Seismic Swarm PS20190714.1: Geological Context and Event Analysis near Sofifi, Indonesia

Seismic swarm PS20190714.1 was recorded 182 km south-southeast of Sofifi, Indonesia, in the tectonically complex Molucca Sea region. The sequence began at 09:10 on 14 July 2019 and concluded at 10:35 on 15 July 2019, spanning 25 hours and 24 minutes. During this interval, 11 earthquakes were detected, with the largest event registering magnitude 7.2 at a depth of 18 km. Subsequent events ranged from magnitude 4.8 to 5.8 and occurred predominantly at depths of 10 km.

The Molucca Sea lies within the Pacific Ring of Fire, where convergence among the Philippine Sea Plate, Sunda Plate, and several microplates produces frequent seismic activity. Halmahera Island and surrounding waters form part of an active arc system influenced by oblique subduction and collision processes. This setting generates both shallow crustal events and deeper Wadati-Benioff zone seismicity, consistent with the observed depths in the swarm.

Historical records since 2000 indicate only two prior swarms in the immediate area: one in 2000 and another in 2007. The 2019 sequence aligns with this low frequency of swarm-type activity, although isolated strong earthquakes occur more regularly. A magnitude 7.2 event on 14 July 2019, located 155 km south-southeast of Sofifi and approximately 22 km from the swarm centroid, represents the dominant shock within the documented sequence.

Event timing shows an initial large-magnitude release followed by a series of moderate aftershocks distributed over roughly one day. Magnitudes declined rapidly after the first hour, with the final event recorded the next morning. Depths remained shallow, suggesting rupture within the upper crust or near the plate interface.

Such swarms contribute to understanding stress transfer in collision zones. The 2019 activity underscores ongoing deformation along the western margin of the Halmahera arc, where accumulated strain from regional plate motions is periodically released. No significant surface rupture or tsunami generation was associated with this swarm, consistent with its moderate magnitudes and depths.

References

USGS Earthquake Catalog (historical seismicity data)
Global CMT Project (tectonic setting parameters)
SeismoSight internal swarm classification records