Co-existence of active E-W normal faulting and NE-SW strike-slip faulting in the Eastern Aegean Islands; evidence from offshore studies in Lesvos and Samos, Greece.

Morphotectonic analysis of the offshore margins of the Aegean Islands in combination with onshore structures offers a rather complete image of the ongoing deformation within the Aegean micro-plate and especially along its eastern border zone with the Anatolian micro-plate. The swath data, off Lesvos and Samos islands, have been acquired by the hull-mounted RESON SeaBat 7160 on the oceanographic vessel NAFTILOS of the Hellenic Navy Hydrographic Service and gridded at 15m spatial resolution. Active tectonics affect both areas, as recorded by the intense seismic activity along with pronounced erosional and mass wasting processes.

The southern margin of Lesvos Island is divided into three sub-basins. The main feature is the central elongated sub-basin extending nearly parallel to the coast, reaching 700m water depth. Its northern margin is bounded by an abrupt WNW-ESE normal fault with morphological slopes up to 41^o, whereas its southern one is smoother with 5^o of slope and the overall structure corresponds to a half-graben. At its eastern edge, the basin is interrupted by a narrow steep channel, trending NW-SE, and progressively becomes shallower. At the western part of the Lesvos margin, a shallow basin forms an assymetric tectonic graben. Along the northwestern margin, three E-W basins lying approximately at 300-400 m water depth, constitute pull-apart basins within the complex ENE-WSW shear zone of the southern strand of the North Anatolian Fault, bounded by the sub-parallel Skyros and Adramytion Faults. Seismic activity in 2017 comprised a 6.3 magnitude earthquake on the WSW-ESE normal fault of the Lesvos Basin and two major aftershocks of magnitude 5.2 and 5.0 at the NW-SE strike-slip faults of the channel. During 2020 and 2021 normal WNW-ESE faulting with magnitude 5.1 and ENE-WSW dextral strike-slip faulting with magnitudes 4.8, and 5.0 occurred at the western and northwestern basins. However, a magnitude 7.0 earthquake had occurred onshore at the NE-SW  Kalloni-Aghia Paraskevi strike-slip fault in 1867.

The northern margin of Samos Island is bounded by a normal north dipping E-W fault that generated the strong earthquake of magnitude 7.0 on 30October 2020.The Samos Basin forms a half-graben of 690m water depth with morphological slopes of 31^o along the fault zone. Several canyons trending N-S, carve the northern margin ending up between 100m and 600m water depth, and several mass wasting events can be identified alongside the Samos coastline. Westwards, the Ikaria Basin is significantly deeper, reaching 1100m water depth and is delineated by an abrupt zone of nearly 51⁰ slope values, corresponding to the NE-SW Samos active western margin, probably related to strike-slip faulting. Additionally, an impressive retrogressive erosional structure occupies the area between Samos and Ikaria islands, with two prominent meandering narrow canyons debouching at the Ikaria Basin.

The combination of E-W to WNW-ESE normal faulting and NE-SW to ENE-WSW dextral strike-slip faulting with minor NW-SE sinistral strike-slip faulting is observed all over the North Aegean Sea, acommodating the southwestward motion of the Aegean micro-plate, relative to the Eurasian plate in the north and the Anatolian micro-plate in the East.