Earthquake Swarm Analysis: Sulawesi, Indonesia, February-March 2023

Sulawesi, Indonesia, occupies a complex tectonic setting at the convergence of the Australian, Eurasian, Sunda, and Philippine Sea plates. This configuration produces frequent seismic activity through a combination of subduction along the Sulawesi Trench and major strike-slip faults such as the Palu-Koro and Matano systems. The island’s geology features young volcanic arcs and fragmented continental crust, resulting in shallow to intermediate-depth earthquakes that can cluster in time and space.

Between 01:26 UTC on 27 February 2023 and 02:03 UTC on 1 March 2023, a swarm of 48 earthquakes was recorded in Sulawesi. The sequence began with a magnitude 5.5 event at 38 km depth, followed by 47 smaller events whose magnitudes ranged from 2.5 to 3.6 and whose focal depths were predominantly 10 km. The largest aftershocks reached magnitude 3.6 on both 28 February and 1 March. Most activity occurred within the first 24 hours, after which the rate declined steadily until the swarm terminated.

Such swarms are characterized by the absence of a single dominant mainshock-aftershock sequence and by the rapid occurrence of numerous events of similar size. In this case the initial 5.5 quake appears to have triggered a cascade of smaller ruptures along nearby fault segments. The concentration of events at shallow depths suggests activation of upper-crustal structures rather than deeper subduction-related seismicity.

Historical records indicate that comparable swarms have been rare in Sulawesi since 2000. Only three previous episodes have been identified: one in 2005, one in 2017, and one in 2018. Each prior swarm consisted of a single documented cluster, underscoring the episodic nature of swarm activity in the region. The 2023 sequence therefore represents the fourth such event in more than two decades and the first to exceed 40 recorded earthquakes.

The tectonic framework of Sulawesi implies that future swarms remain possible wherever strain accumulates along the Palu-Koro fault or subsidiary structures. Continuous monitoring with dense seismic networks is essential for distinguishing swarm behavior from foreshock sequences that might precede a larger mainshock. While the 2023 swarm released only modest energy, it highlights the need for updated hazard assessments that incorporate the potential for clustered seismicity in this highly active plate-boundary zone.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geological Survey of Indonesia (puskim.pu.go.id)