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Dynamic Fragility of a Slender Rock Pillar in a Carbonate Rock Mass - the Ram0n Pillar Case

Jbaara, A. (1) Tsesarsky, M (1,2)

(1) Department of Structural Engineering, Ben Gurion University of the Negev, Beer-Sheva 84105

(2) Department of Earth and Environmental Sciences, Ben Gurion University of the Negev, Beer Sheva 84105

Fragile geological features (FGFs) provide critical empirical data for validating 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 consideration of rock mass properties in their fragility assessments. Sedimentary FGFs received limited attention from the geological and engineering communities. This study presents a detailed dynamic fragility analysis of a 42-meter-high pillar Ramon Pillar. Composed of a sedimentary rock mass with various discontinuities, the pillar was modeled using a high-resolution finite elements (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 pillars’ 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 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 PEER strong motion database. Results show that an M7 on the DST (45 km away) will not comprise the pillar integrity, whereas an M6 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 ky, these findings challenge the assumption that the SNSZ can produce an M 6 event

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