Seismic Swarm in the Molucca Sea: Analysis of Event PS20190106.1

The Molucca Sea region, situated between the Indonesian islands of Sulawesi and Halmahera, represents one of the most tectonically complex areas on Earth. This marginal sea lies at the convergence of the Philippine Sea Plate, the Sunda Plate, and the Molucca Sea Plate, resulting in intricate subduction dynamics and frequent seismic activity. The double subduction system, with slabs dipping in opposing directions, generates significant stress accumulation and release, contributing to the area's elevated earthquake hazard.

Seismic swarms, characterized by clusters of earthquakes occurring in close temporal and spatial proximity without a dominant mainshock, are recurrent phenomena in this setting. Swarm PS20190106.1, recorded in the Molucca Sea, began at 17:27 on 6 January 2019 and concluded at 14:11 on 7 January 2019. Over this 20-hour, 44-minute interval, eight earthquakes were detected. The sequence initiated with a magnitude 6.6 event at 43 km depth, followed by seven additional shocks ranging from magnitude 4.5 to 5.2, with focal depths between 19 km and 52 km. These events clustered tightly, illustrating the swarm's diffuse energy release pattern typical of fluid migration or aseismic slip influences in subduction environments.

Historical records indicate that ten such swarms have occurred in the Molucca Sea since 1 January 2000. Prior episodes took place in 2001 (three swarms), 2007 (two swarms), 2009 (two swarms), and 2014 (three swarms). This periodicity underscores the region's persistent seismic unrest driven by ongoing plate interactions.

Notable strong earthquakes have also struck nearby since 2000. A magnitude 7.0 event occurred 156 km northwest of Tobelo on 18 January 2023, approximately 44 km from the swarm center. Earlier, magnitude 7.1 quakes were recorded 141 km northwest of Ternate on 14 November 2019 (96 km from the center) and 155 km northwest of Ternate on 15 November 2014 (64 km from the center). These larger events highlight the potential for the Molucca Sea's subduction zones to produce destructive shaking, often accompanied by tsunami risks given the offshore setting.

Geological context from regional studies confirms the Molucca Sea's high seismicity stems from rapid convergence rates exceeding 10 cm per year in places, coupled with slab deformation and possible tear faults. Updated monitoring by global networks continues to refine hypocentral parameters, aiding hazard assessment for nearby populations in North Maluku province.

References
USGS Earthquake Catalog
Global Centroid Moment Tensor Project
SeismoSight internal swarm classification data