Seismic Swarm S20250417.1 in Minahasa, Sulawesi: Geological Context and Event Analysis

Minahasa, located on the northern peninsula of Sulawesi, Indonesia, lies within one of the most tectonically active regions on Earth. The area forms part of a complex plate boundary zone involving the Eurasian, Indo-Australian, and Philippine Sea plates, along with smaller microplates such as the Molucca Sea Plate. This setting produces frequent seismic activity through subduction, strike-slip faulting, and crustal deformation along the North Sulawesi Trench and associated structures.

Sulawesi's geology reflects millions of years of plate convergence, resulting in volcanic arcs, thrust faults, and extensional basins. Minahasa specifically experiences earthquakes from both shallow crustal sources and deeper subduction-related events. Historical records document numerous significant quakes, underscoring the region's persistent hazard. Updated monitoring indicates ongoing microseismicity consistent with these tectonic processes.

Swarm S20250417.1 began at 18:17 on 16 April 2025 and concluded at 01:09 on 19 April 2025, spanning 54 hours and 52 minutes. During this period, 42 earthquakes were recorded in Minahasa. The sequence opened with a magnitude 3.1 event at 10 km depth. A magnitude 5.5 earthquake at 21 km depth occurred early on 17 April, followed by multiple events clustered between magnitudes 2.5 and 4.4, predominantly at depths of 5–21 km. The largest aftershocks reached 4.4, with the majority remaining below 4.0 and concentrated near 10 km depth.

This swarm pattern featured an initial moderate event succeeded by a dense series of smaller shocks without a single dominant mainshock-aftershock decay. Activity peaked within the first several hours before gradually diminishing over the following two days. Depths remained shallow to intermediate, aligning with known fault systems in the Minahasa Peninsula.

Such swarms are not uncommon in Sulawesi's dynamic environment. Historical data since 2000 show only two prior swarms in the region: one in 2014 and the current 2025 event. This limited frequency highlights the episodic nature of clustered seismicity amid background tectonic strain.

The swarm provides insight into local stress release along secondary faults, possibly triggered by fluid migration or aseismic slip in the subduction interface. No damage or casualties were associated with these events, consistent with their moderate magnitudes.

• USGS Earthquake Catalog
• Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) reports
• SeismoSight internal swarm classification database