Seismic Swarm PS20190202.1 Near Sungai Penuh, Indonesia: Geological Context and Event Analysis

The seismic swarm designated PS20190202.1 occurred on 2 February 2019, approximately 196 km west-southwest of Sungai Penuh in western Sumatra, Indonesia. The sequence began at 09:03 local time and concluded by 13:10, spanning 4 hours and 7 minutes during which seven earthquakes were recorded. This activity unfolded within the tectonically active Sunda subduction zone, where the Indo-Australian Plate converges with the Eurasian Plate at rates of 5–6 cm per year, producing frequent seismic events along the Sumatran fault system and associated megathrust segments.

The swarm exhibited a range of magnitudes and focal depths consistent with shallow crustal and upper-plate seismicity typical of the region. Events included a magnitude 5.2 quake at 10 km depth at 09:03, followed by the largest event of magnitude 6.0 at 20 km depth at 09:27. Subsequent shocks comprised magnitudes 5.3 (10 km), 4.1 (10 km), 5.9 (18 km), 5.8 (10 km), and a final magnitude 5.1 (10 km) at 13:10. Depths clustered predominantly between 10 and 20 km, suggesting activation along secondary faults above the main subduction interface rather than direct megathrust rupture.

Such swarms represent episodic clusters of earthquakes without a dominant mainshock-aftershock pattern, often linked to fluid migration or stress transfer in subduction-related environments. Historical records since 2000 indicate 12 comparable swarms in the vicinity, with notable concentrations in 2007 (seven events) and single instances in 2005, 2008, 2009, 2010, and 2018. This pattern underscores persistent background seismicity driven by ongoing plate convergence.

The area has also hosted several strong earthquakes since 2000. The 25 October 2010 magnitude 7.8 event struck 215 km southwest of Sungai Penuh, 67 km from the 2019 swarm center. Earlier, a magnitude 7.2 quake occurred on 25 February 2008 at 164 km west-southwest of Sungai Penuh, 47 km from the swarm epicentral zone. The 12 September 2007 magnitude 7.9 event was located 87 km southwest of Sungai Penuh, 83 km from the swarm center. These larger events illustrate the region's capacity for great earthquakes capable of generating tsunamis, consistent with the Sunda megathrust's history of producing magnitude 8+ ruptures.

Geologically, western Sumatra lies at the edge of the overriding Eurasian Plate, where oblique subduction accommodates both thrust and strike-slip motion. The 2019 swarm's timing and distribution align with stress accumulation along the Mentawai segment of the subduction zone, a region known for segmented rupture behavior. Monitoring such swarms provides insight into precursory activity that may precede larger events, although no immediate escalation followed this sequence.

References

• USGS Earthquake Catalog (earthquake.usgs.gov)
• SeismoSight internal swarm classification data