Site effects in the Val d'Agri basin (Southern Italy)

Abstract

The Val d’Agri area (VA), which is one of the areas with the highest seismic hazard in Italy, hosts the largest on-shore gas-/oilfield in Europe. Since 2002, the operator installed a local seismic network to monitor the seismicity of the area and to study the potential influence of industrial activities on it. Recently, the Italian Government decided to assign the monitoring duties to an independent external consultant (SPM) and published experimental guidelines (ILG), which describe the recommendations to follow for the geophysical monitoring of hydrocarbon extraction, waste-water injection and gas storage. Exclusively in the event of incompressible fluid injection, the ILG propose the application of an alert system, which entails specific measurements, in case that seismic parameters exceed specific thresholds levels. The reference parameters that have to be monitored inside an area called Inner Domain (DI) - a 3D-volume 5 km around the operation area are PGV, PGA and magnitude. VA basin is a quaternary tectonic basin located in the Lucanian Apennines, filled by Plio-Pleistocene deposits overlapping Mesozoic-Paleogene Lagonegro unit and Miocene flysch succession. The Lucanian Apennine is characterized by a thin-skinned thrust system tectonically superimposed on a deformed carbonate substratum (Apulia platform). All these seismic parameters are sensitive to the complex geological setting of the area, hence, using a fixed-threshold activation system can be a misleading approach. During this ILG test-period, one of the purposes of the SPM is to adjust for the specific site the threshold values imposed by the guidelines. Therefore, this study aims at: the evaluation of the network quality by analyzing the noise level at each seismic station; estimating the contribution of the local geological structure on the site response at each station by applying spectral-ratios techniques, on earthquakes and microtremor recorded by the VA network.