Seismic Swarm Near Manokwari, Indonesia, January 2009

A notable seismic swarm occurred 76 km west-northwest of Manokwari, Indonesia, from 19:43 on 3 January 2009 to 13:06 on 4 January 2009. In 17 hours and 22 minutes, 27 earthquakes were recorded, with magnitudes ranging from 4.9 to 7.7 and focal depths between 17 km and 55 km. The sequence began with two large events of magnitude 7.7 and 7.6 within seconds of each other, followed by numerous aftershocks and additional moderate-to-strong shocks, including a magnitude 7.4 event later that evening.

The swarm exhibited typical characteristics of clustered seismic activity, with the majority of events concentrated at depths around 35 km. Later shocks included magnitudes of 5.9, 5.7, and several 5.0–5.6 events distributed throughout the night and into the following morning. Such swarms reflect rapid stress release along active fault segments without a single dominant mainshock-aftershock pattern.

Manokwari lies on the Bird’s Head Peninsula of western New Guinea, a region situated within the complex collision zone between the Australian and Pacific plates. This tectonic setting produces frequent seismic activity through a combination of subduction along the New Guinea Trench and strike-slip motion along regional fault systems such as the Sorong Fault. The area forms part of the Pacific Ring of Fire and has experienced repeated moderate-to-large earthquakes throughout recorded history.

Historical records since 2000 indicate three prior swarms in the vicinity: two events in 2002 and one in 2004. The January 2009 swarm was accompanied by three significant independent earthquakes of magnitude 7.4–7.7 located between 16 km and 44 km from the swarm centroid, underscoring the elevated seismic hazard of the Manokwari region during that period.

References

USGS Earthquake Catalog
Global CMT Catalog
Scientific literature on New Guinea tectonics (2000–2023)