A New Strategy for the Electromagnetic Monitoring of Seismic Areas: the Case-Study of Agri Valley (Southern Italy).

The Agri valley is one of the most active areas of Southern Apennine chain that was hidden by destructive events in historical and recent periods (i.e. the 1857 Great Neapolitan earthquake). The geological environment is extremely complex and the location of the main faults are still debated. The Agriu valley is a N- W elongated basin filled by quaternary deposits covering the pre-quaternary rock of the Apennine chain. The area is characterised by a very low man-made electromagnetic noise and it represents an ideal "outdoor laboratory" to test new strategies for geophysical monitoring of active faults. In this work we present a novel approach based on the integration of Deep Electrical Resistivity Tomography (DERT), Self-Potential (SP) and Magnetotelluric (MT) time-continuous measurements. The basic idea underlying this approach is to jointly analyse and modelling the electrical signals, observed on earth-surface, and the time-dependent changes of subsurface resistivity patterns. We are firmly convinced that any conclusions about the relationship between anomalous electrical signals and earthquake activity cannot be achieved without a good knowledge of subsurface resistivity structures and an accurate localisation of the electrical sources. In our work DERT has been applied for illuminating in-depth the geological structures and giving a contribute to better define the thickness and the shape of alluvial deposits covering the pre- quaternary bedrock of Agri valley. SP surveying and time-continuous monitoring have been carried to study electrokinetic effects due to groundwater patterns and to identify possible fingerprints of fluid migration phenomena. New SP stations with sensors in boreholes (200m) have been installed. MT time-continuous soundings have been performed to analyse the time-dependent changes in deep resistivity patterns. All data measured in the study area have been processed and filtered using robust statistical methodologies (DFA, wavelet, multi-fractal analysis etc..) and the 2D resistivity patterns have been estimated using novel tomographic tools for geophysical data inversion. This approach will disclose the way for obtaining a "global electromagnetic monitoring" of Agri valley: a time series of subsurface resistivity images with an investigation depth varying in the range 0-10Km and information about the spatial-temporal dynamics of electrical accumulation zones possibly-related to seismogenetic phenomena. To-date, the more interesting results regard the identification of near-surface fault evidences also confirmed by paleo-seismological studies, the assessment of the effective stability of resistivity estimates and the analysis of the influence of groundwater circulation system. Finally, we are working to integrate these electromagnetic observations with satellite data (TIR and InSAR) available for the Agri valley.