Morphotectonic analysis along the neotectonic faults of Geras Gulf of Lesvos Island (NЕ Aegean, Greece)

The tectonic structure of Lesvos is characterized mainly by an extensional regime acting from Neogene to recent while it is also under the influence of the westward migration of the southern branches of the North Anatolian Fault and the North Aegean Trench (NAT). Some of the main active faults of Lesvos Island are extended along the Geras gulf, which form an area of particular importance due to its proximity to the town of Mytilene. At a primal study of faults, a rift zone was found by neotectonic mapping, with deep-slip to oblique-slip normal faults of general direction NW - SE and W – E, respectively. Afterwards, based on rural measurements, the stress pattern of the area was studied as the main directions of the strain-stress field trends (σ1, σ2, σ3) were calculated. In some specific sites of fault surfaces overlapping generations of slickenside striae were observed, meaning that more than one field tectonic trends acted in the same position in different time periods. The results include two main distinct tectonic phases; the oldest one with extensional axis directed NESW and the newest trending NNW - SSE. The tectonic analysis and the interpretation of digital relief model (DEM), as well as the use of satellite imagery of the study area, have contributed significantly to the quantitative and qualitative analysis of morphotectonic characteristics of the faults. On the basis of the digital relief model, morphological sections were constructed perpendicular to the faults in order to extract information on the morphology of the slope. Moreover, profiles of morphological slope gradients were constructed along faults, based on the slip map of the digital relief model, with mean gradient ranging from 14° to 16° for most of the faults. These values seem to be related to the lithology of the rising block and the uplift rate. Shaded relief maps and three-dimensional imaging helped identifying faults. The determination of the effect of the tectonic geomorphological phenomena can be defined and quantified with morphotectonics indicators. In the present study five (5) morphotectonics indicators were applied: Stream Length – Gradient Index (SL), Drainage Basin Asymmetry Factor (AF), Hypsometric Integral (HI), Ratio of Valley – Floor Width to Valley Height (Vf), Mountain – front Sinuosity (S). The calculation of morphotectonic indicators in the regional faults confirmed the activity, and the recent action, which gave rise to the tectonic structures observed today. The high values of the SL index are found in morphological slope of the faults. The AF index shows a river spin, possibly due to the influence of faults, which either lift or humiliate the respective pieces of the rift zone. The index Vf exhibits relatively low values indicating a strong, deep erosion of the streams rising in the piece. The estimates of Mountain – front Sinuosity index are ranging from 1.1 to 1.6
and characterize active faults, though not associated with any known historical earthquake. Finally using empirical magnitude - fault length relationships, for the Gera’s Gulf area, the maximum expected magnitude earthquake for each fault or fault zone is calculated to Ms = 5.7 – 6.3.