Seismic Swarm PS20070913.1: Analysis of the September 2007 Events Southwest of Padang

A seismic swarm designated PS20070913.1 occurred on 13 September 2007, centered 169 km southwest of Padang, Indonesia. The sequence began at 00:09 and concluded at 20:52 local time, encompassing 16 earthquakes over 20 hours and 43 minutes. This activity unfolded within the tectonically active Sumatran subduction zone, where the Indo-Australian plate converges with the Eurasian plate at rates of approximately 5–6 cm per year, generating frequent seismicity along the Sunda megathrust.

The swarm featured a range of magnitudes and focal depths. The sequence initiated with a magnitude 5.0 event at 35 km depth, followed shortly by a 5.2 at 14 km and a 6.1 at 67 km. Subsequent events included a 5.7 at 16 km and two magnitude 5.1 shocks at 29 km and 24 km. A notable magnitude 6.5 occurred at 28 km depth, succeeded by a 5.4 at similar depth. The largest event reached magnitude 7.0 at 22 km, with later activity comprising multiple magnitude 5.0–5.5 events clustered between 28 km and 35 km. Depths predominantly ranged from 14 km to 67 km, reflecting both shallow crustal and intermediate-depth processes typical of subduction environments.

Swarm sequences like this often indicate stress redistribution along fault segments without a single dominant mainshock-aftershock pattern. In this case, the events clustered temporally and spatially, suggesting possible fluid migration or aseismic slip triggering successive ruptures. Such patterns provide insight into the preparatory phases of larger subduction earthquakes, as the swarm coincided with a magnitude 7.0 event that released significant energy within the sequence.

Regional geology underscores the area's high seismic hazard. The subduction interface here has produced great earthquakes historically, including the 2004 Indian Ocean event and subsequent large ruptures. Since 2000, three swarms have been documented in the vicinity, with two occurring in 2005 and one in 2007, highlighting episodic clustered activity amid steady plate convergence.

Strong earthquakes since 2000 further illustrate the setting. A magnitude 7.2 event struck 164 km west-southwest of Sungai Penuh on 25 February 2008, located 82 km from the swarm center. The magnitude 7.0 shock of 13 September 2007 itself occurred just 27 km from the swarm epicenter, underscoring the proximity of major releases to these clustered sequences.

This swarm contributes to understanding short-term seismic clustering in subduction zones, where multiple moderate-to-large events can occur within hours, potentially modulating stress on adjacent fault patches.

References

• SeismoSight internal swarm classification records
• USGS Earthquake Catalog (events since 2000)