Seismic Swarm Activity in the Molucca Sea: Analysis of the January 2019 Event

The Molucca Sea, situated between the Indonesian islands of Sulawesi, Halmahera, and the Sangihe archipelago, represents one of the most tectonically complex regions on Earth. This marginal sea lies at the intersection of the Eurasian, Philippine Sea, and Australian plates, where opposing subduction zones create a distinctive collision zone. The Sangihe arc subducts westward beneath the Eurasian plate, while the Halmahera arc subducts eastward beneath the Philippine Sea plate, resulting in intense crustal deformation and frequent seismic activity.

Seismic swarms—clusters of earthquakes occurring in rapid succession without a dominant mainshock—are characteristic of this environment. They often arise from fluid migration along faults or stress adjustments within the colliding arcs. Historical records indicate five such swarms in the Molucca Sea since 2000, occurring in 2001, 2014 (three events), and 2019.

The swarm designated S20190107.1 began at 17:41 on 6 January 2019 and concluded at 07:55 on 8 January 2019, spanning 38 hours and 14 minutes. During this period, 33 earthquakes were recorded, with magnitudes ranging from 3.7 to 4.9 and focal depths between 10 and 94 km. The sequence commenced with a 3.7-magnitude event at 31 km depth, followed rapidly by a 4.2-magnitude shock at 10 km. Subsequent events included multiple magnitude 4.7 and 4.9 quakes clustered around 35–52 km depth, reflecting activity along both shallow crustal faults and deeper portions of the subducting slabs.

Temporal distribution showed peak intensity during the first 12 hours, with nine events exceeding magnitude 4.0. Activity then moderated, featuring intermittent bursts on 7 January, including deeper events at 56 km and 94 km. The final events on 8 January were shallower, at 10–35 km, suggesting a gradual migration of stress release toward the surface. Magnitudes remained moderate throughout, consistent with swarm behavior rather than a foreshock-mainshock-aftershock pattern.

Geological context underscores the Molucca Sea’s elevated seismicity. The region has produced several significant earthquakes in recent decades due to the ongoing arc-arc collision, which began in the Miocene and continues today. This setting promotes both thrust faulting along subduction interfaces and strike-slip motion within the overriding plates. Depths recorded in the 2019 swarm align with known seismogenic zones: shallower events (10–20 km) likely occurred in the accretionary complex, while deeper ones (40–94 km) correspond to the Wadati-Benioff zones of the dual subduction system.

Such swarms provide valuable data for understanding stress transfer in convergent margins. The 2019 event’s moderate magnitudes and varied depths illustrate how distributed deformation accommodates plate convergence rates estimated at 5–8 cm per year in this sector. Ongoing monitoring by regional networks enhances early detection capabilities in this high-hazard area.

References

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