Seismic Swarm Activity in Northern Sumatra: February 2022

A seismic swarm designated S20220225.1 occurred in northern Sumatra, Indonesia, beginning at 01:35 on 25 February 2022 and concluding at 10:31 on 28 February 2022. Over approximately 80 hours and 55 minutes, 51 earthquakes were recorded. The events were predominantly shallow, with most hypocenters at depths of 10 km and one at 5 km and another at 11 km. Magnitudes ranged from 2.5 to 6.1, with the largest event—a magnitude 6.1 earthquake at 4 km depth—occurring early in the sequence on 25 February at 01:39. Subsequent notable shocks included a magnitude 4.9 at 10 km depth later that morning and a magnitude 4.7 at 5 km depth on 26 February. The swarm exhibited a typical pattern of clustered activity with an initial strong mainshock followed by numerous aftershocks that gradually decreased in frequency and magnitude.

Northern Sumatra lies within a tectonically active region shaped by the subduction of the Indo-Australian Plate beneath the Eurasian Plate along the Sunda Trench. This convergent margin generates frequent seismicity, including both interplate thrust events and intraplate strike-slip activity associated with the Great Sumatran Fault. The February 2022 swarm aligns with this setting, where shallow crustal stresses can produce sequences of moderate earthquakes without a single dominant mainshock. Historical records document major events in the region, such as the 2004 magnitude 9.1–9.3 Sumatra-Andaman earthquake, which originated west of northern Sumatra and triggered a devastating tsunami. More recent activity includes the 2012 magnitude 8.6 Indian Ocean earthquake, an intraplate strike-slip event farther offshore, and ongoing moderate seismicity along the Sumatran fault system.

The swarm's shallow depths and rapid succession of events suggest localized stress release within the overriding plate or near the subduction interface. Such swarms are not uncommon in subduction zones and may reflect fluid migration or aseismic slip triggering brittle failure. No significant damage or casualties were associated with this particular sequence, consistent with the moderate magnitudes involved.

References

USGS Earthquake Catalog
Global CMT Catalog
Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) reports