The Sumba region of Indonesia experienced a seismic swarm designated PS20200808.1 between 10:17 on 8 August 2020 and 04:52 on 9 August 2020. Over 18 hours and 35 minutes, six earthquakes were recorded, all at a focal depth of 10 km. Magnitudes ranged from 2.6 to 5.3, with the strongest event occurring at 10:45:49 on 8 August.
The sequence unfolded as follows: two magnitude-5.1 events within seven minutes of swarm onset, followed by the peak magnitude-5.3 shock. Later activity included a magnitude-5.0 event in the afternoon, a smaller magnitude-2.6 shock, and a final magnitude-5.0 event that marked the swarm’s conclusion. Such clustered, shallow seismicity is characteristic of swarm behavior, where multiple events occur without a single dominant mainshock-aftershock pattern.
Sumba lies within the Lesser Sunda Islands along the Sunda subduction zone, where the Australian plate converges with the Sunda plate at rates of approximately 70 mm per year. This tectonic setting produces frequent shallow crustal earthquakes and occasional larger megathrust events. The island’s geology reflects a complex history of accretionary processes, with sedimentary and volcanic rocks overlying a basement that records the region’s long-term exposure to plate-boundary stresses.
Historical records since 2000 indicate only two prior swarms in the immediate area—one in 2018 and one in 2019—suggesting that swarm-type activity remains infrequent relative to isolated tectonic earthquakes. The 2020 swarm’s parameters align with the region’s typical shallow seismicity, reinforcing the ongoing influence of subduction-related deformation.
Seismic swarms in subduction margins often reflect fluid migration or stress redistribution along fault networks rather than a single large rupture. The consistent 10 km depth of the 2020 events points to activity within the upper crust, where brittle failure predominates. No damage or tsunami reports were associated with this swarm, consistent with its moderate magnitudes.
Continued monitoring of the Sumba region remains essential given its position on an active plate boundary. Future swarms may provide additional insight into local stress regimes and the potential for larger events.
References: USGS Earthquake Catalog Global CMT Project Indonesian Meteorological, Climatological, and Geophysical Agency (BMKG) reports