Fault zone signatures from ambient vibration measurements: a case study in the region of Visp (Valais, Switzerland)

Abstract

Investigations of tectonic features, such as faults, are important challenges for geologists and engineers. Although direct investigational methods, such as boreholes and trenches, have the potential to provide accurate data, these direct methods are usually expensive and time consuming, and give only punctual insights into subsurface structures. Geophysical methods, for example electric surveys and ground penetrating radar, are less expensive and faster to implement. However, these geophysical methods may be difficult or sometimes even impossible to apply in regions with rough topography or regions which are highly urbanized. In this study, we propose an easy-to-use and affordable method to detect fault zones based on ambient vibration observations. We apply this method in the region between Visp and Unterstalden (canton Valais, Switzerland) on a small fault branch, which has no explicit surface expression, and which is linked to a major fault zone, the Simplon Fault Zone. The assumption is that the fault of interest is surrounded by damage zone consisting of fractured rock, and that this results in lateral changes of both seismic velocity and attenuation. The objective was, first, to identify such lateral changes in the observed seismic wave-field, and second, to map any anomalies and combine them with the available geological information. In this way, we were able to follow the fault trace even without a clear surface expression of the fault. Our observations showed the existence of a signature in the power spectra of the seismic noise that may correspond to a damage zone. Such signature is observed along the trace of the expected fault.

ShakeMap-based prediction of earthquake-induced mass movements in Switzerland calibrated on historical observations

“…1 account for regional amplification effects as explained in Cauzzi et al (2015), clearly apparent in the Swiss Foreland (north of the Alps, along the east-west axis of the country) and along the Alpine valleys. The approximate finite-fault geometries for the events of Basel 1356 and Stalden-Visp 1855 were derived from literature sources (Ferry et al 2005;Fritsche 2008;Baumann et al 2013;Cauzzi et al 2015). Possible significant finite-fault effects, e.g.…”