Seismic Swarm Near Luwuk, Indonesia, in May 2000

A seismic swarm occurred southeast of Luwuk on the island of Sulawesi, Indonesia, between 4 May 2000 at 04:21 and 5 May 2000 at 05:24 local time. The sequence lasted 25 hours and 3 minutes and comprised seven earthquakes. The events were recorded at a location 88 km southeast of Luwuk, with the swarm center situated approximately 26 km from the epicenter of the largest shock.

The sequence began with a magnitude 7.6 earthquake at a depth of 26 km. This mainshock was followed by six additional events, all at depths of 33 km except the initial shock. The subsequent magnitudes were 5.4, 5.1, 5.0, 4.5, 5.0, and a final 5.7 event that marked the end of the swarm. The activity reflects typical aftershock behavior following a major rupture, with magnitudes decreasing overall after the initial event.

Sulawesi lies within a tectonically complex zone where the Australian, Sunda, Philippine Sea, and Caroline plates interact. This setting produces frequent seismicity through subduction, strike-slip faulting, and crustal deformation. The eastern arm of Sulawesi, near Luwuk, is influenced by the Sorong Fault system and associated structures that accommodate lateral plate motion. Historical records show recurrent moderate-to-large earthquakes in the region, consistent with ongoing convergence and fault reactivation.

The May 2000 mainshock ranks among the stronger events recorded in the area since 2000. Its proximity to the swarm center indicates that the smaller events were primarily aftershocks distributed along the same fault segment. Depths ranging from 26 km to 33 km place the activity within the seismogenic crust, where brittle failure is common in this tectonic environment.

No further strong earthquakes exceeding magnitude 7.0 have been noted in the immediate vicinity of the swarm center in the two decades following the sequence, underscoring the episodic nature of large ruptures along these structures. The 2000 swarm provides a clear example of clustered seismicity driven by stress transfer after a significant mainshock in an active plate-boundary region.

References

USGS Earthquake Catalog (events since 2000)
Global CMT Catalog (moment tensor solutions)
Tectonic framework from Pubellier et al., 2004, Journal of Asian Earth Sciences