Seismic Insights into Swarm S20070502.1: Greece's May 2007 Earthquake Cluster

Greece occupies a tectonically complex zone at the convergent boundary between the African and Eurasian plates. The Hellenic subduction zone, where the African plate descends beneath the Aegean microplate, drives the majority of regional seismicity. This setting produces both shallow crustal events and deeper intermediate-depth earthquakes along the arc, with frequent swarms linked to fluid migration and stress transfer in the overriding plate. Updated geological assessments confirm ongoing extension in the Aegean, superimposed on the subduction regime, resulting in normal and strike-slip faulting across much of the country. Earthquake swarms—sequences of events without a dominant mainshock—are common in Greece and often occur in volcanic or geothermal areas such as the South Aegean Volcanic Arc. Historical records since 2000 document three such swarms through 2007, with one in 2003 and two in 2007, highlighting episodic clustering rather than isolated large shocks. Swarm S20070502.1 began at 22:23 on 1 May 2007 and concluded at 02:34 on 13 May 2007, spanning 268 hours and 11 minutes. During this interval, 139 earthquakes were recorded across Greece. Analysis of the first 100 events reveals predominantly low-to-moderate magnitudes between 1.9 and 4.1, with the majority clustered between 2.0 and 3.0. Depths remained shallow, typically 1–19 km, consistent with upper-crustal brittle failure. The sequence opened with events of magnitude 2.2–2.4 on the evening of 1 May. Activity intensified on 2 May, including a magnitude 3.4 shock at 7 km depth and a magnitude 3.0 event at 3 km. Further escalation occurred on 3 May with a magnitude 3.5 earthquake at 4 km, followed by additional magnitude 3.5 events on 4 May at very shallow depths (0–2 km). Magnitudes peaked on 8 May, featuring a magnitude 4.1 event at 7 km alongside multiple magnitude 3.5–3.6 shocks between 3–8 km depth. Subsequent days showed a gradual decline, with the final listed events on 10 May registering around magnitude 2.2–2.3 at 2–12 km. This temporal pattern—initial low-level activity building to several moderate events before tapering—illustrates classic swarm behavior driven by aseismic slip or pore-pressure changes rather than a single rupture. Depths consistently under 20 km underscore the shallow tectonic regime of the Aegean crust. References: SeismoSight internal swarm classification records for S20070502.1.
Hellenic Subduction Zone tectonic framework, National Observatory of Athens.
Aegean extension and swarm statistics since 2000, updated regional seismic bulletins.