Seismic Swarm PS20080604.1 Near Ruteng, Indonesia

A seismic swarm designated PS20080604.1 occurred approximately 53 km north-northwest of Ruteng on Flores Island, Indonesia. The sequence began at 17:31 on 3 June 2008 and concluded at 10:47 on 4 June 2008, spanning 17 hours and 15 minutes. During this interval, five earthquakes were recorded with magnitudes ranging from 4.6 to 6.0 and focal depths between 7 and 14 km.

The events unfolded as follows: an initial magnitude 5.9 earthquake at 17:31 on 3 June at 14 km depth, followed by a 5.8 event at 21:03 at 7 km depth. A magnitude 6.0 shock occurred at 22:04 at 14 km depth, succeeded by a magnitude 5.1 event at 23:43 at 10 km depth. The swarm ended with a magnitude 4.6 earthquake at 10:47 on 4 June at 10 km depth. These shallow depths indicate activity within the upper crust, consistent with swarm behavior where multiple events cluster temporally and spatially without a single dominant mainshock-aftershock pattern.

Flores Island lies within the tectonically complex Lesser Sunda Islands, situated at the boundary between the Eurasian and Indo-Australian plates. The region experiences convergence along the Sunda subduction zone, with additional influence from back-arc thrusting associated with the Flores Thrust. This setting produces frequent shallow to intermediate-depth seismicity. Historical records document significant events, including the 1992 magnitude 7.8 Flores earthquake, which generated a destructive tsunami, and numerous moderate events linked to thrust faulting.

Seismic swarms in this area often reflect fluid migration or stress redistribution along active faults rather than long-term strain accumulation. Depths of 7–14 km for the 2008 swarm align with typical crustal faulting in the Flores back-arc region. Such sequences contribute to understanding local hazard potential, as repeated moderate shaking can affect infrastructure on the island.

Updated tectonic models confirm ongoing activity driven by oblique subduction and arc-continent collision processes east of Flores. Monitoring by regional networks continues to track similar clusters, aiding in refined seismic hazard assessments for East Nusa Tenggara province.

References

USGS Earthquake Hazards Program catalog
Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports
Hamilton, W. (1979). Tectonics of the Indonesian Region. U.S. Geological Survey Professional Paper 1078
Audley-Charles, M.G. (2011). Tectonic development of the Banda Arc. Geological Society of London Special Publications