Swiss Journal of Geosciences

, Volume 106, Issue 3, pp 529–541 | Cite as

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

  • Cyrill Baumann
  • Jan Burjánek
  • Clotaire Michel
  • Donat Fäh
  • Luis A. Dalguer
Article
  • 157 Downloads

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.

Keywords

Central Alps Rhone–Simplon fault zone Neotectonics Hotée fault Geophysical prospecting Visp earthquake 1855 

Zusammenfassung

Untersuchungen von tektonischen Einheiten, wie zum Beispiel Verwerfungen, stellen wichtige Herausforderungen für Geologen und Ingenieure dar. Obwohl direkte Untersuchungsmethoden, wie beispielsweise Bohrlöcher oder Grabungen das Potential in sich bergen, genaue Daten zu liefern, sind sie doch gewöhnlich sehr teuer, zeitintensiv und geben nur einen punktuellen Einblick ins Erdinnere. Geophysikalische Methoden wie elektrische Bodenuntersuchungen oder das Georadar sind zwar weniger teuer und einfacher in der Handhabung, diese Methoden sind aber oft nur sehr schwer oder teilweise gar unmöglich einzusetzen, vor allem im schweren Gelände oder in stark überbautem Gebiet. In dieser Studie präsentieren wir eine einfach anwendbare und vor allem kostengünstige Methode, um Verwerfungen mittels seismischer Bodenunruhe detektieren können. Wir wenden diese Technik in der Region zwischen Visp und Unterstalden an einer Nebenverwerfung der Simplon Linie an, die keinen expliziten Oberflächenausdruck aufweist. Dabei gehen wir von der Vermutung aus, dass die zu untersuchende Verwerfung das Gestein in seiner unmittelbaren Umgebung stark zerbrochen hat, was wiederum eine laterale Modifikation der seismischen Geschwindigkeit und Dämpfung zur Folge hätte. Das Ziel war es, eine solche laterale Veränderung des beobachteten Wellenfeldes zu ermitteln, diese Anomalie zu kartieren und in einen geologischen Kontext zu setzen. Auf diese Weise konnten wir der Spur der Verwerfung auch ohne klaren Oberflächenausdruck folgen. Unsere Beobachtungen zeigen die Existenz einer Signatur im seismischen Spektrum des Hintergrundrauschens, die der gesuchten Verwerfung entsprechen könnte. Eine solche Signatur konnte über eine grössere Distanz hinweg identifiziert werden. Diese Beobachtungen, zusammen mit geomorphologischen und geologischen Hinweisen, stützen die Hypothese einer Verwerfung im entsprechenden Untersuchungsgebiet.

Résumé

Les enquêtes sur les unités tectoniques, tels que des failles, sont des défis importants pour les géologues et les ingénieurs. Bien que les méthodes d’investigation directe, tels que les forages ou les tranchées aient le potentiel de fournir des données exactes, elles sont généralement très coûteuses, chronophages et ne donnent qu’un aperçu ponctuel des sous-sols. Les méthodes géophysiques telles que les sondages électriques ou les enquêtes géoradar sont moins chères et plus faciles à manipuler, mais ces méthodes sont souvent très difficiles, voir même impossibles à utiliser, surtout en régions urbanisées ou présentant un fort relief. Dans cette étude, nous proposons une méthode qui nous permet de détecter des failles au moyen du bruit de fond sismique. Nous utilisons cette technique dans la région entre Viège et Unterstalden sur une faille mineure, probablement associée au système extensif de la ligne de faille du Simplon, et qui n’a pas d’expression explicite en surface. Nous supposons que la faille à analyser est entourée de roches fracturées, ce qui aurait pour conséquence une modification latérale de la vitesse et de l’atténuation sismique. Le but est d’identifier une telle modification latérale du champ d’ondes observé, cartographier cette anomalie et de la replacer dans le contexte géologique relatif à la faille cherchée. Ainsi nous avons pu suivre la trace de la faille cherchée sans expression claire à la surface. Nos observations montrent l’existence d’une signature dans le spectre du bruit de fond sismique qui pourrait correspondre à la zone endomagée de la faille cherchée. Une telle signature a pu être identifiée le long de la faille supposée. Ces observations et les indices géomorphologiques soutiennent l’hypothèse d’une faille dans la région analysée.

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Copyright information

© Swiss Geological Society 2013

Authors and Affiliations

  • Cyrill Baumann
    • 1
  • Jan Burjánek
    • 1
  • Clotaire Michel
    • 1
  • Donat Fäh
    • 1
  • Luis A. Dalguer
    • 1
  1. 1.Swiss Seismological Service, ETH ZurichZurichSwitzerland

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