Journal Article

Fundamental and higher two-dimensional resonance modes of an Alpine valley

Laura Ermert, Valerio Poggi, Jan Burjánek and Donat Fäh

in Geophysical Journal International

Volume 198, issue 2, pages 795-811
ISSN: 0956-540X
Published online June 2014 | e-ISSN: 1365-246X | DOI: http://dx.doi.org/10.1093/gji/ggu072
Fundamental and higher two-dimensional resonance modes of an Alpine valley

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We investigated the sequence of 2-D resonance modes of the sediment fill of Rhône Valley, Southern Swiss Alps, a strongly overdeepened, glacially carved basin with a sediment fill reaching a thickness of up to 900 m. From synchronous array recordings of ambient vibrations at six locations between Martigny and Sion we were able to identify several resonance modes, in particular, previously unmeasured higher modes. Data processing was performed with frequency domain decomposition of the cross-spectral density matrices of the recordings and with time-frequency dependent polarization analysis. 2-D finite element modal analysis was performed to support the interpretation of processing results and to investigate mode shapes at depth. In addition, several models of realistic bedrock geometries and velocity structures could be used to qualitatively assess the sensitivity of mode shape and particle motion dip angle to subsurface properties. The variability of modal characteristics due to subsurface properties makes an interpretation of the modes purely from surface observations challenging. We conclude that while a wealth of information on subsurface structure is contained in the modal characteristics, a careful strategy for their interpretation is needed to retrieve this information.

Keywords: Site effects; Wave propagation

Journal Article.  9693 words.  Illustrated.

Subjects: Volcanology and Seismology

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