Seismic Swarm in Minahasa, Sulawesi: February 2025 Event Analysis

A seismic swarm designated S20250226.1 struck the Minahasa region of Sulawesi, Indonesia, beginning at 21:38 on 25 February 2025 and concluding at 00:38 on 28 February 2025. Over 51 hours, the sequence produced 117 earthquakes. Analysis of the first 100 events reveals predominantly shallow focal depths clustered around 10 km, with magnitudes ranging from 2.6 to 6.1. The sequence initiated with a magnitude 3.2 event, followed rapidly by a magnitude 6.1 shock at 21 km depth. Subsequent activity included multiple events above magnitude 4.0, such as 4.9, 5.0, and 4.9 within the first day, alongside numerous smaller tremors between 3.0 and 3.9.

Temporal distribution showed highest rates in the initial 12 hours, with events occurring at intervals of minutes. Depths remained consistent near 10 km for the majority of shocks, though a few reached 30 km or as shallow as 5 km. The largest event, magnitude 6.1, occurred early in the sequence, after which activity transitioned to a decaying pattern of lower-magnitude releases. No events exceeded magnitude 6.1 in the recorded initial 100.

Minahasa lies on the northern arm of Sulawesi within the tectonically complex junction of the Sunda, Australian, and Philippine Sea plates. The region experiences convergence along the Minahasa subduction zone, where the Philippine Sea plate subducts beneath the Sunda plate at rates of approximately 4–6 cm per year. This setting produces frequent moderate to large earthquakes and supports active volcanism, including the nearby Soputan and Lokon volcanoes. Crustal faults such as the Palu-Koro and Matano systems further contribute to regional seismicity through strike-slip motion.

Earthquake swarms remain uncommon in Minahasa. Historical records since 2000 indicate only one prior swarm, which occurred in 2014. The 2025 sequence therefore represents a notable departure from typical isolated mainshock-aftershock patterns observed in the area.

The shallow depths of most events suggest activation within the upper crust, consistent with fluid migration or stress transfer along local fault networks. Magnitudes were generally modest after the initial peak, limiting potential for widespread damage, though the magnitude 6.1 event would have been widely felt across northern Sulawesi.

Ongoing monitoring by regional seismic networks remains essential given the area's position on the Pacific Ring of Fire. Continued observation will clarify whether the swarm signals increased background stress or represents an isolated episode.

References

USGS Earthquake Catalog
Global CMT Project
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports
Tectonic framework summaries from the Geological Survey of Indonesia