SCEC Award Number 15038 View PDF
Proposal Category Travel Only Proposal (SCEC Annual Meeting)
Proposal Title Fragile Geologic Features: Constraints on Near-Fault Ground Motions
Investigator(s)
Name Organization
Mark Stirling GNS Science (New Zealand) Rob Buxton GNS Science (New Zealand) Glenn Biasi University of Nevada, Reno
Other Participants
SCEC Priorities 6e, 4b, 4d SCEC Groups Geology, EFP, GMP
Report Due Date 10/16/2015 Date Report Submitted 10/14/2015
Project Abstract
 SCEC-related activities in 2015 have been threefold: (1) We have achieved the first-ever appli-cation of precariously-balanced rocks (PBRs) to constrain design ground motions at a critical facility. The PBRs have been used to evaluate the preliminary Safety Evaluation Spectrum (SEE; 10,000 year return period) for the Clyde Dam, the largest concrete dam in New Zealand. The age and fragility of seven (out of a population of > 45) PBRs located within 2km of the dam, and at a similar distance to the southwest end of the active Dunstan Fault, have been quantified. The resulting fragility ages are consistent with the PBRs having survived multiple large earthquakes, implying that ground motions at the southwest end of the fault (and dam site) have been considerably less than those estimated for the preliminary SEE which incorporated fault hanging wall effects. Considerable effort went into understanding PBR geomorphic de-velopment and 10Be ages. Our revised and recommended SEE has been reduced by removing hanging wall effects from the SEE estimation. The SEE is still conservative with respect to the PBR data, but ap-propriately so given uncertainties in the PBR fragility ages; (2) We have incorporated the PBR data into the new SCEC PBR database during a research visit by Stirling to Biasi at UNR, and; (3) Stirling has pub-lished a paper with Fernando Della Pasqua titled “Erosion Rates for Pinnacles Formed in Soft Sediments and implications for Constraining Seismic-Hazard Models” in the Bulletin of the Seismological Society of America (SCEC Contribution number 1960).
Intellectual Merit Our project represents the first time PBRs have successfully moved from the domain of scientific research into that of real-world industry application. The work therefore substantially address-es SCEC’s Disciplinary Activity C/Problem 6 “Validation of ground motion prediction through analysis and dating of precariously balanced rocks and other fragile geomorphic features”. The resulting PBR age and fragility data also constitute a significant input to the new SCEC PBR database.
Broader Impacts Successful transition of PBRs from scientific research into industry application will set a precedent for further PBR applications, as is now the case at Diablo Canyon power plant. PBRs (and other fragile geologic features) are still the only criteria that have been identified for validation of seismic hazard models at long return periods (>100 years).
Exemplary Figure Comparison of peak ground acceleration (PGA) hazard curves to PBRs for the Clyde Dam, New Zealand. PBRs are plotted as crosses (four sets of crosses for each PBR, reflecting the range of fragilities and ages, in which the inverse of age is plotted on the y-axis, and toppling acceleration is plotted on the x-axis), and the hazard curves are colour coded according to percentile level. Hanging wall effects for the major local fault (Dunstan Fault) have been removed from the hazard curves in light of the PBR data. The resulting (reduced) hazard curves still show conservatively high PGA estimates given the age and fragility of the PBRs, but this was judged to be an appropriate level of conservatism given the importance of the application. The PBR data are also a significant recent contribution to the developing SCEC PBR database.