Precariously balanced rocks in northern Utah: are Wasatch Fault earthquakes worse than expected?

Where they persist in seismically active regions, precariously balanced rock [PBR] formations can be used to provide constraint on the strongest co-seismic ground motions which occurred during the largest prehistoric earthquakes. The Wasatch Fault system in Northern Utah consists of at least ten active segments each capable of producing M>7.0 earthquakes with recurrence intervals on the order of 500-1000 years based on paleoseismic trenching, active-source seismic, Lidar, and a variety of other geological data. This fault system is among the most hazardous in North America based on probabilistic hazard assessment and deterministic ground motion simulations, the latter of which show acceleration of 1g in the most densely-populated valley regions of Utah. To provide quantitative data for direct comparison to such simulations we identify, map, and model PBRs in the vicinity of the Wasatch Fault zone. We ultimately measure the force required to topple each formation by comparing these observations to previous work and through ongoing drone-based photogrammetry and multiphysics modeling, from which we infer the maximum possible co-seismic ground motion at each site. By identifying the geological units and geomorphological conditions that lead to PBR formation, we also map regions where we would expect but fail to find any PBRs. Based on extensive field mapping in which we've located 27 PBRs, we observe a systematic pattern: we find no PBRs at all within 15km of the Wasatch fault, a few semi-precarious rocks (<0.7g toppling force) from 15km to 30km distance, with true PBRs (<0.4g) only at distances greater than 30km. These combined results suggest that the ground motion simulations are underestimating the regional extent or amplitude of co-seismic shaking due to Wasatch fault earthquakes.