Seismic Swarm PS20010827.1: Analysis of Activity Near Ternate, Indonesia

The seismic swarm designated PS20010827.1 occurred 118 km west-northwest of Ternate, Indonesia, in the tectonically active Molucca Sea region. It began at 04:17 on 26 August 2001 and concluded at 23:43 on 27 August 2001, spanning 43 hours and 26 minutes. During this interval, nine earthquakes were recorded, all at a focal depth of 33 km.

The sequence commenced with a magnitude 5.1 event at 04:17:03 on 26 August, followed by a magnitude 5.0 shock at 19:34:10 the same day. Activity intensified on 27 August with a peak magnitude 6.1 earthquake at 01:16:47, succeeded by magnitude 5.1 events at 01:21:48 and 01:30:55. Additional shocks included magnitudes 5.0 at 02:21:00, 5.3 at 02:31:41, 5.0 at 14:41:40, and a final magnitude 5.1 at 23:43:09. This pattern reflects a typical swarm progression, with an initial trigger followed by clustered aftershocks without a single dominant mainshock.

The Ternate area lies within a complex collision zone where the Philippine Sea Plate interacts with the Sunda Plate and the Halmahera arc system. This setting produces frequent intermediate-depth seismicity along subduction interfaces and associated fault networks. Depths consistently recorded at 33 km align with regional crustal characteristics in the Molucca Sea Collision Zone.

Historical records since 1 January 2000 indicate only two swarms in the vicinity, with PS20010827.1 representing the first. Notable strong earthquakes nearby include a magnitude 7.1 event 137 km west-northwest of Ternate on 24 February 2001, 25 km from the swarm center. Subsequent activity featured a magnitude 7.5 shock 126 km west-northwest on 21 January 2007 (9 km from center), a magnitude 7.1 event 155 km northwest on 15 November 2014 (92 km from center), a magnitude 7.1 shock 141 km northwest on 14 November 2019 (61 km from center), and a magnitude 7.4 event 127 km west-northwest on 1 April 2026 (13 km from center). These occurrences underscore persistent strain accumulation and release along regional structures.

Such swarms provide insight into episodic stress redistribution in subduction-related environments, often linked to fluid migration or aseismic slip preceding larger events. Continued monitoring remains essential given the area's history of clustered seismicity.

References

SeismoSight internal swarm classification records
USGS Earthquake Catalog (regional tectonic summaries)