GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.1 41 First results from microseismic monitoring of a rockslide at Madonna del Sasso (Verbania, Italy) C. Colombero 1 , C. Comina 1 , A. Fiaschi 2 , G. Saccorotti 3 , S. Vinciguerra 1,4,5 1 Dipartimento di Scienze della Terra, Università degli Studi di Torino, Italy 2 Fondazione Prato Ricerche, Prato, Italy 3 Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy 4 Department of Geology, University of Leicester, Leicester, UK 5 British Geological Survey, UK Introduction. The hazard mitigation of rockslides and the understanding of their time evolution are a main task particularly when a pre-existing fracture system affects the rock mass. Instability can rapidly accelerate to failure without clear precursory warnings. Traditional monitoring techniques (inclinometers, tiltmeters, extensometers, ...) may be inadequate to forecast the dynamics of evolution and provide quantitative elements for the development of an early warning system. Since a persistent fracture system originates from the genesis, growth and progressive coalescence of microcracks, it is key to constrain the initial stages of rupture. The energy release, in the form of elastic waves, associated with the formation of microcracks can be detected by a microseismic monitoring network. The location of the microseismic events and the monitoring of their time and spatial evolution with particular reference to potential destabilizing factors, such as rainfalls and temperature changes, can provide information with respect to the understanding of the unstable sectors within the rock mass and the prediction of the possible acceleration to the failure. To understand the failure processes and to search for precursory patterns to failure, the microseismic technique has been extensively used in rock samples at the laboratory scale and at intermediate scales in quarries, mines or tunnels. Conversely, only a few applications exist at the landslides/rockslides scale, and thus it is a priority testing the ability of microseismic monitoring in giving interesting insights into gravitational instability and, in very few cases, in providing precursory patterns for failure forecasting and hazard assessments. Interesting applications on unstable rock slopes and rockfalls have been developed by Spillman et al. (2007), Amitrano et al. (2010), Helmstetter and Garambois (2010) and Levy et al. (2011), studies on rocky cliff instabilities (Amitrano et al. , 2005) and mudslides (Amitrano et al. , 2007) are also reported. In this paper we present the first results from a microseismic monitoring network installed at Madonna del Sasso (NW Italy) to monitor rock instability phenomenon highlighted by neat and long lasting episodes of slow deformation recorded by standard monitoring devices (topographic measurements, borehole inclinometers, crackmeters and extensometers). Adetailed geophysical and geomechanical characterization of the site has been preliminarily undertaken in order to define the overall geometry and the fracturing state of the instable rock mass and to establish the best position of the nodes for the monitoring network. Furthermore the definition of the seismic velocity field inside the rock mass is a fundamental parameter for the processing of the recorded microseismic signals, in order to localize the microseismic events. We also analyzed the signals acquired during the first six months of monitoring. We present the first results, and we aim to develop objective and automatic procedures for signal classification and event localization. The test site. The cliff of Madonna del Sasso (45° 79’ N, 8° 37’ E) is located in NW Italy, on the western shore of the Orta Lake. It is a high massive granite outcrop bordered on three sides (N, E and S) by roughly vertical walls, with a height of about 200 m (Fig. 1a). The top of the relief is located at an altitude of approximately 650 m a.s.l. and it is occupied by a panoramic square, in front of the XVIII-century sanctuary from which the place takes its name. From a geological point of view, the area is totally characterized by a granitic rock mass, known as Granito di Alzo. The actual morphology has been heavily affected from intense min- ing activity carried out until a few decades ago. A preliminary geomechanical characterization