Seismic Swarm PS20070221.1: Analysis of Activity South of Sofifi, Indonesia

The seismic swarm designated PS20070221.1 occurred in a tectonically dynamic region approximately 170 km south of Sofifi, Indonesia. This area lies within the Halmahera arc system, where complex plate interactions between the Philippine Sea Plate and surrounding crustal blocks generate frequent seismic events. Shallow to intermediate-depth earthquakes are common due to ongoing subduction and strike-slip faulting along regional structures.

The swarm initiated at 08:04 on 20 February 2007 and concluded at 03:20 on 22 February 2007, spanning 43 hours and 15 minutes. During this period, seven earthquakes were recorded, with the strongest event reaching magnitude 6.7. Depths ranged from 10 km to 28 km, indicating predominantly upper-crustal activity consistent with the region's active fault networks.

The sequence began with the magnitude 6.7 mainshock at a depth of 12 km. Subsequent events included a magnitude 5.0 at 15 km depth later the same day, followed by a magnitude 6.0 at 11 km depth. On 21 February, activity continued with magnitudes of 5.5 (28 km), 5.0 (10 km), and 4.6 (10 km). The final event, magnitude 5.0 at 24 km depth, marked the swarm's end. This temporal clustering without a single dominant aftershock sequence highlights the swarm's diffuse nature, typical of fluid-influenced or stress-transfer processes in arc environments.

Such swarms contribute to the broader seismic history of the Halmahera region. Indonesia's position along the Pacific Ring of Fire results in elevated seismicity, with the Halmahera sector experiencing both volcanic and tectonic earthquakes. A notable recent event was the magnitude 7.2 earthquake on 14 July 2019, located 155 km southeast of Sofifi and roughly 92 km from the swarm epicentral area. This demonstrates persistent strain accumulation along nearby fault systems.

Analysis of the 2007 swarm provides insight into short-term seismic patterns. The rapid succession of moderate events within a confined timeframe suggests localized stress release rather than a classic foreshock-mainshock-aftershock progression. Depths mostly below 15 km align with brittle failure in the shallow crust, while the deeper 28 km event may reflect transitional deformation. Monitoring such episodes aids in refining seismic hazard assessments for nearby populations and infrastructure.

Overall, the PS20070221.1 swarm exemplifies the recurrent moderate seismicity that characterizes this segment of the Indonesian archipelago. Continued observation supports improved understanding of tectonic processes driving regional earthquake occurrence.

References

SeismoSight internal swarm classification records.