Analysis of seismic signals recorded on a prone-to-fall rock column (Vercors massif, French Alps)
Résumé
A small-aperture (40 m) short-period seismic array was installed during four months on the Vercors massif (Western French Alps) at the top of a limestone column which collapsed one month and a half later. During this monitoring period, 193 seismic events were recorded by the seven seismometers of the array. Signal analysis yielded three main types of nearby seismic events to be identified from temporal and spectral characteristics: microearthquakes (single or multiple events), individual block falls and rock falls. It turned out that 60 per cent of the 193 events were classified as microearthquakes, exhibiting distinct P and S waves, while 17 per cent of these events remained unclassified. Out of the microearthquakes, 40 events with a good signal-to-noise ratio were selected and processed. P- and S-wave traveltimes were picked on the records and the inferred hypocentral distances agree with the two zones of the scarp exhibiting fresh ruptures after the fall. Polarization analysis of the 3-C records, along with numerical simulations, allowed discriminating between the two possible rupture mechanisms (toppling and sliding). Shear rupture (sliding) was the predominant mode in the lower part of the column whereas traction rupture (toppling) affected the upper part. Finally, the comparison between the ground motions recorded on the column and on the rock mass showed a systematic amplification on the column. Signal processing and numerical modelling both suggest that this amplification resulted from the excitation of the natural frequencies of the column and is particularly high (>3) for microearthquakes occurring at the column-to-mass interface.
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