Southern Sumatra lies along the Sunda subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year. This tectonic setting produces frequent seismic activity, including both interface thrust events and intraslab earthquakes. The region has a well-documented history of large-magnitude ruptures, such as the 1833 and 1861 events, as well as more recent activity including the 2007 Bengkulu sequence and the 2010 Mentawai tsunami earthquake. Ongoing monitoring by regional networks confirms persistent background seismicity along the megathrust and associated strike-slip faults such as the Sumatran Fault System. Between 22:15 UTC on 30 June 2021 and 10:47 UTC on 3 July 2021, a swarm of 32 earthquakes was recorded in southern Sumatra. All events occurred at a focal depth of 10 km, with magnitudes ranging from 2.6 to 4.2. The sequence began with a 3.6 event and included several events above magnitude 3.5, notably a 4.2 and a 4.1 on 1 July. Activity remained elevated for roughly 60 hours before declining, with the final recorded event registering 2.8. The temporal distribution shows clustered bursts rather than a classic foreshock–mainshock–aftershock pattern. Peak rates occurred on 1 July, when 15 events were detected within a 24-hour window. Magnitudes remained modest throughout, and no single dominant mainshock exceeded 4.2, consistent with swarm-type behavior often linked to fluid migration or localized stress redistribution along pre-existing fault structures. Such swarms are not uncommon in the Sumatran subduction environment and typically do not herald immediate large-magnitude rupture. Nevertheless, they contribute to cumulative stress transfer along the plate interface and merit continued observation by national seismic agencies. References