Location:

Period:

10 Jul 2021 12:02:21 - 12 Jul 2021 22:38:09 (2 days 10 hours 35 minutes)

Volcanoes in 100km radius:

Methana(79km)

Earthquakes:

4 swarms found nearby.

2009

S20090902.1(27.8km)

2 Sep

5 days 4 hours

95 earthquakes

2011

S20110127.2(18.3km)

27 Jan

22 hours

44 earthquakes

S20110422.1(25.8km)

22 Apr

1 day 1 hours

28 earthquakes

2021

S20210720.2(0.8km)

20 Jul

1 day 23 hours

38 earthquakes

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Seismic Activity: The July 2021 Earthquake Swarm in Greece

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, making it one of Europe’s most seismically active regions. The Hellenic Arc, a subduction zone extending from the Ionian Sea to southwestern Turkey, drives frequent earthquakes as the African plate descends beneath the Aegean plate. This tectonic setting produces both shallow crustal events and deeper subduction-related seismicity, with the Aegean region experiencing ongoing extension and back-arc spreading.

The swarm designated S20210711.1 occurred in this dynamic environment. It began at 12:02 on 10 July 2021 and concluded at 22:38 on 12 July 2021, spanning 58 hours and 35 minutes. During this period, 58 earthquakes were recorded, with magnitudes ranging from 2.5 to a peak of 4.3. Depths were predominantly shallow, between 1 km and 12 km, consistent with activity along upper-crustal faults influenced by the regional extensional regime.

Event timing showed clustering in the first 24 hours, followed by a gradual decline. The largest shock (magnitude 4.3 at 10 km depth) occurred early on 11 July, after which activity shifted toward smaller events at depths of 6–9 km. Such patterns are typical of swarm sequences in Greece, where fluid migration or aseismic slip can sustain elevated seismicity without a single dominant mainshock.

Since 2000, only three swarms have been documented in the national catalogue. Earlier episodes took place in 2009 (one swarm) and 2011 (two swarms). The 2021 sequence therefore represents the third swarm of the instrumental era, underscoring the episodic nature of clustered seismicity along Greece’s active fault systems.

These observations align with Greece’s long-term seismic character. Instrumental and historical records confirm that moderate-magnitude swarms frequently occur without producing destructive shaking, although they contribute to cumulative stress redistribution along the Hellenic plate boundary. Continued monitoring remains essential given the region’s high strain rates and proximity to populated areas.

References

Hellenic Seismic Network (NOA) earthquake catalogue
United States Geological Survey (USGS) tectonic summaries for the Eastern Mediterranean
McKenzie, D. (1972). Active tectonics of the Mediterranean region. Geophysical Journal International.