Seismic Swarm S20110913.1: Analysis of Earthquake Activity in Greece

Seismic swarm S20110913.1 was recorded in Greece between 13:36 on 12 September 2011 and 12:51 on 14 September 2011. Over this 47-hour and 15-minute period, a total of 43 earthquakes were detected. The events clustered in both time and space, with magnitudes ranging from 1.1 to 3.7 and focal depths primarily between 5 and 23 km. Such swarms represent sequences of earthquakes occurring in rapid succession without a single dominant mainshock, often linked to fluid migration or stress adjustments along fault systems.

The strongest event reached magnitude 3.7 at a depth of 8 km on 12 September at 14:29:22 UTC. Additional notable shocks included magnitudes of 3.3 at 18:15:45 and 18:20:50 on the same day, both at 10 km depth. On 13 September, peaks of 3.3 at 03:42:14 (5 km depth) and 3.1 at 01:56:12 (10 km depth) were observed. Depths remained shallow overall, consistent with activity in the upper crust, while smaller events (magnitudes 1.1–2.5) filled the sequence and indicated ongoing microfracturing.

Greece lies at the convergent boundary between the African and Eurasian plates, where the African plate subducts beneath the Aegean plate along the Hellenic Arc. This tectonic setting produces frequent seismic activity, including both large thrust earthquakes and shallower swarms associated with back-arc extension and strike-slip faulting in the Aegean region. The Hellenic subduction zone has shaped the region's geology for millions of years, contributing to volcanic arcs such as the South Aegean Volcanic Arc and elevated seismicity rates compared to stable continental interiors.

Historical records maintained by SeismoSight indicate that only two prior swarms have occurred in the monitored area since 1 January 2000. These took place in 2003 (one swarm) and 2008 (one swarm). The 2011 sequence therefore represents a rare recurrence within this timeframe, highlighting episodic rather than continuous swarm behavior in the locale.

Analysis of the 2011 data reveals a gradual decay in event rate after the initial 24 hours, with the majority of larger magnitudes occurring early in the sequence. Depths showed modest variation, clustering around 10–15 km for many events, which may reflect a common seismogenic layer. No single event exceeded magnitude 4.0, underscoring the moderate energy release typical of swarm-type activity rather than a classic foreshock-mainshock-aftershock pattern.

Continued monitoring of such sequences aids in understanding local stress regimes and potential triggers, including pore-pressure changes within the crust. The provided swarm parameters offer a detailed temporal and magnitude distribution that supports refined models of Aegean tectonics.

References

SeismoSight internal swarm classification records for S20110913.1.
Hellenic Arc tectonic framework, U.S. Geological Survey Earthquake Hazards Program.
Aegean plate boundary studies, European-Mediterranean Seismological Centre.