Constraints on Probabilistic Seismic-Hazard Models from Unstable Landform Features in New Zealand

Abstract

We undertake the first New Zealand-based pilot study to investigate the use of ancient precariously balanced rocks (rocks that are unstably balanced on top of a pedestal) as a criteria for testing estimates of earthquake shaking from probabilistic seismic hazard models for long-return periods. To date, research to test seismic-hazard models in New Zealand has been restricted to the short historical record of earthquakes. Our survey of five sites in the South Island of New Zealand has yielded a total of 28 precariously balanced rocks which, on the basis of established methodology, are used to provide estimates of the maximum ground motions that could have occurred at the sites since the rocks became precarious. Age estimates for the precariously balanced rocks (40,400 to 55,300 years for central Otago schist rocks, and 77,300 years for northwest Nelson granitic rocks) are made from a limited number of cosmogenic dates obtained from bedrock removed from the pedestals of the rocks. Comparisons of the maximum peak ground accelerations and ages of the precariously balanced rocks with seismic-hazard curves derived from the New Zealand national seismic-hazard model show that the rocks indicate considerably lower hazard than the seismic-hazard model at sites located within 5 km of active faults, whereas the agreement is favorable for sites located away from active faults. The variability about the median estimates of peak ground acceleration for the fault sources and/or the median accelerations for the fault sources assumed in the seismic- hazard model may therefore be overestimated for the sites near active faults.