A seismic swarm designated S20210721.1 was recorded in Sulawesi, Indonesia, between 21:33 UTC on 20 July 2021 and 18:26 UTC on 22 July 2021. In approximately 45 hours, 36 earthquakes were detected, with the majority occurring at shallow depths near 10 km and one event reaching 18 km. Magnitudes ranged from 2.5 to 5.2, highlighting a concentrated episode of crustal activity without a single dominant mainshock.
The sequence began with a 2.7-magnitude event at 21:33 on 20 July, followed rapidly by events of 3.7 and 4.1 within the first hour. Activity continued through the night with smaller tremors before a notable 4.1-magnitude shock at 05:39 on 21 July. The largest event, magnitude 5.2, occurred at 17:44 on 21 July at 12 km depth, after which the swarm intensified with multiple events above magnitude 3.0, including several 4.1- and 4.2-magnitude shocks. The final recorded event was a 3.9-magnitude quake at 18:26 on 22 July. Depths remained predominantly shallow, consistent with upper-crustal faulting.
Sulawesi occupies a tectonically complex region at the convergence of the Eurasian, Australian, and Philippine Sea plates. The island’s distinctive K-shaped geometry results from multiple subduction zones and strike-slip faults, notably the Palu-Koro fault system traversing central Sulawesi. This fault accommodates rapid left-lateral motion and has produced destructive earthquakes historically. The 2018 magnitude-7.5 Palu earthquake demonstrated the region’s capacity for both strong shaking and associated tsunami generation due to underwater landslides.
Seismic swarms in Sulawesi often reflect distributed strain release along secondary faults or fluid migration within fractured crust rather than classic foreshock-mainshock-aftershock sequences. The 2021 swarm’s lack of a clear magnitude progression and its short duration align with such patterns. Most events clustered within a narrow time window and spatial footprint, suggesting localized stress adjustment.
Indonesia maintains one of the world’s highest rates of seismicity because of its position along the Pacific Ring of Fire. Sulawesi experiences frequent moderate earthquakes, with the national agency BMKG and international networks such as the USGS providing continuous monitoring. Updated tectonic models incorporate GPS data showing ongoing convergence rates of several centimeters per year, sustaining the potential for future swarm activity or larger events.
The S20210721.1 swarm adds to the instrumental record of Sulawesi’s microseismicity and underscores the importance of real-time detection for hazard assessment in densely populated areas near active faults.
USGS Earthquake Catalog (earthquake.usgs.gov)
Geological Survey of Indonesia (Badan Geologi) reports on Sulawesi tectonics
BMKG seismic bulletins for 2021 activity in Sulawesi