Investigation of the 1867 Lesbos (NE Aegean) earthquake fault pattern based on soil-gas geochemical data

Abstract In this work, the soil-gas method is being used to compare geochemical data with the geological and structural information on Lesbos Island (NE Aegean) in particular in the Aghia Paraskevi–Kalloni region, the epicentral area of an earthquake in 1867. The island belongs to an seismically active zone, part of the right-lateral and normal fault systems of North Anatolia and North and Central Aegean. Field observations, aerial photo interpretation and microearthquake distribution show two main fault systems trending NE–SW to NNE–SSW and NW–SE. The strongest known destructive earthquake hit the island in 1867 with its epicentral area in the central part of Lesbos (estimated intensity IX–X for the most central and eastern part of the island and VI–VII for the rest). The soil-gas sampling was performed in August 1996 in the Aghia Paraskevi area. By this soil-gas survey, an attempt was made for the recognition of active faults, using the soil-gases as fault tracers. High values of all the investigated gases (He, 222Rn, CO2 and CH4) characterize the central and southern sector of the area. The soil-gas anomalous concentrations are probably linked to the very active extensional tectonic structures, that cause the characteristic seismicity of Lesbos (especially those of 222Rn and CO2). After statistical processing of the data and the use of contour and dot-maps of each investigated gas, an important accordance was found between the interpreted linear gas anomalies and the observed faults which possibly generated the 1867 shock.