Seismological contribution to Passo della Morte landslide characterization (North Eastern Italy)

In the North-East of Italy, along the left side of the Tagliamento River, a large area, named Passo della Morte, is involved in several landslides. The western part is characterized by sub-vertical limestone layers and a dense system of discontinuities and fractures, which create ideal conditions for rapid rockfalls. The eastern part of this area is interested by several slides which differ in movement direction and velocity. Their origin is linked to the reactivation of an ancient landslide. The most active blocks move up to 5 cm/year. The main risks are represented by the possible consequence of landslides interaction with the National Road that crosses this area and the Tagliamento River. A microseismicity and acoustic emission monitoring system was installed in the western part of Passo della Morte, inside a tunnel, to monitor the signals generated by rock slope deformation and to recognize precursory phenomena. The system consists of five sensors, two for seismic signal identification and three for acoustic emissions detection. A surface seismometer and a borehole accelerometer arrange the seismic station. Three waveguide with the relative piezoelectric arrange the acoustic emission stations. The orientation of the borehole accelerometer was evaluated indirectly. A program that applied the maximum cross-correlation method and used the surface seismometer as a reference for relative orientation estimation was developed in Fortran90. The analysis of some earthquake allowed the reconstruction of the accelerometer orientation. The microseisms and the acoustic emissions detected by the monitoring system were analyzed and correlated to rainfalls in order evaluate their effects on the rock mass. A direct relation between different rainfall events and sharp increases in microseisms and acoustic emissions was found. Site effects were investigated analyzing seismic noise and earthquake recordings with the Horizontal to Vertical Spectral Ratio method. The seismic noise recorded at different site along the reactivated landslide was analyzed in order to detect the landslide slip surface and reconstruct its geometry. Data processing not give the expected results. The analysis of earthquakes recorded by the surface seismometer showed the absence of amplification able to bring in resonance the unstable rock mass. The cooperation of our research group with Civil Protection Department of Rome allowed to apply at San Giuliano di Puglia seismic stations the same methods used at Passo della Morte in order to estimate relative orientation of the accelerometers and evaluate site effects. The correct orientation of the borehole accelerometers was verified using the program developed. The surface accelerometer was used as reference. Nakamura method was applied at each station to determine the relative resonant frequencies. The resonant frequency found for one site was linked to the bedrock depth. H/V spectral ratio and reference station methods were applied at each station in order to investigate the frequencies strongly amplified. For one site, the amplifications found were very high.