In situ seismic investigations of fault zones in the Leventina Gneiss Complex of the Swiss Central Alps

Abstract

AbstractUnderground seismic tomography investigations have been carried out in the Faido access tunnel of the Gotthard Base Tunnel, Switzerland. Velocity measurements were made over a total length of 2651 m of the adit with the tunnel seismic prediction system, ISIS (Integrated Seismic Imaging System). ISIS provides high-resolution seismic imaging, using an array of rock anchors equipped with 3D-geophones.The first onsets of the compressional and shear waves were used for tomographic inversion. Two-dimensional seismic-velocity models reveal a disturbed zone between 2 and 3 m inward from the tunnel wall, characterized by strong variations from 3500 to 5800 m s−1 in compressional wave velocity VP, and from 2000 to 3000 m s−1 in shear-wave velocity VS. High-velocity zones correspond to quartz lenses, and low velocities mainly indicate fractured rock. Beyond the excavation disturbance zone, the variations in seismic velocities are generally smaller. The tomographic image of the rock mass also revealed two major fault zones composed of cataclastic shear planes surrounded by wider fracture zones. These structural characteristics are also useful for the prediction of cataclastic zones at other sites.