Image processing and roughness analysis of exposed bedrock fault planes as a tool for paleoseismological analysis: results from the Campo Felice fault (central Apennines, Italy)

Abstract

Morphologic investigations along the Campo Felice (CF) fault (central Apennines, Italy) have been made in order to develop a procedure for the paleoseismological analysis of bedrock fault scarps. The CF fault has been responsible for the formation of an impressive limestone fault scarp. Geomorphologic work on the CF basin and related fault indicated that the scarp originated from tectonic fault displacements. Three morphologic units have been distinguished along the fault scarp and defined as morphosome M1 (lowest part of the scarp), M2 and M3 (the uppermost part). These units display different karstic features, which are the result of their different duration of exposure to weathering. Micromorphologic analyses focused on the morphosome M1, along which the CF fault plane is exposed for a height ranging between 4 and 7 m. These analyses were aimed at defining differently weathered bands located at various heights, and parallel to the fault scarp top and base. The presence of these bands suggests repeated fault movements. The exposed fault surface displays a low-grade biokarstic weathering due to the action of epilithic and endolithic organisms. The biokarst distribution is, however, inhomogeneous and conditioned by the presence of nourishing elements, moisture and by light intensity. An area preferentially affected by the biokarstic processes develops as a band at the bedrock–soil contact at the base of the scarp. Roughness and colour analyses were made to identify uplifted bands which previously formed at the bedrock–soil contact. The roughness analysis was made using a microroughness-meter along 20-cm long horizontal transects repeated each 20 cm of fault height for the entire morphosome M1, at various sites along the scarp. The roughness variance data, plotted vs. the fault height, failed to identify differently weathered bands of paleoseismological interest. This result is probably due to the complex distribution of biokarst along the investigated fault plane. More reliable results have been obtained by areal analysis of the variation of the colour rendering of the rocks exposed along the fault plane at different sites. Photographic images of large portions of fault surfaces have been processed with standard graphic computer programs. The variations of colour indicated the presence of bands at various heights along the fault plane. Two uplifted bands have been recognised at all the investigated sites suggesting two displacement events (E1 and E2). A preliminary chronological framework for these two events, the youngest of which affected the CF fault, can be derived from the paleoseismological data available for the southernmost branch of the regional fault system that includes the CF fault. According to these data, E1 may have occurred between 860 and 1300 AD, while E2 may have occurred at about 1900 BC. Work is in progress to define surface exposure ages of different parts of the fault plane by means of in situ produced cosmogenic 36Cl. This procedure will give further chronological constraints for the age of E1 and E2 and will also permit to test the validity of the micromorphologic analysis of bedrock fault scarps for paleoseismological aims.