Dynamic fragility of slender and discontinuous rock pillars – an example from the Negev Desert, Israel

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Fragile geologic features have been previously identified as potentially useful for validating un-exceeded ground motions estimated from PSHA models. The two potential seismic sources in southern Israel (Negev Desert) are the Dead Sea Transform (DST), an active tectonic border, and the Sinai-Negev Shear Zone (SNSZ). For the DST, an M 7 with a return period of 500 years was proposed, and for the SNS, an upper bound of M 6.2 was suggested. Here we present a study of the dynamic fragility of a freestanding rock pillar in the Negev Desert to access the peak ground motion in the region. The Ramon pillar is 42 m high, with a 0.12 slenderness ratio, comprised of discontinuous, hard carbonate rock. The fragility age (OSL dating) of the pillar is 11,000 years. Free vibrations of the Ramon pillar were measured using a broadband seismometer placed on its top, with a simultaneous measurement on the parent cliff. The pillar was aerially scanned using UAV-born Lidar and photogrammetry to produce an accurate 3-D model. Rock mass stiffness was measured in-situ for the entire height of the pillar. Based on the scan and measurements, a Finite Element (FE) model of the pillar was developed. The model was validated by comparing FE modal analysis and free vibration measurements. Following validation, a dynamic analysis of selected M 7 and M 6.2 earthquakes was performed. It was found that an M 6.2 on the SNSZ (R_Rup < 10 km) would most probably lead to the collapse of the pillar, and therefore the occurrence of such an earthquake during the past 11,000 years is not probable.