Seismic Swarm PS20040211.1: Analysis of Activity Near Nabire, Indonesia

On 11 February 2004, a seismic swarm designated PS20040211.1 was recorded 31 km south of Nabire, Indonesia. The sequence began at 05:08 and concluded at 19:08 local time, encompassing five earthquakes within a 14-hour window. This event represents the sole swarm documented in the region since 1 January 2000.

The swarm unfolded with the following sequence of events. The initial shock at 05:08:20 registered magnitude 5.7 at a depth of 32 km. A second event of identical magnitude 5.7 followed at 05:28:25, originating from 22 km depth. At 06:37:45, a magnitude 5.4 earthquake occurred at 30 km depth. Later activity included a magnitude 5.0 event at 13:44:21 from 10 km depth, and the final recorded shock of magnitude 4.1 at 19:08:34, also at 10 km depth. Depths ranged from 10 km to 32 km, with the majority of energy release concentrated in the first three hours.

Nabire lies within the tectonically complex western New Guinea region, where the Australian Plate converges with the Pacific Plate along a zone of oblique subduction and strike-slip faulting. This setting produces frequent moderate to large earthquakes, driven by the northward motion of the Australian Plate at rates of approximately 70–100 mm per year. The area south of Nabire sits near the transition between the New Guinea Trench and inland fault systems, including segments of the Sorong Fault and related structures that accommodate lateral shear.

In the weeks surrounding the swarm, several major earthquakes struck nearby. A magnitude 7.0 event occurred on 5 February 2004 just 6 km from the swarm center. Two days later, on 7 February, a magnitude 7.3 earthquake took place 75 km away. Later in the year, on 26 November, two magnitude 7.1 earthquakes were recorded 15 km and 68 km from the swarm location, respectively. These events underscore the elevated seismic hazard in the immediate vicinity during 2004.

The 2004 swarm illustrates typical characteristics of clustered seismicity in subduction-related environments, where stress transfer along adjacent fault segments can trigger multiple events without a single dominant mainshock. Depths indicate activity within both the upper crust and the subducting slab interface. No additional swarms have been identified in the catalog since this occurrence, highlighting the episodic nature of such sequences in the Nabire area.