Candlestick stalagmites, a tool to better understand the influence of past earthquakes on natural caves

Candlestick stalagmites exhibit resonance in the frequency-band of regional earthquake ground motions. An earthquake can break such elongated structures if the ground movement is strong enough. Therefore, the existence of intact stalagmites in caves would indicates that a certain level of ground movement has not been exceeded since they exist. Field surveys were carried out in the Han-sur-Lesse Cave (Belgian Ardennes) to estimate the eigenfrequencies of stalagmites of different sizes and shapes (e.g., Minaret Stalagmite, “Verviétois” gallery) and to explore their reactions to external events such as quarry blasts or earthquakes. The eigenfrequencies of the stalagmites are obtained from direct measurements of ambient seismic noise caused by human activities, microseisms, etc. Seismic sensors were placed on the stalagmites and on the nearby cave floor as well as at various other locations inside and outside the cave. Noise and transient events (e.g., quarry blasts) were recorded during weeks of continuous measurements. 3D laser scans of the stalagmites and Finite Element Modelling allowed constraining the link between their complex shape and their eigenfrequencies.