M 7.1; 141 km NW of Ternate, Indonesia; (14 Nov 2019) (55km from the swarm center)
M 7.1; 155 km NW of Ternate, Indonesia; (15 Nov 2014) (23km from the swarm center)
M 7.1; 137 km WNW of Ternate, Indonesia; (24 Feb 2001) (98km from the swarm center)
Seismic Swarm PS20141221.1 in the Halmahera Region, Indonesia
On 21 December 2014, a seismic swarm designated PS20141221.1 was recorded 164 km west-northwest of Tobelo, Indonesia. The sequence began at 11:34 UTC and concluded at 14:18 UTC, encompassing five earthquakes within a span of two hours and forty-four minutes. Event parameters indicate a primary shock of magnitude 6.3 at 41 km depth, followed by four aftershocks ranging from magnitude 4.7 to 5.2 at depths between 48 km and 56 km.
This swarm reflects typical intermediate-depth seismicity associated with the complex plate interactions in the Molucca Sea Collision Zone. The region lies at the convergence of the Philippine Sea Plate, the Australian Plate, and the Sunda Plate, where subduction and collision processes generate frequent earthquake clusters. Depths recorded during the swarm align with activity along the subducting slab beneath the Halmahera arc.
Historical records since 2000 document eight prior swarms in the vicinity, occurring in 2001 (three events), 2007 (two events), 2009 (one event), and 2014 (two events). These episodes underscore the persistent seismic character of the area, often linked to stress accumulation along regional fault systems.
Notable strong earthquakes since 2000 further illustrate the tectonic setting. A magnitude 7.0 event struck 156 km northwest of Tobelo on 18 January 2023, approximately 85 km from the swarm center. Earlier, magnitude 7.1 earthquakes occurred on 14 November 2019 (141 km northwest of Ternate, 55 km from the center), 15 November 2014 (155 km northwest of Ternate, 23 km from the center), and 24 February 2001 (137 km west-northwest of Ternate, 98 km from the center). Such events highlight the capacity for larger ruptures within the same structural framework.
Analysis of the 2014 swarm suggests it represents a localized release of strain without escalation to a major mainshock-aftershock sequence. Depths exceeding 40 km indicate the events originated within the downgoing slab rather than shallow crustal faults, consistent with the subduction dynamics of the Halmahera margin. Monitoring of similar swarms aids in refining seismic hazard assessments for eastern Indonesia.
References
SeismoSight internal swarm classification records.
USGS earthquake catalog (regional events since 2000).