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| Titel |
Modelling Sediment Thickness for Site-Effect Characterisation using H/V Spectral Ratio Analysis and Electrical Resistivity Tomography |
| VerfasserIn |
Koen Van Noten, Thomas Lecocq, Arnaud Watlet, Thierry Camelbeeck |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250097648
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| Publikation (Nr.) |
 EGU/EGU2014-13252.pdf |
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| Zusammenfassung |
| The H/V Spectral Ratio (HVSR) analysis of ambient seismic noise has been widely used
to estimate the fundamental site resonance frequency in the context of site-effect
characterisation. In regions of unknown subsurface structure, in which there is a significant
acoustic impedance contrast between sediments and the underlying bedrock, HVSR can be a
very powerful tool to map bedrock morphology and sediment thickness. Calibrating
the power-law relationship between the variation in fundamental frequency and
sediment thickness around these unknown sites is crucial for sediment thickness
mapping. This empirical relationship can be easily calculated by conducting HVSR
analysis of ambient noise measurements above boreholes with known bedrock depth.
Additional local H/V measurements above near-surface geophysical profiles, for
instance created by Electrical Resistivity Tomography (ERT), allow training and
improving the power-law relationship for sites with a shallow bedrock depth. As the
compaction of sediments influences the Vs, one has however to take into account that this
empirical relationship can only be applied in relative small areas with a similar local
geology.
Between 2008 and 2010, a seismic swarm (MLmax = 3.2) occurred in a hilly area, 20 km
SE of Brussels (Belgium). 60 of the 300 recorded events were felt/heard by the local residents
and were reported in the corresponding ‘Did You Feel It’ internet inquiries held by Royal
Observatory of Belgium. Several low-magnitude events show a distinct macroseismic
intensity variation that can be explained by the geological site effect, i.e. the local sediment
thickness, affecting the human perception of these earthquake-induced ground motions.
In this presentation, we apply the above described methodology and discuss the
results of a geophysical survey including ERT-profiling, ambient noise recording,
HVSR analysis in Geopsy and DEM-modelling to characterise the local site effects.
The resulting sediment thickness model (between 0m and 80m) allows us to
demonstrate that the highest macroseismic intensities are reported by respondents
living on eroded valley flanks above shallow bedrock, whereas lower macroseismic
intensities are reported on hill sites where respondents live on a thicker sedimentary
pile.
This site-effect characterisation shows that the lower macroseismic intensities of felt
events on hill tops can only be explained by the absorption of high frequency seismic energy
radiated by the local source if a considerable thickness of the local sediment column is
present. |
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