Comparison of microseismic-event and ambient-noise strategies for rock-mass monitoring

Unstable rock mass in highly populated areas draw attention to the need for appropriate monitoring techniques due to their sudden energetic release and large destructive power. In this work we present the results of a seismic-based characterization and monitoring campaigns, carried out on a prone-to-fall granitic cliff in northwestern Italy (Madonna del Sasso, VB). The frontal portion of this cliff is free on three sides (Fig. 1a) with high vertical walls (150 m) directly exposed on the roads, houses and small factories located on the western shore of Orta Lake. The top of this area is affected by four main fracture families, which almost isolate two unstable sectors (A and B, with an estimated volume of 12,000 and 7,500 m3 respectively) and show a complex intersecting 3-D pattern (Fig. 1b). The steep morphology and the limited accessibility of the cliff led to a site-specific approach to the understanding of its behavior. Considering the growing people’s awareness of the natural risk related to the site, a monitoring campaign with unconventional methodologies, devised on the basis of the preliminary seismic characterization, appeared the best choice to control the evolution of the phenomenon. Four triaxial geophones were installed on sector A, with ST1 and ST2 located at the foot of the block, ST3 placed at the top of the rock column and ST4 settled outside the fractured area, in order to be used as a reference station (see again Fig. 1a for the location of the stations). Particularly, in this work we focus on the comparison between the outcomes of microseismicity (event detection, location and time rate) and ambient seismic noise (spectral analysis and cross-correlation) techniques.