An earthquake swarm designated S20220930.3 was recorded in northern Sumatra, Indonesia, beginning at 13:36 on 30 September 2022 and concluding at 16:47 on 2 October 2022. Over this 51-hour period, 56 earthquakes were detected. The sequence featured a prominent mainshock of magnitude 5.9 at 18 km depth on 30 September at 19:28:40 UTC, accompanied by a magnitude 5.0 event at 10 km depth later that evening. Subsequent activity consisted predominantly of smaller events, with magnitudes ranging from 2.5 to 4.6 and focal depths clustered around 10 km.
The swarm initiated with a magnitude 2.5 foreshock at 10 km depth, rapidly escalating to the largest event within hours. Aftershocks continued at a declining rate through 1 October and into the early hours of 2 October, terminating with a final magnitude 2.6 event. Depths remained shallow throughout, consistent with activity along upper-crustal structures.
Northern Sumatra occupies a tectonically active segment of the Sunda subduction zone, where the Indo-Australian plate converges with and subducts beneath the Eurasian plate at rates of approximately 5–6 cm per year. This oblique convergence produces both megathrust earthquakes along the plate interface and strike-slip motion along the Great Sumatran Fault, a major right-lateral system traversing the island. The region’s high seismicity reflects ongoing strain accumulation and release at these boundaries.
Historical records document recurrent large earthquakes in northern Sumatra. The 2004 Mw 9.1–9.3 Sumatra-Andaman event, centered west of the island, generated a devastating trans-oceanic tsunami and remains one of the largest instrumentally recorded quakes. Subsequent activity includes the 2012 Mw 8.6 Indian Ocean strike-slip sequence and numerous moderate events linked to the Sumatran Fault. Updated seismic monitoring by regional networks continues to track background seismicity and swarm episodes that punctuate longer-term strain release.
The 2022 swarm’s shallow depths and rapid succession of events align with typical swarm behavior observed along the Sumatran Fault and adjacent secondary structures, where fluid migration or aseismic slip may trigger clustered seismicity without a single dominant mainshock-aftershock sequence. No surface rupture or significant damage was associated with this particular episode.
References
USGS Earthquake Catalog (earthquake.usgs.gov)
Global CMT Catalog (globalcmt.org)
Geological Survey of Indonesia (puskim.pu.go.id)
Sieh, K. & Natawidjaja, D. (2000). Neotectonics of the Sumatran fault, Indonesia. Journal of Geophysical Research.