SCEC Award Number 12088 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Fragile Geologic Features in New Zealand and Southern California: New Age Constraints and Comparison to Seismic Hazard Models
Name Organization
Mark Stirling GNS Science (New Zealand) Dylan Rood University of Glasgow (United Kingdom)
Other Participants Hanks, Tom
SCEC Priorities 6e, 4a, 4b SCEC Groups Geology, EFP, GMP
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
Our 2012 SCEC funding went towards attending the SCEC annual meeting, and undertaking a field reconnaissance of “well studied” sites of precariously-balanced rocks (PBRs) in Southern California. The objective was to undertake a review of the PBRs, data and interpretations, identify the strengths and issues of the work, and where effort should be concentrated in the future. The main review findings are as follows:

PBRs appear to be rare, well-studied features. In all sites visited, the PBRs appear to be no more than a few in number (usually just one), yet the surrounding rockmass is of suitable form (morphology, joint spacing etc) for formation of such features. We instead saw abundant semi-precarious outcrops and semi PBRs in these areas, and could therefore see a “representative fragility” in these landforms. Our recommendation is that that the rare, isolated PBRs should not be used for constraining PSH models and physics based seismic hazard analyses (e.g. Cybershake) unless the PBR areas contain tens to hundreds of PBRs. Since no such site exists (to our knowledge), we consider that these rare PBRs may be less meaningful for constraining the upper bounds of ground motions than the abundant semi fragile landforms that exist across the outcrops. Constraining seismic hazard models to fit the rare fragile PBRs may result in serious underestimation of the hazard. Future work should instead be focussed on assessment of fragility and age of abundant semi-fragile PBRs in suitable terrains.
Intellectual Merit This project is meritorious in identifying a need for SCEC to urgently review it's seismic hazard validation methods. We have given a substantial cautioning on the use of PBRs for constraining seismic hazard models and related products (eg Cybershake). We consider that the PBR data may be overinterpreted if so few observations of rare PBR features are used to constrain important SCEC end-user models and products in the future. We recommend that more work go into characterising the more numerous semi-PBR features observed in the PBR sites before any significant steps are take to use data from the most fragile features.
Broader Impacts Our work will ultimately lead to more realistic and reliable validation methods for seismic hazard models and products. Logically, improvements to the actual models/products will follow.
Exemplary Figure Figure 5. The “Big Rock” precariously-balanced rock (PBR), Lovejoy Buttes, California. Numerous semi-PBRs are also present on the surrounding hillslopes, and these provide a more representative fragility for the butte than do the rare PBRs.