2024 – Exposure of Australian Dams to Earthquakes on Proximal Faults

Mark Quigley, Tim Werner, Yuxiang Tang, Dan Clark, Jonathan Griffin, Haibin Yang

The purpose of this study is to obtain estimates of ground surface rupture and ground motion hazards at standardised scales useful for general reference and regional comparisons amongst ANCOLD-registered dams (n=548). Geospatial and statistical methods are used to investigate the exposure of dams to ground surface rupture and shaking hazard from 409 faults in the Geoscience Australia Neotectonic Features Database (NFD). We identify 216 faults at distances less than 100 km from 428 dams and measure 4055 fault-to-dam distances. At least 31 dams are located within 1 km of NFD fault traces and at least 16 of these dams could reside within NFD primary fault zones. Estimates of NFD maximum moment magnitudes (Mw,max) from fault area and length regressions range from 5.6 ≤ Mw ≤ 7.9. Average (AD) and maximum (MD) NFD fault displacements for Mw,max events range from 0.3 m ≤ AD ≤ 4.4 m and 0.9 m ≤ MD ≤ 8.4 m. Distanceprobability regressions for distributed ground surface rupture suggest approximately 40 dams have a ≥ 10% probability of ground surface rupture occurring in the area encompassing the dam in a Mw,max event. Median pseudo-spectral accelerations (PSA) for 3.0-second period (PSA[3.0s])are estimated for Mw,max at dam sites using SGC09 (Somerville, et al. 2009) and A12 (Allen, 2012) ground motion models assuming engineering rock site conditions with an average shear wave velocity between the surface and 30 m depth (VS30 ) of 760 m/s. Five-hundred and sixty-three SGC09 and 154 A12 Mw,max scenarios produce median 3.0-sec PSA ≥ 0.1g at dam sites. Comparison with 1:5,000 annual exceedance probability (AEP) PSA[3.0s] estimates from the 2023 Australian National Seismic Hazard Assessment (NSHA23) indicate there are 2310 (SGC09) to 1087 (A12) instances where NFD Mw,max PSA[3.0s]exceed NSHA23 values, encompassing 310 (SGC09) to 196 (A12) dams. Proximity to NFD faults imparts a first-order control on relative hazard. A large increase in the number of identified NFD faults over the last decade suggests further research will continue to increase NFD fault populations. NFD fault slip rates and Mw,max recurrence intervals are important parameters in seismic hazard analysis for many dams but exhibit large epistemic and aleatoric uncertainties. Improving the characterisation of NFD faults through acquisition of direct fault-specific seismic hazard information (e.g., Late Quaternary single-event displacements, slip rates, inter-event times, frequency-Mw distributions, segmentation scenarios) will assist with hazard profiling for many ANCOLD dams.