This study reviews seismograms from 10 rock-fall events recorded between 1992 and 2001 by the permanent seismological network Sismalp in the French Alps. A new seismic magnitude scale was defined, which allowed us to compare and classify ground-motion vibrations generated by these Alpine rock-falls. Each rock-fall has also been characterized by its ground-motion duration t 30 at an epicentral distance of 30 km. No relation was found between rock-fall parameters (fall height, runout distance, volume, potential energy) and rock-fall seismic magnitudes derived from seismogram amplitudes. On the other hand, the signal duration t 30 shows a rough correlation with the potential energy and the runout distance, highlighting the control of the propagation phase on the signal length.The signal analysis suggests the existence of at least two distinct seismic sources: one corresponding to the initial rupture associated with an elastic rebound during the detachment and the other one generated by the rock impact on the slope. Although the fall phenomenon includes other complex processes (fragmentation of the block, interaction with topography, plastic deformation during and after impact) 2D finite-element simulations of these two seismic sources are able to retrieve the main seismogram characteristics.2

No abstract

[1] This paper investigates the variation of the first resonance frequency of the Chamousset limestone column (21,000 m 3 , Vercors, French Alps) before its collapse in November 2007. The site was instrumented with seismometers and extensometers during a 4-month period with some gaps in the measurements. Experimental results and numerical modeling showed that the resonance frequency of a prone-to-fall column can be derived from the spectra of continuous seismic noise records. At the Chamousset site, the evolution of the resonance frequency appeared to be strongly controlled by the temperature. When temperatures were positive, slight resonance frequency variations correlated well with thermal fluctuations. Irreversible damage can occur during freezethaw cycles and to a lesser extent during strong wind. It coincided with a significant drop in resonance frequency, which was interpreted as the result of rock bridge breakage. This hypothesis is supported by fresh rupture observations after the collapse, seismic event records, and numerical modeling. This study suggests that seismic noise recording could be used for assessing the potential failure of unstable columns in rigid rocks.

No abstract

French Alps) delineated by an open rear fracture was continuously instrumented with two three-component seismic sensors from mid-May 2009 to mid-October 2011. Spectral analysis of seismic noise allowed several resonance frequencies to be determined, ranging from 6 to 21 Hz. The frequency domain decomposition (FDD) technique was applied to the ambient vibrations recorded on the top of the rock column. Three vibration modes were identified at 6, 7.5 and 9 Hz, describing the upper part of corresponding modal shapes. Finite element numerical modelling of the column dynamic response confirmed that the first two modes are bending modes perpendicular and parallel to the fracture, respectively, while the third one corresponds to torsion. Seismic noise monitoring also pointed out that resonance frequencies fluctuate with time, under thermomechanical control. For seasonal cycles, changes in frequency are due to the variations of the bulk elastic properties with temperature. At daily scale, increase in fundamental frequency with temperature has been interpreted as resulting from the rock expansion inducing a closure of the rear fracture rock bridges, hence stiffening the contact between the column and the rock mass. Conversely, the rock contraction induces a fracture opening and a decrease in resonance frequency. In winter, when the temperature drops below 0 • C, a dramatic increase in fundamental frequency is observed from 6 Hz to more than 25 Hz, resulting from ice formation in the fracture. During spring, the resonance frequency gradually diminishes with ice melting to reach the value measured before winter.

International audienceThis paper presents an experimental analysis performed on a simplified brake apparatus. In past years a common approach for squeal prediction was the complex eigenvalues analysis. The squeal phenomenon is treated like a dynamic instability: when two modes of the brake system couple at the same frequency, one of them becomes unstable, leading to increasing vibration. The presented experimental analysis is focused on correlating squeal characteristics with the dynamic behavior of the system. The experimental modal identification of the setup is performed and different squeal conditions and frequencies are reproduced and analyzed. Squeal events are correlated with the modal behavior of the system as a function of the main parameters. A clear distinction between squeal events involving the dynamics of the pad and squeal events involving the dynamics of the caliper is performed. The effect of the adding of damping is also investigated on the squeal phenomenon. Two opposite roles of the modal damping are described: a large modal damping can either prevent the rise of squeal instabilities or enlarge the squeal propensity of the brake apparatus. The robustness of the obtained squeal events permits a further analysis on the triggering mechanism of the squeal instability during braking

S U M M A R YA 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.

Abstract. The influence of meteorological conditions on rockfall occurrence has been often highlighted, but knowledge of it is still not sufficient due to the lack of exhaustive and precise rockfall databases. In this study, rockfalls have been detected in a limestone cliff by annual terrestrial laser scanning, and dated by photographic survey over a period of 2.5 years. A near-continuous survey (one photo every 10 min) with a wide-angle lens has made it possible to date 214 rockfalls larger than 0.1 m 3 , and a monthly survey with a telephoto lens has dated 854 rockfalls larger than 0.01 m 3 . Analysis of the two databases shows that the rockfall frequency can be multiplied by a factor as high as 7 during freeze-thaw episodes and 26 when the mean rainfall intensity (since the beginning of the rainfall episode) is higher than 5 mm h −1 . Based on these results, a three-level scale has been proposed for predicting the temporal variations of rockfall frequency. The more precise database and freeze-thaw episode definition make it possible to distinguish different phases in freeze-thaw episodes: negative temperature cooling periods, negative temperature warming periods and thawing periods. It appears that rockfalls occur more frequently during warming and thawing periods than during cooling periods. It can be inferred that rockfalls are caused by thermal ice dilatation rather than by dilatation due to the phase transition. But they may occur only when the ice melts, because the cohesion of the ice-rock interface can be sufficient to hold the rock compartment which has been cut.