Dynamic fragility of a slender rock pillar in a carbonate rock mass

Fragile geological features (FGFs) provide critical empirical data for the validation of probabilistic seismic hazard models over prehistoric timescales. Among FGFs, precariously balanced rocks (PBRs) are the most widely studied, with fragility analyses based on simple rigid-body rocking dynamics. FGFs formed from sedimentary rock masses differ from PBRs and require the consideration of rock mass properties in their fragility assessments. Sedimentary FGFs have received limited attention from the geological and engineering communities. This study presents a detailed dynamic fragility analysis of a 42 m high Ramon pillar (Negev Desert, Israel). Composed of a sedimentary rock mass with various discontinuities, the pillar was modeled using a high-resolution finite-element (FE) model, with 1.25×106 elements. The model was constructed using high-resolution aerial lidar scanning and in situ measurements of rock elastic modulus along the pillar's height. Validation was achieved by comparing computational modal analysis with in situ measurements of natural vibrations, accurately predicting the first mode (1.3 Hz) and estimating the second mode (2.7 Hz) with a 10 % deviation from the observed values (3 Hz). The assumption of uniform rock elastic moduli (back-calculated) or simplified geometries yielded unsatisfactory results, highlighting the importance of precise characterization. Situated near two significant seismic sources, the Sinai-Negev Shear Zone (SNSZ), with a potential M 6 earthquake, and the Dead Sea Transform (DST), with a potential M 7 earthquake, both with sub-millennial return periods, the pillar's fragility was used to test regional seismic hazard estimates. Two methodologies were employed: a simplified spectral analysis based on empiric ground motion models and a fully dynamic FE analysis incorporating recorded ground motions from the Pacific Earthquake Engineering Research Center (PEER) strong-motion database. Results show that an M 7 on the DST (45 km away) will not compromise the pillar integrity, whereas an M 6 earthquake on the SNSZ (6 to 20 km away) would likely lead to breakage at its base due to tensile stresses exceeding its basal strength. Given the pillar fragility age of 11.4 kyr, these findings challenge the assumption that the SNSZ can produce an M 6 event.