A seismic swarm designated PS20190412.1 was recorded 118 km south-southwest of Luwuk, Indonesia, beginning at 11:40 on 12 April 2019 and concluding at 05:26 on 13 April 2019. Over 17 hours and 45 minutes, the sequence included six earthquakes. The events occurred in a tectonically complex region of eastern Sulawesi, where interactions among the Sunda, Australian, and Philippine Sea plates drive frequent seismic activity.
The swarm initiated with a magnitude 6.8 earthquake at a depth of 15 km on 12 April 2019 at 11:40:49. This was followed within minutes by a magnitude 5.7 event at 10 km depth at 11:45:36. Subsequent shocks included a magnitude 5.3 at 11:50:11, a magnitude 4.7 at 12:03:54, and a magnitude 5.4 at 12:12:12, all at depths of 10 km. The sequence ended with a magnitude 5.0 earthquake at 05:26:14 on 13 April 2019, also at 10 km depth. Such clustered activity without a dominant mainshock-mainshock pattern is characteristic of swarms in subduction-influenced zones.
Eastern Sulawesi lies within the Indonesian archipelago, part of the Pacific Ring of Fire, where convergence of major plates produces both shallow crustal events and deeper subduction-related seismicity. The area around Luwuk features active thrust faults and strike-slip structures associated with the Banggai-Sula microplate. Historical records indicate elevated seismicity since at least the early 20th century, with notable events including strong shaking in the Banggai Islands region during the mid-2000s. Since 2000, this marks only the second documented swarm in the immediate vicinity, following one in 2000, underscoring the episodic nature of clustered seismicity here amid otherwise steady background rates.
Geological mapping of the region reveals sedimentary basins overlying metamorphic basement rocks, with fault systems accommodating oblique convergence. Depths of 10–15 km for the 2019 events align with typical shallow crustal faulting in this setting. Updated monitoring by regional networks confirms ongoing potential for similar sequences, though no significant aftershocks or escalation occurred post-swarm.
This analysis draws on internal seismic classification data alongside established tectonic frameworks for Sulawesi. Continued observation supports improved hazard assessment in this high-activity corridor.