A SMALL, EARTHQUAKE-TRIGGERED SUBMARINE LANDSLIDE AT GIALTRIA, NORTH EUBOEAN GULF, GREECE (JUNE 2023)

On June 8, 2023, a 4.8M earthquake occurred near Atalanti, Central Greece, at 12:33 PM GMT, with minimal damage locally but felt over 100 km away. Shortly after, local residents 30-km to the north noticed that a section of beach in Gialtra (Evia) had disappeared into the embayment to the north of the beach.

This site is located at a point on the western part of Evia on the south side of Gialtra Bay on the Edipsos-Lichada peninsula, in a western portion of the Euboean Gulf graben. The point of land has formed at the northern convergence of NW- and NE-facing beaches, with longshore transport shifting due to wind fields to consistently accumulate sediment to the point. The slope from the beach, after a 5-10 m wide beachface, drops from approximately 2 m to 30 m depth within 75 m of the beach, continuing gently to approximately 40 m depth 200m offshore.

The site was visited twice, on 6 and 23 June. Direct observations, combined with RTK-GNSS profiles, aerial and submarine photogrammetry, sidescan sonar, interferometric bathymetry, and sediment samples were taken. Elevations range from +1 m to approximately -40 m water depth. A prominent headscarp scalloped a portion of the beach inland approximately 3 m across a 20 m wide scarp. The headscarp slopes approximately 26° on the slide base, reducing the slope of the oversteepened sediment-seagrass layered materials. Exposed strata slide-left and slide-right indicate a failure depth to approximately 3 m with most visible materials down to approximately -15 m disintegrative in nature with a few small cohesive blocks (<50 cm) visible. The sediments are quartzose to feldspathic sands with abundant sea grass (Posidonia oceanica) in layered accumulations in the nearshore zone. The slope becomes less steep approximately 50 m offshore as the runout extends to the basin floor from approximately from -20 to -40-m out. Given the nature of sediment transport, P. spp. cohesion, and tectonics in the area, it is clear that this region will continue to shed both inorganic and organic matter into the basin in the future through this process.

This site is an excellent example of earthquake-triggered submarine landslides in a relatively closed gulf which may further produce a tsunami. It is also an interesting example of how morphodynamics can lead to beach reconstitution and for this reason the team will continue to monitor this coastline.