Seismic Swarm PS20180613.1: Offshore Activity West of Padang, Indonesia

A seismic swarm designated PS20180613.1 occurred approximately 236 km west-southwest of Padang, Indonesia, between 23:08 UTC on 12 June 2018 and 14:23 UTC on 13 June 2018. The sequence lasted 15 hours and 14 minutes and produced five earthquakes. All events were located in the offshore region of the Sunda subduction zone, where the Indo-Australian Plate converges with the Sunda Plate at a rate of roughly 5–6 cm per year.

The five recorded events are as follows: a magnitude 5.9 earthquake at 23:08:28 on 12 June at 9 km depth; a magnitude 5.6 event at 23:46:11 on 12 June at 10 km depth; a magnitude 5.4 shock at 02:07:57 on 13 June at 10 km depth; a magnitude 5.5 earthquake at 06:59:02 on 13 June at 19 km depth; and a magnitude 4.4 event at 14:23:15 on 13 June at 17 km depth. Depths remained shallow, consistent with activity near the plate interface or within the overriding plate.

Swarm sequences in subduction settings commonly reflect transient stress changes along the megathrust or on subsidiary faults. The clustered timing and moderate magnitudes observed here align with patterns seen in other subduction-related swarms, where fluid migration or slow slip episodes can trigger multiple events without a single dominant mainshock-aftershock decay.

Since 1 January 2000, five swarms have been documented in the same offshore sector. Earlier episodes occurred in 2005 (one swarm), 2007 (two swarms), 2008 (one swarm), and 2009 (one swarm). These recurrent clusters underscore the persistent seismic productivity of the western Sumatra margin.

The broader geological framework features the Mentawai Islands and adjacent forearc basins, underlain by a locked portion of the subduction interface capable of generating great earthquakes. Historical megathrust ruptures, such as the 1797 and 1833 events, produced significant tsunamis along the Padang coast. Modern instrumentation has recorded frequent moderate seismicity, confirming ongoing strain accumulation.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Sieh et al., 2008, Tectonic framework of Sumatra (various peer-reviewed summaries)