Seismic Swarm S20191114.2: Analysis of Activity Northwest of Ternate, Indonesia

Seismic swarm S20191114.2 occurred approximately 138 km northwest of Ternate in Indonesia's North Maluku province. The sequence began at 16:30 on 14 November 2019 and concluded at 00:51 on 17 November 2019, spanning 56 hours and 20 minutes. During this period, 77 earthquakes were recorded, with magnitudes ranging from 3.7 to 4.9 and focal depths between 10 km and 38 km.

The swarm exhibited a rapid onset with several events above magnitude 4.5 in the first hours, including a 4.9 magnitude earthquake at 16:30:57 on 14 November at 35 km depth. Subsequent activity included multiple 4.8 and 4.9 events clustered around 35 km depth, interspersed with shallower events near 10 km. Later phases showed continued moderate shaking, with the final notable event being a magnitude 4.8 at 00:32:44 on 17 November at 35 km depth. Depths remained predominantly in the upper crust, consistent with regional tectonic patterns.

This swarm coincided closely with a magnitude 7.1 earthquake on 14 November 2019, located 141 km northwest of Ternate and only 9 km from the swarm epicentral area. The temporal and spatial proximity suggests the larger event may have influenced or triggered the subsequent swarm activity through stress redistribution along nearby faults.

Regional Geological Context

The area northwest of Ternate lies within the tectonically complex Molucca Sea region, where the Philippine Sea Plate interacts with the Halmahera Arc and surrounding microplates. Subduction processes drive frequent seismicity, with the Molucca Sea Plate fragment contributing to both shallow crustal events and intermediate-depth activity. Ternate itself sits near the active Gamalama volcano, part of the Halmahera volcanic chain influenced by westward subduction.

Historical records indicate recurrent swarm behavior in this zone. Since 1 January 2000, eleven swarms have been documented, occurring in clusters during 2001 (three events), 2007 (two events), 2014 (three events), and 2019 (three events). These episodes reflect episodic strain release along the arc's fault systems rather than isolated mainshock-aftershock sequences.

Insights from Swarm Characteristics

Analysis of the 77 events reveals a high rate of moderate-magnitude earthquakes without a single dominant mainshock within the swarm itself. Depths cluster around 35 km for many larger events, while shallower 10 km events suggest involvement of both deeper thrust faults and upper crustal structures. Magnitudes above 4.0 constituted a significant portion of the sequence, indicating sustained energy release over the two-and-a-half-day period.

Such swarms in subduction-related arcs often arise from fluid migration or aseismic slip that loads adjacent fault patches. The 2019 activity aligns with this pattern, occurring amid the broader tectonic regime of oblique convergence in eastern Indonesia.

References

• SeismoSight internal swarm classification records for S20191114.2.
• Regional tectonic summaries from the U.S. Geological Survey Earthquake Hazards Program.