A seismic swarm designated PS20190122.1 occurred approximately 150 km west-southwest of Waingapu on Sumba Island, Indonesia, between 23:59 on 21 January 2019 and 05:34 on 22 January 2019. The sequence lasted five hours and thirty-five minutes and comprised five earthquakes. Magnitudes ranged from 4.4 to 6.3, with focal depths between 10 km and 26 km.
The events unfolded as follows. The initial shock registered magnitude 6.0 at 26 km depth. Four minutes later, a magnitude 5.0 event occurred at 10 km depth. Roughly one hour and forty-eight minutes after that, a magnitude 5.1 earthquake struck at 10 km depth. The largest event, magnitude 6.3, took place at 24 km depth near the end of the swarm window. The final recorded shock measured magnitude 4.4 at 10 km depth.
This swarm reflects typical shallow crustal seismicity along the plate boundary south of Sumba. The region forms part of the Sunda-Banda arc system, where the Indo-Australian Plate subducts northward beneath the Eurasian Plate. Convergence rates average 70–80 mm per year, producing frequent moderate-to-large earthquakes. Sumba itself occupies an uplifted forearc position influenced by both subduction and collision with the Australian continental margin.
Historical records indicate low swarm frequency in the immediate area. Since 1 January 2000, only one prior swarm has been documented in the same source zone, occurring in 2018. Background seismicity consists mainly of isolated thrust and strike-slip events rather than prolonged clusters.
The 2019 sequence released energy through multiple moderate shocks without a single dominant mainshock, consistent with swarm behavior driven by fluid migration or stress triggering along pre-existing faults. Depths remained within the seismogenic zone above the subduction interface, suggesting activation of upper-plate structures.
Continued monitoring is warranted given the tectonic setting. The arc segment near Sumba has produced damaging earthquakes in the past century, underscoring the importance of regional hazard assessment.
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Hamilton, W. (1979). Tectonics of the Indonesian Region. USGS Professional Paper 1078.