Seismic Swarm PS20070121.1: Analysis of the January 2007 Molucca Sea Sequence

The seismic swarm designated PS20070121.1 occurred in the tectonically complex Molucca Sea region of Indonesia, centered approximately 132 km east-southeast of Laikit, Laikit II (Dimembe). The sequence initiated at 11:27 UTC on 21 January 2007 and concluded at 00:36 UTC on 25 January 2007, spanning 85 hours and 8 minutes. During this interval, 42 earthquakes were recorded, with magnitudes ranging from 5.0 to 7.5 and focal depths predominantly between 10 and 40 km.

The swarm commenced with a magnitude 7.5 mainshock at a depth of 22 km. This event was followed by numerous aftershocks clustered at shallow depths around 10 km, including multiple events of magnitude 5.0–5.6 within the first 24 hours. Notable subsequent activity included a magnitude 6.2 earthquake at 23 km depth on 21 January and additional magnitude 5.5 events on 23 and 24 January. Depths remained generally shallow, consistent with crustal deformation in an active collision zone.

The Molucca Sea lies at the junction of the Philippine Sea Plate, the Australian Plate, and the Eurasian Plate, forming part of the western Pacific Ring of Fire. This area features the Molucca Sea Collision Zone, where opposing subduction systems—the Sangihe Arc to the west and the Halmahera Arc to the east—converge. Such double subduction produces intense seismicity, with frequent moderate-to-large earthquakes driven by plate convergence rates exceeding 10 cm per year in places. Historical records document repeated seismic swarms in the region since at least 2001, with three such sequences identified since 1 January 2000.

Strong regional earthquakes since 2000 include the magnitude 7.5 event of 21 January 2007 itself (13 km from the swarm center), a magnitude 7.4 event in 2016 (18 km distant), and magnitude 7.1 events in 2019 (65 km), 2014 (97 km), and 2001 (22 km). These events underscore the persistent seismic hazard associated with ongoing plate boundary interactions.

Analysis of the swarm timing and magnitude distribution indicates a typical aftershock decay pattern following the initial 7.5 mainshock, with the majority of activity concentrated in the first two days. Shallow focal depths suggest rupture within the upper crust, potentially linked to thrust faulting along the Halmahera Arc. No surface rupture was reported, as is common for offshore sequences in this setting.

This swarm highlights the value of continuous seismic monitoring for understanding short-term clustering in high-strain environments. Continued observation remains essential given the region's history of generating destructive earthquakes.

References

USGS Earthquake Catalog Global CMT Catalog SeismoSight internal swarm classification data