Description

Seismic hazard analyses form the basis for the forecast of earthquake-induced damage to regions prone to seismic risk. Precariously balanced rocks (PBRs) are balanced boulders that serve as in situ negative indicators for earthquake-generated strong ground motions and can physically validate seismic hazard analyses over multiple earthquake cycles. Understanding what controls where PBRs form and how long they remain balanced is critical to their utility in seismic hazard analyses. The geologic and geomorphic settings of PBRs are explored using PBR surveys, stability analysis from digital photographs, joint density analysis, and landscape morphometry. An efficient field methodology for documenting PBRs was designed and applied to 261 precarious rocks in Arizona. An interactive computer program that estimates 2- dimensional (2D) PBR stability from digital photographs was developed and tested against 3-dimensional (3D) photogrammetrically generated PBR models. 2D stability estimates are sufficiently accurate compared to their 3D equivalents, attaining <8.8% error in estimated PBR centers of mass heights. Joint analysis reveals a mean PBR joint density of 0.39 m-1 with few PBRs formed in joint densities <0.22 m-1 and >0.55 m-1. Landscape morphometry shows that PBRs are situated in upper reaches of drainage basins near divides and hillslope crests. Surveyed PBRs are preserved on local hillslope gradients between 10° and 45°, and contributing areas between 1 m and 30 m. The close comparison between the 2D and 3D PBR stability estimates indicates that the program developed here may be used to quickly estimate PBR stabilities from digital photographs taken in the field within reasonable accuracy. The joint density results indicate that structural control on the bedrock from which PBRs are produced is critical vi to their formation and preservation; high joint densities create small boulders that completely decompose prior to exhumation while low joint densities create relatively large boulders that are stable. Morphometry results may help predict expected locations of PBRs in landscapes. More importantly, they caution that construction of PBR exhumation rates from surface exposure ages needs to account for their geomorphic location in a drainage basin given that spatial variation in PBR exhumation rates are controlled by the landscape’s geomorphic state.