Stability Analysis of Fragile Rock Pillars and Insights on Fault Activity in the Negev, Israel

Fragile geologic features (FGFs) are used as negative indicators of strong ground motion. By evaluating the stability of FGFs and determining their age, it is possible to constrain the local maximum seismic ground acceleration that has occurred during their lifetime. This methodology was originally developed to analyze precariously balanced rocks (a subset of FGFs) and has been used to assess long-term fault activity and improve seismic hazard analyses. In the Negev desert of Israel, several in situ, slender rock pillars exhibit natural frequencies within the range of seismic waves (1–10 Hz) and therefore constitute an important FGF subset. However, the motion of such pillars may be complex with an initial stage of swaying followed by basal detachment, rocking and toppling, or failure within the pillar. To demonstrate that pillar stability can be analyzed using the existing FGF methodology, we show that the tensional stresses developed at the base of a pillar swaying at its fundamental modes of motion are comparable or larger than the typical tensile strength of Negev pillars. Finally, we demonstrate how a newly documented data set of Negev FGFs can be used to provide new insights on fault activity along the Negev-Sinai Shear Zone and the Arava Fault. Assuming a plausible range of motion amplification, the stability analysis of long-standing FGFs yields significant constraints on fault seismicity parameters (M_max < 7 for a section of the Arava Fault). Extending the regional data set would provide important insights for regional seismic hazard along the Dead Sea Transform.