Dynamic Fragility of a Slender Rock Pillar in a Sedimentary Rock Mass – from rock mechanics to seismic hazard

Abstract. Fragile geological features (FGF) are the only empirical data to validate seismic hazard analysis over prehistoric timescales. Precariously balanced rocks (PBR) are the most common FGF in practice, with fragility analysis based on rigid body rocking dynamics. FGFs evolved from sedimentary rock masses cannot be treated as rigid blocks, and rock mass properties should be considered in their fragility analysis. Contrary to PBRs, sedimentary FGFs received limited attention from the geological and engineering communities. This paper presents a comprehensive dynamic fragility analysis of a 42-meter-high pillar – the Ramon Pillar (Negev Desert, Israel). The pillar is comprised of a sedimentary rock mass with various discontinuities. An accurate finite elements (FE) model of the pillar (1.25^.10⁶ elements) was developed based on high-resolution aerial LiDAR scanning and in-situ measurements of rock elastic modulus along its entire height. The model was validated by comparing computational modal analysis with in-situ measurements of natural vibrations. The first mode of 1.3 Hz was precisely predicted. The second mode was predicted with a 10 % difference: 2.7 Hz calculated compared to 3 Hz measured. An a-priory assumption of rock elastic modulus (or back-calculation) or simplified geometries yielded unsatisfactory results. Following the successful validation, a fully dynamic fragility analysis of the pillar was performed, using recorded ground motions, to study the basal tensile stresses. Located in a region with two seismic sources, the Sinai Negev Shear Zone (SNSZ) and the Dead Sea Transform (DST), the pillar's fragility analysis was used to test regional seismic hazard estimates. It was found that an M 6 earthquake on the SNSZ will probably lead to breakage of the pillar at its base due to stresses exceeding its basal strength. Given a fragility age of 11.4 ky, our analysis challenges the assumption that the SNSZ can produce an M 6 event.