Seismic Swarm S20030619.1 Near Néa Artáki, Greece: June 2003 Analysis

A notable earthquake swarm, designated S20030619.1, was recorded 5 km east-northeast of Néa Artáki on the island of Evia, Greece. The sequence commenced at 05:25 on 18 June 2003 and concluded at 03:29 on 21 June 2003, encompassing 41 events over 70 hours and 4 minutes. This swarm reflects the persistent seismic activity in the region driven by regional tectonics.

The broader geological setting of Evia places it within the Aegean extensional province. The area experiences normal faulting linked to back-arc spreading behind the Hellenic subduction zone, where the African plate subducts beneath the Eurasian plate. Evia lies near the transition between the Gulf of Evia graben and the North Aegean trough, influenced by dextral shear from the North Anatolian Fault system. These structures produce shallow to intermediate-depth seismicity, consistent with the swarm's focal depths ranging mostly between 10 and 26 km.

Historical records document recurrent moderate seismicity in central Greece and Evia. The region has experienced destructive events, including the 1894 Atalanti earthquake sequence along the Atalanti fault and earlier activity associated with the Gulf of Corinth rift system. Modern instrumental monitoring since the late 20th century has captured numerous swarms and mainshock-aftershock sequences, underscoring the area's capacity for clustered earthquake activity without a single dominant rupture.

Within swarm S20030619.1, the largest event reached magnitude 4.5 at 05:25 on 18 June 2003 at 21 km depth. Subsequent notable shocks included a magnitude 4.4 event at 03:38 on 19 June (20 km depth), a magnitude 4.1 at 01:03 on 19 June (25 km depth), and a magnitude 4.0 at 01:00 on 19 June (20 km depth). The majority of events clustered between magnitudes 2.0 and 3.5, with depths predominantly in the 15–26 km range and a few outliers at shallower (2–12 km) or deeper (60 km) levels. The sequence displayed a typical swarm pattern of gradual onset, peak activity on 18–19 June, and progressive decay by 21 June.

Such swarms in the Aegean often arise from fluid migration or aseismic slip along fault networks rather than a single large rupture. The spatial concentration near Néa Artáki aligns with mapped normal faults bounding the southern Gulf of Evia. No surface rupture or significant damage was associated with this low-to-moderate magnitude sequence.

Ongoing monitoring by Greek and European seismic networks continues to refine understanding of these clusters, contributing to probabilistic hazard assessments for central Greece. The 2003 swarm exemplifies the background seismicity that characterizes this tectonically dynamic margin.

References

• SeismoSight internal swarm classification S20030619.1
• Hellenic Seismic Network bulletins (2003)
• Geological maps of Evia, Institute of Geology and Mineral Exploration (IGME), Greece
• Tectonic framework of the Aegean, updated regional studies (post-2020 compilations)