Seismic swarm PS20181002.1 occurred 94 km south of Waingapu on Sumba Island, Indonesia. The sequence began at 23:12 on 1 October 2018 and concluded at 04:49 on 2 October 2018, spanning 5 hours and 37 minutes. During this interval, five earthquakes were recorded with the following parameters: a magnitude 5.0 event at 23:12:02 on 1 October at 10 km depth; a magnitude 6.0 event at 23:59:42 on 1 October at 29 km depth; a magnitude 5.9 event at 00:16:45 on 2 October at 26 km depth; a magnitude 4.4 event at 04:11:47 on 2 October at 26 km depth; and a magnitude 5.7 event at 04:49:31 on 2 October at 17 km depth.
This swarm exemplifies a cluster of seismic events occurring in rapid succession without a single dominant mainshock. Such patterns often reflect localized stress adjustments along fault systems or fluid migration within the crust. The events clustered at shallow to intermediate depths between 10 km and 29 km, consistent with activity in tectonically active subduction-influenced zones.
Sumba Island lies within the tectonically complex region of the Lesser Sunda Islands, where the Indo-Australian Plate converges with the Sunda Plate. This interaction produces frequent seismic activity along thrust faults and strike-slip structures. Historical records indicate recurrent moderate to strong earthquakes in the vicinity, driven by ongoing plate boundary deformation. The 2018 swarm aligns with this established pattern of episodic clustering rather than isolated large-magnitude releases.
Geological mapping of the area shows Sumba as an emergent portion of the Australian continental margin, featuring sedimentary basins and volcanic arcs that amplify seismic wave propagation. Depths recorded in the swarm suggest rupture within the upper crust, where brittle failure predominates. No surface rupture was associated with these events, typical for moderate swarms in this setting.
Monitoring of similar swarms in the broader Indonesian archipelago demonstrates their value for understanding stress transfer and potential foreshock sequences. The PS20181002.1 data provide a compact temporal window for examining energy release distribution, with the largest event reaching magnitude 6.0.
SeismoSight internal classification records for swarm PS20181002.1.
Tectonic framework descriptions from regional geological surveys of the Sunda Arc system.