Seismic Swarm S20151117.2: Analysis of Activity in Greece
Seismic swarm S20151117.2 occurred in Greece from 07:57 on 17 November 2015 to 15:49 on 23 November 2015. Over 151 hours and 52 minutes, the sequence produced 252 earthquakes. Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 14 km, with magnitudes ranging from 2.0 to 5.0. The strongest shocks included a magnitude 4.8 event at 10 km depth on 17 November at 12:37:55, a magnitude 5.0 event at 10 km on 18 November at 12:15:37, and a magnitude 4.9 event at 8 km on 18 November at 13:03:14. Earlier activity featured several magnitude 4+ events clustered within the first 24 hours, followed by a gradual decline in both frequency and peak magnitudes through 19 November.
The temporal distribution shows intense initial productivity on 17 November, with multiple events exceeding magnitude 3.0 within hours of onset. Depths remained consistently crustal, indicating brittle failure in the upper lithosphere without deeper involvement. This pattern aligns with typical swarm behavior driven by fluid migration or stress redistribution rather than a single mainshock-aftershock cascade.
Greece occupies a tectonically active zone at the convergent boundary between the African and Eurasian plates. The Hellenic subduction zone, where the African plate descends beneath the Aegean plate, generates frequent seismicity across the region. The broader Hellenic Arc accommodates convergence rates of approximately 35–40 mm per year, resulting in a combination of thrust, strike-slip, and normal faulting. Historical records document numerous destructive earthquakes, including the 365 AD Crete event and the 1856 Rhodes shock, underscoring the long-term seismic hazard.
Since 1 January 2000, only two seismic swarms have been recorded in the national catalog, with the first occurring in 2003. Swarm S20151117.2 therefore represents a rare clustered episode within an otherwise steady background of distributed seismicity.
References
- Hellenic National Seismic Network (HNSN) bulletins
- European-Mediterranean Seismological Centre (EMSC) catalog
- USGS Earthquake Hazards Program regional reports
- McKenzie, D. (1972). Active tectonics of the Mediterranean region. Geophysical Journal International.