Study of the eigenfrequencies of stalagmites to better understand paleoseismicity

The frequency-band of regional earthquake ground motions generally does not exceed 20 Hz. In this frequency band, structures (such as candlestick stalagmites) can resonate and break at a lower acceleration than expected during an earthquake, if the ground movement is strong enough. Therefore, the absence of breakage in these structures would indicate that a certain level of ground movement has not been exceeded since they existed. The existence of intact candlestick stalagmites in the caves is therefore an indicator of the upper limit of ground movements (e.g., Gribovszki et al. 2017) that a specific site may have encountered, and the natural frequency is a fundamental parameter in the study of the response of these stalagmites to seismic movement. In this context, field surveys were carried out in the Han-sur-Lesse Cave (Belgian Ardennes) to identify the stalagmites for which an analytical approach (based on cantilever beam theory for free vibration, a specific case of the Euler–Bernoulli theory) gives an eigenfrequency less than 20 Hz. In this formula, the stalagmite is considered as a perfect cylinder and therefore depends only on the mechanical properties of the stalagmite and their height and diameter. These two parameters were measured in the cave. This study identified 27 stalagmites distributed in different areas of the cave (Synanthropes and Cornet room; Vervietois Gallery) and on different substratum (stalagmitic floor, fallen rocks, ...). Thanks to a three-component seismic sensor placed on the stalagmite itself, the eigenfrequencies of these eligible stalagmites are then obtained by the direct measurement of the ambient seismic noise caused by human activities, microseisms, etc. This acquisition campaign is underway and already gives an idea of the frequency of 6 stalagmites thanks to acquisitions of 15 to 30 minutes each. To explore the reactions of these structures to external events such as quarry blasts or earthquakes, seismic measurements will be carried out over a longer period (weeks) as was the case for the Minaret studies (Martin et al. 2020). With sensors located at the base of the stalagmites, in the middle of the cave room and at the surface outside the cave, this will allow to compare the results and identify a possible amplification of movement at eigenfrequencies as was shown for the Minaret (Martin et al. 2020).