Multiphase speleothem damage and passage collapse in Niedźwiedzia Cave (E: 16°50′36,65″, N: 50°14′04,26″) NE Bohemian Massif, Poland, were studied with an integrative approach to decipher the cause of damage during the last 320 ka. Since the most likely trigger of damage in the cave was an earthquake, we support our interpretation with a probable seismic intensity prediction to define a seismogenic source and to calculate the expected macroseismic intensity and peak ground acceleration (PGA) at the research site. Finally, we evaluate probable seismic effects in the cave with the elastic properties of speleothems to ascertain a threshold value of failure.
The relative range of earthquake hazards for a particular site depends significantly on the decrease in the ground motion amplitude (or intensity) with distance from all surrounding potential seismic sources, event size, and type of acting mechanism. The ground motion relation generally depends on the depth of focus, earthquake mechanism, and geological conditions along the path (Schenková et al., 1981). Therefore, to discuss probable seismic effects in Niedźwiedzia caves and their vicinities, we used the attenuation curves presented previously for the research area (Schenková et al., 1981; see the coefficient for the 13th zone). Dataset 2020TC006459_ds_to fig.5 contains a calculation of the attenuation curves, i.e., decrees of the intensity (on the MSK scale) and PGA (m/s2) with distance from the epicenter. We used these data to estimate (or to predict) the expected values of intensities ranging with distance from the seismogenic source region. This approach is a steady-state approximation because we assumed that isoseismal spread is regularly distributed around the epicenter and that the local influence of discontinuities is omitted.
The failure criteria resulting in speleothem damage were evaluated on a prismatic cantilever beam supported at one end. The parameters describing these criteria are the speleothem natural frequency and the corresponding critical horizontal ground acceleration (m/s2), which depend on elastic properties of the speleothem and the following geometric dimensions: height, H, and diameter, D, measured in situ (we acquired geometric measurements for 50 broken speleothems in Niedźwiedzia Cave). The horizontal ground acceleration is the limit value when at which speleothems breaks. The natural frequency (Hz) indicates that the failure process is more likely to occur if the natural frequency (or resonance frequency) is in accordance with the seismic wave frequency affecting the speleothem. If both frequencies match, the speleothem resonates, enhancing the oscillation effect, which may lead to damage. Dataset 2020TC006459_ds_to fig.6 contains H and D measurements of 50 broke speleothems and calculated natural frequency and critical horizontal ground acceleration for each measured speleothem. The values of both failure parameters depend on elastic properties that can be assessed in laboratory tests in which the following parameters are estimated: P- and S-wave velocities, Vp and Vs, respectively, using an acoustic measurement system, density, ρ, (from the measured mass and volume), Young's modulus, E, and tensile stress, σt. The AMS tests were carried out at the Institute of Earth Sciences, University of Silesia, Sosnowiec, Poland. The failure stress was estimated based on the uniaxial compressive stress test carried out in the external laboratory of AP GEOTECHNIKA, Katowice, Poland, using a universal testing machine. The strength tests were conducted for two 0.05x0.05 m (height/dimension, h/d = 1) speleothems samples.
