Seismic Swarm PS20190708.1 in the Molucca Sea: Analysis and Regional Context

The Molucca Sea, situated in eastern Indonesia between the islands of Sulawesi and Halmahera, forms part of the tectonically complex boundary between the Philippine Sea Plate and the Australian Plate. This region experiences frequent seismic activity due to oblique subduction and collision processes involving multiple microplates, resulting in a high density of earthquakes at varying depths.

Seismic swarm PS20190708.1 occurred entirely on 8 July 2019 within the Molucca Sea. The sequence began at 02:24:36 UTC and concluded at 23:58:34 UTC, spanning 21 hours and 33 minutes. Five earthquakes were recorded during this period, with magnitudes ranging from 4.9 to 5.2 and focal depths between 19 km and 136 km. The events included a magnitude 5.1 quake at 34 km depth, followed by a magnitude 5.0 event at 19 km, a magnitude 5.2 event at 19 km, another magnitude 5.0 at 28 km, and a final magnitude 4.9 at 136 km.

Such swarms represent clusters of earthquakes without a single dominant mainshock, often linked to fluid migration or stress adjustments along fault systems in subduction zones. Historical records indicate 11 swarms have occurred in the area since 1 January 2000, with notable clusters in 2001 (three events), 2007 (two events), 2014 (four events), and 2019 (two events including the present case).

The Molucca Sea has also hosted several strong earthquakes since 2000. These include a magnitude 7.4 event 127 km west-northwest of Ternate on 1 April 2026 (53 km from the swarm center), a magnitude 7.5 event 126 km west-northwest of Ternate on 21 January 2007 (41 km from the swarm center), and a magnitude 7.1 event 137 km west-northwest of Ternate on 24 February 2001 (64 km from the swarm center). These larger events underscore the persistent seismic hazard in the region driven by ongoing plate convergence.

Geological monitoring of the Molucca Sea benefits from regional networks that track both shallow crustal and deeper intraslab seismicity. Depths exceeding 100 km, as observed in one swarm event, typically indicate activity within the subducting slab, while shallower events align with thrust or strike-slip faulting near the plate interface.

References

• United States Geological Survey Earthquake Catalog
• Incorporated Research Institutions for Seismology (IRIS) Data Services