by regional compression. No surface rupture or significant damage was associated with the sequence, consistent with the modest energy release.
Such swarms provide insight into fault mechanics in subduction margins, highlighting distributed strain rather than stick-slip behavior on major interfaces. Monitoring continues through regional networks to assess any evolution toward larger events.
References
SeismoSight internal classification data for swarm S20220110.2.
USGS Earthquake Hazards Program catalog for Halmahera tectonics.
Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) regional seismic reports.