Seismic Swarm PS20021010.1: Insights into Activity Near Manokwari, Indonesia

On 10 October 2002, a significant seismic swarm designated PS20021010.1 was recorded 97 km south-southeast of Manokwari, Indonesia. The sequence began at 10:50 local time and concluded around midnight on 11 October, spanning 13 hours and 10 minutes. During this period, 15 earthquakes were detected, with the largest event registering magnitude 7.6 at a shallow depth of 10 km. Subsequent events ranged from magnitude 5.0 to 6.7, all occurring at similar depths, indicating a concentrated release of stress within a limited crustal volume.

The swarm initiated with the magnitude 7.6 mainshock, followed rapidly by events of magnitudes 5.5, 5.4, and 5.3 within the first hour. Additional notable shocks included magnitudes 6.7 and 6.0 at 12:28 and 12:29, respectively. The sequence tapered with smaller events through the evening, ending with a magnitude 5.2 quake just after midnight. This pattern reflects typical swarm behavior, where multiple events of comparable size occur without a single dominant mainshock-aftershock decay, often linked to fluid migration or aseismic slip along faults.

Manokwari lies in West Papua, at the western margin of New Guinea, where the tectonics are governed by the oblique collision between the Australian and Pacific plates. This region forms part of the broader Papua Fold and Thrust Belt and is influenced by the New Guinea Trench to the north. Shallow crustal seismicity predominates due to active thrust and strike-slip faults accommodating convergence rates of approximately 10–12 cm per year. Historical records show recurrent moderate-to-large earthquakes, consistent with the area's position within the Pacific Ring of Fire.

Since 2000, only one swarm has been documented in this locale prior to later monitoring expansions, underscoring the episodic nature of clustered activity here. The 2002 sequence occurred near a prior magnitude 7.6 event centered 102 km south-southeast of Manokwari, highlighting persistent seismic hazard from nearby fault segments. Updated regional assessments confirm ongoing monitoring by Indonesian and international networks, with no major swarms reported in the immediate vicinity through recent years.

Such swarms provide critical data for understanding stress transfer in collision zones. The uniform 10 km depths across events suggest activation along a shallow detachment or splay fault within the accretionary complex. Continued observation aids in refining probabilistic hazard models for Manokwari and surrounding communities.

References

• USGS Earthquake Catalog (events 2002)
• Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) regional reports
• Tectonic framework from Global Earthquake Model (GEM) fault database