Seismic Swarm PS20180928.1: Analysis of Events North of Palu, Indonesia

On 28 September 2018, a seismic swarm designated PS20180928.1 was recorded 72 km north of Palu, Indonesia. The sequence began at 06:59 UTC and concluded at 14:26 UTC, encompassing 15 earthquakes over 7 hours and 26 minutes. The swarm occurred within a tectonically active zone characterized by complex plate interactions at the boundary of the Sunda, Australian, and Philippine Sea plates. Sulawesi island lies at the convergence of multiple microplates, resulting in high seismicity driven by strike-slip and thrust faulting.

The swarm initiated with a magnitude 6.1 event at 4 km depth, followed rapidly by a magnitude 5.4 earthquake at 10 km depth. Subsequent events included a magnitude 5.1 at 10 km depth before the mainshock, a magnitude 7.5 earthquake at 20 km depth occurring at 10:02 UTC. This largest event aligned with the Palu-Koro fault system, a major left-lateral strike-slip structure extending through central Sulawesi. Aftershocks followed, with magnitudes ranging from 5.2 to 5.8, predominantly at depths of 10 km, indicating shallow crustal activity consistent with the regional fault mechanics.

Geologically, the Palu region features the Palu-Koro and Matano faults, which accommodate oblique convergence between the Australian and Sunda plates at rates of approximately 4–6 cm per year. The 2018 swarm's spatial distribution, centered near the swarm epicenters, reflects stress transfer along this fault network. Historical data indicate limited prior swarm activity in the area since 2000, with only two documented swarms in 2005 and 2017. Strong earthquakes have been infrequent, with the magnitude 7.5 event representing the most significant since 2000 and occurring 22 km from the swarm centroid.

The sequence highlights typical aftershock patterns following a major strike-slip rupture, where initial foreshocks may have contributed to fault loading before the mainshock. Depths remained shallow throughout, amplifying surface effects in the sedimentary basins around Palu Bay. Updated tectonic models confirm ongoing deformation in this segment of the fault, underscoring the potential for recurrent seismic sequences.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• SeismoSight internal swarm classification records
• Tectonic framework of Sulawesi from peer-reviewed geophysical studies (e.g., updates through 2023)