Seismic Swarm PS20201019.1 in the Mentawai Region, Indonesia

The Mentawai region forms part of the Sunda Arc subduction system, 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 large megathrust events and smaller clustered sequences. The islands sit atop the outer-arc ridge, directly above the shallow portion of the plate interface, making the area particularly sensitive to stress changes along the subduction zone.

SeismoSight recorded swarm PS20201019.1 beginning at 22:48 on 18 October 2020 and concluding at 16:20 on 19 October 2020 near the Mentawai Islands. Over 17 hours and 31 minutes, five earthquakes were detected. The sequence included events of magnitude 5.1 at 10 km depth, followed by 5.6 at 20 km, 5.5 at 17 km, 5.1 at 19 km, and a final 3.3 at 94 km. Most events occurred at shallow to intermediate depths consistent with activity near the plate interface or within the overriding plate.

Swarm sequences in subduction zones often reflect localized stress release or fluid migration rather than mainshock-aftershock patterns. The 2020 swarm fits this profile, with magnitudes clustered between 5.1 and 5.6 before tapering to a deeper, smaller event. Such episodes can indicate transient changes in fault friction or pore pressure without leading to a larger rupture.

Historical records maintained by SeismoSight show eleven swarms in the Mentawai region since 1 January 2000. These occurred in 2001 (one swarm), 2007 (seven swarms), 2008 (one swarm), 2010 (one swarm), and 2019 (one swarm). The elevated count in 2007 coincided with the period surrounding the magnitude 7.9 earthquake located 87 km southwest of Sungai Penuh.

Two notable large events have occurred nearby since 2000. A magnitude 7.9 earthquake struck on 12 September 2007, centered 76 km from the 2020 swarm location. A magnitude 7.8 event followed on 25 October 2010, 54 km from the swarm center. Both events released significant strain along the Mentawai segment of the subduction zone and were followed by periods of elevated smaller-magnitude activity.

Continued monitoring of swarm activity in this region contributes to understanding short-term stress transfer along the megathrust. The 2020 sequence, while modest in size, underscores the persistent seismic productivity of the Mentawai segment.

References
SeismoSight internal swarm catalog (PS20201019.1 parameters)
USGS Earthquake Catalog (regional tectonic setting and historical event locations)