Erosion Rates for Pinnacles Formed in Soft Sediments and Implications for Constraining Seismic‐Hazard Models

Abstract We provide estimates of the rates of natural modification of badlands pinnacles formed in soft sediments at two sites in New Zealand and assess the utility of pinnacles for constraining seismic‐hazard models. One of the sites is in a humid temperate environment, and the other is in a semiarid environment. Rates of pinnacle modification of 30–270  mm/yr (equivalent to 3–27  m/century) are estimated by comparison of images acquired one‐to‐two decades apart. The primary mode of modification is the progressive narrowing of the pinnacle column by precipitation‐induced erosion, along with consequent loss of height as the columns become thin and unstable. Additional damage may have been produced by earthquake shaking for specific pinnacles, but in general this is likely to be trivial, given that (1) estimates of the levels of peak ground acceleration (PGA) required to shake down the pinnacles at the two sites are at least a factor of 5–13 greater than the PGAs predicted or measured from regional earthquakes during the time periods of observation (in other words they would only be significantly damaged by major local earthquakes) and (2) a fragile pinnacle has actually formed in the time periods of observation. The pinnacle modification rates are rapid enough that they would not have existed one‐to‐two centuries ago or would have been of substantially different geometries. As such, pinnacles are unlikely to be useful for constraining seismic‐hazard models for return periods greater than a few decades, at least in the context of the New Zealand environment.