Underground Traffic‐Induced Body Waves Used to Quantify Seismic Attenuation Properties of a Bimaterial Interface Nearby a Main Fault

Abstract

AbstractUnderground traffic activities spread ground‐borne vibrations in a complex way. The present work focuses on 173 individual vehicles tracked as moving sources of vibrations in the Mont Terri rock laboratory, located 95 m away from a motorway tunnel. Two neighboring geophones record the vertical ground velocity of traffic‐induced events with peak particle velocities ranging in 0.15–0.35 m/s. A dynamic cross‐correlation of the waveforms is used to align the individual events for coherent and robust analysis. A root mean square (rms) method allows identifying the main feature, centered at the Main Fault, and an unexpected feature located about 140 m apart. The dominant seismic frequencies are 15 and 10 Hz, respectively. The vehicle speeds (70–90 km/h), estimated from the time‐delay between the two features, and the seismic velocity (800–2,300 m/s), assessed from a simple kinematic model, are used to convert time to position along the tunnel, allowing modeling the local rms with the Bornitz's equation. The resolved frequency‐independent attenuation coefficients are 2.61 s/km in the Opalinus Clay including the gallery network, and 1.23 s/km in the Limestone, a contrast of elastic properties that defines a bimaterial interface. A particle motion analysis highlights body waves, with dominant vertically polarized shear waves above the Main Fault. The origin of the unexpected feature is discussed in terms of site effects and seismic propagation in a heterogeneous fracture network. Traffic‐induced events can be used as reproducible, low‐frequency, and non‐destructive sources with potential interest in long‐term monitoring at the scale of an underground gallery.