Seismic Swarm PS20010226.1: Analysis of Events Near Ternate, Indonesia

The seismic swarm designated PS20010226.1 was recorded on 26 February 2001, approximately 136 km west-northwest of Ternate, Indonesia. The sequence began at 05:45 and concluded at 18:19 local time, encompassing a total duration of 12 hours and 34 minutes. During this interval, five earthquakes were detected, providing a focused window into short-term seismic clustering within a tectonically active zone.

The events unfolded as follows. The initial shock at 05:45:15 registered a magnitude of 5.4 at a depth of 33 km. A second event of identical magnitude occurred at 09:18:19, also at 33 km depth. At 09:31:09, a magnitude 5.1 earthquake struck at the same depth. Activity continued with a magnitude 5.1 event at 15:16:35, this time at 60 km depth. The swarm concluded with a magnitude 4.6 shock at 18:19:54, returning to 33 km depth. These closely timed occurrences, predominantly at shallow to intermediate depths, illustrate typical swarm behavior where energy release remains distributed rather than dominated by a single mainshock-aftershock pattern.

This swarm represents the sole recorded instance since 1 January 2000, underscoring the infrequent nature of such clustered activity in the immediate vicinity. Notably, it followed a magnitude 7.1 earthquake on 24 February 2001, centered 137 km west-northwest of Ternate and only 23 km from the swarm epicenter. Subsequent strong events have included a magnitude 7.5 quake on 21 January 2007 (44 km from the swarm center), a magnitude 7.1 event on 15 November 2014 (51 km distant), another magnitude 7.1 shock on 14 November 2019 (19 km away), and a magnitude 7.4 earthquake on 1 April 2026 (31 km from the center). These larger ruptures highlight persistent strain accumulation along regional fault systems.

Geologically, the area lies within the Molucca Sea collision zone, where interactions among the Philippine Sea, Australian, and Sunda plates generate elevated seismicity. The Halmahera arc and associated volcanic features contribute to a dynamic environment marked by both shallow crustal events and deeper Wadati-Benioff zone activity. Historical patterns indicate that swarms may serve as indicators of stress redistribution preceding or following major releases, although this 2001 sequence remained moderate in scale.

Overall, the PS20010226.1 swarm offers a concise example of transient seismic clustering in a high-hazard setting, consistent with the broader tectonic framework of eastern Indonesia.

References:
SeismoSight internal classification data for swarm PS20010226.1 and associated historical earthquake records.