Seismic Swarm PS20000828.1 Near Ambon, Indonesia: Geological Context and Event Analysis

On 28 August 2000, a seismic swarm designated PS20000828.1 was recorded 74 km west-southwest of Ambon in Indonesia’s Maluku province. The sequence began at 15:05 and concluded at 19:29 local time, encompassing seven earthquakes within a span of four hours and twenty-three minutes. Magnitudes ranged from 4.8 to 6.8, with focal depths between 10 km and 33 km. The largest event, magnitude 6.8, occurred at 15:05:47 at a depth of 16 km, followed by a magnitude 6.1 shock thirteen minutes later. Subsequent events included magnitudes 5.3, 5.0, 4.8, 6.0, and 6.3, the final two registering at 33 km depth.

This swarm unfolded within the tectonically complex Banda Arc region, where the Indo-Australian, Eurasian, and Philippine Sea plates interact through oblique subduction and arc-continent collision. The area experiences frequent seismic activity due to the convergence of these plates along the Seram Trough and surrounding fault systems. Ambon lies near the western margin of the Banda Sea, a zone characterized by rapid crustal deformation and intermediate-depth seismicity associated with subducting slabs.

Historically, the Maluku Islands have recorded multiple destructive earthquakes linked to the same tectonic framework. Notable events include the 1899 Ambon earthquake and subsequent shocks in the early twentieth century, underscoring the region’s long-term susceptibility to both isolated large-magnitude quakes and clustered seismic sequences. Modern monitoring confirms that swarm-like activity often occurs here when stress accumulates along segmented faults or within the overriding plate, releasing energy in rapid succession rather than through a single mainshock-aftershock pattern.

Analysis of the 2000 swarm reveals a progression from shallower to deeper events, suggesting possible fluid migration or cascading stress transfer within the crust. Such swarms provide valuable data on local fault mechanics and can help refine seismic hazard models for nearby population centers. Although no single swarm defines long-term risk, repeated episodes in this setting highlight the need for continued geophysical observation and resilient infrastructure planning.

References

• United States Geological Survey Earthquake Catalog
• Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) regional reports
• Tectonic framework studies of the Banda Arc, Journal of Geophysical Research