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

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 N orth and Central Aegean. Field observations, aerial photo interpretation an d microearthquake distribution show two main fault systems trending NE-SW t o NNE-SSW and NW-SE. The strongest known destructive earthquake hit the isl and in 1867 with its epicentral area in the central part of Lesbos (estimat ed intensity IX-X for the most central and eastern part of the island and V I-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. Hi gh values of all the investigated gases (He, Rn-222, CO2 and CH4) character ize the central and southern sector of the area. The soil-gas anomalous con centrations are probably linked to the very active extensional tectonic str uctures, that cause the characteristic seismicity of Lesbos (especially tho se of Rn-222 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 fau lts which possibly generated the 1867 shock. (C) 1999 Elsevier Science B.V. All rights reserved.