First Use of Fragile Geologic Features to Set the Design Motions for a Major Existing Engineered Structure

Abstract

We document the first use of fragile geologic features (FGFs) to set formal design earthquake motions for a major existing engineered structure. The safety evaluation earthquake (SEE) spectrum for the Clyde Dam, New Zealand (the mean 10,000 yr, ka, return period response spectrum) is developed in accordance with official guidelines and utilizes constraints provided by seven precariously balanced rocks (PBRs) located 2 km from the dam site and the local active Dunstan fault. The PBRs are located in the hanging wall of the fault. Deterministic PBR fragilities are estimated from field measurements of rock geometries and are the dynamic peak ground accelerations (PGAs) required for toppling. PBR fragility ages are modeled from B10e cosmogenic isotope exposure dating techniques and are in the range of 24–66 ka. The fragility ages are consistent with the PBRs having survived at least two large Dunstan fault earthquakes. We develop a PGA-based fragility distribution from all of the PBRs, which represents the cumulative toppling probability of a theoretical random PBR as a function of PGA. The fragility distribution is then used to eliminate logic-tree branches that produce PGA hazard curves that would topple the random PBR with a greater than 95% probability (i.e., less than 5% survival probability) over a time period of 24 ka (youngest PBR fragility age). The mean 10 ka spectrum of the remaining hazard estimates is then recommended as the SEE spectrum for the dam site. This SEE spectrum has a PGA of 0.55g, which is significantly reduced from the 0.96g obtained for a preliminary version of the SEE spectrum. The reduction is due to the combined effects of the PBR constraints and a substantial update of the probabilistic seismic hazard model. The study serves as an important proof-of-concept for future applications of FGFs in engineering design.

Late Quaternary Slip Rates for the Hyde and Dunstan Faults, Southern New Zealand: Implications for Strain Migration in a Slowly Deforming Continental Plate Margin

“…(2007) and used by Stirling et al. (2012, 2021). Furthermore, because we obtained CRN ages at the Rock Creek trench site, we are able to revise the earthquake chronology for the Hyde Fault using these data, reducing uncertainty on the age of event C significantly (Table 5; Supporting Information S1).…”

“…(2012) and Stirling et al. (2021). These investigations targeted the Devonshire Fan (Figures 5a and 5d), the formation of which was loosely constrained to the interval 75–30 ka using OSL dating, and an adjacent inset terrace in Rochester gully (Figure 5d) formed after a period of stream incision prior to ∼30 ka.…”

“…Minor splays from the main north-west dipping fault have caused additional vertical displacement of lower terrace levels, and there is some evidence of minor folding of the fan surface (Figure 5a; Barrell, 2019). The Neds Creek site is located 5 km south-west of the Devonshire area, where previous paleoseismic investigations have been undertaken (Figures 5a and 5d;Van Dissen et al, 2007), the results of which were summarized in Stirling et al (2012) and Stirling et al (2021). These investigations targeted the Devonshire Fan (Figures 5a and 5d), the formation of which was loosely constrained to the interval 75-30 ka using OSL dating, and an adjacent inset terrace in Rochester gully (Figure 5d) formed after a period of stream incision prior to ∼30 ka.…”

“…The WES is inferred to have little paleorelief, implying that its present-day vertical offset is a good measure of total late Cenozoic tectonic deformation (Stirling, 1990(Stirling, , 1991. Vertical offset of the WES due to motion on the north-west dipping Hyde Fault is up to 1,100 m (Griffin et al, 2021), while vertical offset due to motion on the north-west dipping Dunstan Fault is up to 1,700 m (Stirling et al, 2021). The summed vertical offset of the WES attributable to all faults east of the Nevis-Cardrona Fault System (Figure 1b) is about 10 km (Norris, 2004).…”

The Influence of Basement Terranes on Tectonic Deformation: Joint Earthquake Travel‐Time and Ambient Noise Tomography of the Southern South Island, New Zealand

“…Most of the other range-bounding faults have few if any late Quaternary offset indicators and thus slip rates of the order of ∼0.1 mm/yr are inferred. Some show evidence for episodic rupture behavior, including the Pisa, Dunstan, Titri, and Akatore faults (Barrell et al, 2020;Beanland & Berryman, 1989;Stirling et al, 2021;Taylor-Silva et al, 2020). Fault-related and associated fold deformation has contributed to brittle deformation of the upper crust.…”