Precarious Rocks & Design Ground Motions: From Research To Industry Application

Abstract eng:
We summarise work undertaken to constrain the fragility of three precariously-balanced rocks (fragility = threshold peak ground accelerations, PGAs, for toppling failure) in the vicinity of Diablo Canyon Power Plant (DCPP), coastal California. The precariously-balanced rocks (PBRs) are found on two paleoseastacks (seastacks once actively eroded by the sea, but now uplifted well above sea level) at the Double Rock site, east of the power plant. We develop field-based fragility estimates from the geometries of the PBRs, as well as finite-element-code-based fragility estimates for one of the PBRs. Fragilities in the range of approximately 0.2-0.4g are obtained. These equate to the relatively short return periods of approximately 200-700 years on the DCPP hazard curve, and are consistent with 50th percentile PGAs for Hosgri Fault scenario earthquakes (large magnitude earthquakes at 5 km distance). Our future work will verify the fragilities determined by field and finite-element-code-based methods, and determine the fragility age of at least one of the PBRs to ascertain whether the PBRs are old enough to provide meaningful constraints on the DCPP hazard estimates. Since the PBR fragilities equate to short return periods on the DCPP hazard curve, we consider that PBRs with ages of 1000 years or more would be useful for comparison to DCPP hazard estimates.

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