Seismic Swarm S20260402.4 in the Molucca Sea: Geological Context and Event Analysis

The Molucca Sea, situated between the islands of Sulawesi and Halmahera in eastern Indonesia, represents one of the most tectonically complex regions on Earth. This marginal sea lies at the junction of the Philippine Sea Plate, the Sunda Plate, and several microplates, resulting in intense seismic and volcanic activity driven by multiple subduction zones and strike-slip faults. The area experiences frequent earthquakes due to the rapid convergence rates exceeding 10 cm per year in places, contributing to the broader Pacific Ring of Fire.

Seismic swarm S20260402.4 was recorded in this setting, beginning at 23:36 on 1 April 2026 and concluding at 20:26 on 13 April 2026. Over 284 hours and 49 minutes, the swarm comprised 228 earthquakes. Analysis of the first 100 events reveals a concentration of activity with magnitudes ranging from 2.5 to 5.3. Depths varied from near-surface values of 3–5 km to deeper occurrences exceeding 250 km, though most clustered between 10 km and 50 km. Notable events included a magnitude 4.8 quake on 2 April at 35 km depth and a magnitude 5.3 event on 3 April at 35 km depth. Shallow events predominated in the initial phases, transitioning to a mix of intermediate depths later.

This swarm occurred in proximity to a magnitude 7.4 earthquake on 1 April 2026, located 127 km west-northwest of Ternate and only 6 km from the swarm center. Such spatial correlation suggests possible triggering or stress redistribution effects within the regional fault network.

Historical records indicate 12 seismic swarms in the Molucca Sea since 1 January 2000. These occurred in 2001 (3 swarms), 2007 (2), 2014 (3), 2019 (2), and 2026 (2). The region has also hosted several strong earthquakes during this period, including a magnitude 7.5 event on 21 January 2007 (14 km from the current swarm center) and a magnitude 7.1 quake on 24 February 2001 (9 km from the swarm center). These events underscore the persistent seismic hazard associated with the Molucca Sea's subduction dynamics and associated volcanic arc systems.

The provided swarm data aligns with typical patterns in this high-strain environment, where clustered seismicity often reflects fluid migration or aseismic slip along faults. Depths and magnitudes observed are consistent with both crustal and sub-crustal processes in the area.

References

USGS Earthquake Catalog Global CMT Project Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports