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Preliminary estimate of Holocene slip rate on active normal faults bounding the southern coast of the Gulf of Evia, central Greece

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We investigate the late Quaternary history of slip on the Kamena Vourla and Arkitsa normal faults, which are segments of a fault system bounding the south coast of the Gulf of Evia in central Greece, and which we refer to as the Coastal Fault System. We examine two river terraces, near the village of Molos, which are found within the uplifted footwall of the Kamena Vourla fault. The upper terrace is ~20 m above the present river level and appears to represent fan deposition into the main river channel from surrounding tributaries. The lower terrace, ~8 m above the present-day river bed, represents an interval of river-bed aggradation and correlates with the surface of a delta on the hanging-wall side of the fault. GPS profiles show a 6 ± 0.1 m vertical offset of the lower terrace surface as it crosses the fault. Preliminary dating of the two terrace levels, using both optical luminescence and radiocarbon methods, provides inconclusive results. The lower terrace, however, grades toward the present-day sea level and correlates with the surface of a delta on the hanging-wall side of the fault; it is, therefore, likely to date from ~6 ka, when sea level stabilized at its present-day highstand. With an age of ~6 ka, the 6 m vertical displacement of the lower terrace yields an estimate of ~1.2-2.0 mm/yr for the Holocene rate of slip across the Kamena Vourla fault. This rate of slip is comparable with an estimated rate of ~0.7-2.0 mm/yr for the central (Arkitsa) segment of the Coastal Fault System, and with a 0.4-1.6 mm/yr slip rate measured on the easternmost (Atalanti) segment. These estimates of Holocene slip rates are consistent with the 1-3 mm/yr of present-day extension across the Gulf of Evia measured by GPS, arguing against large changes in rate of extension through the Holocene. Both the Arkitsa and Kamena Vourla faults are clearly active and despite an absence of historical earthquakes on either fault, they should be considered to be a major hazard to local populations. However, further dating studies and palaeoseismic investigations are required before the slip rate and history can be fully quantified.

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