Seismic Swarm PS20141115.1: Analysis of Earthquake Activity Northwest of Ternate, Indonesia

The seismic swarm designated PS20141115.1 occurred northwest of Ternate in Indonesia's North Maluku province, a region characterized by intense tectonic activity. This swarm began at 02:31 UTC on 15 November 2014 and concluded at 11:12 UTC the same day, spanning 8 hours and 40 minutes. Eight earthquakes were recorded during this period, with the sequence featuring a mainshock of magnitude 7.1 at 45 km depth, followed by events of magnitudes 5.9, 5.1, 5.0, 4.3, 5.0, 5.5, and 5.4, predominantly at depths around 35 km. Ternate lies within the Molucca Sea Collision Zone, where the Philippine Sea Plate interacts with the Australian Plate and the smaller Molucca Sea Plate. This complex convergence produces frequent intermediate-depth seismicity along subducting slabs associated with the Halmahera and Sangihe volcanic arcs. The area's geology reflects ongoing plate collision, resulting in a high density of earthquakes and active volcanism, including nearby Gamalama volcano on Ternate island itself. The swarm's temporal pattern shows rapid onset with the largest event, followed by a decay in magnitudes over several hours. Most aftershocks clustered at similar depths near 35 km, suggesting activity within the subducting slab rather than shallow crustal faults. Such swarms indicate stress redistribution in a highly fractured tectonic setting without a single dominant mainshock-aftershock sequence. Historical records since 2000 show five prior swarms in the vicinity, including three events in 2001 and two in 2007. Strong earthquakes have also occurred nearby, such as the magnitude 7.1 event 11 km from the swarm center on 15 November 2014, the magnitude 7.1 event 23 km away on 14 November 2019, the magnitude 7.4 event 72 km distant on 1 April 2026, the magnitude 7.5 event 85 km away on 21 January 2007, and the magnitude 7.1 event 66 km distant on 24 February 2001. These events underscore the persistent seismic hazard in this segment of the Indonesian archipelago. The tectonic framework of the Molucca Sea supports recurrent swarm activity due to slab deformation and plate boundary complexities. Monitoring such episodes provides insight into strain accumulation along the arcs, aiding regional hazard assessment.

References

USGS Earthquake Catalog SeismoSight internal swarm classification data