EGL Ref: 9215 23 June 2022 Page 11 This report shall only be read in its entirety. File: WAI-985-000-REP-LC-0002_Rev0.docx 3.3. Waihi operation site specific seismic hazard The design of all structures, including dams to store tailings, needs to be informed by the natural hazard posed by potential earthquakes. For that purpose, the New Zealand Dam Safety Guidelines (NZDSG) set out the approach that should be taken to the identification of seismic risk. This in turn informs the design of large dam structures so that appropriate safety features are incorporated in the design so structures perform appropriately under seismic loads. This is so even under the worst credible earthquake scenario (which would cause widespread damage in the area) there will not be a loss of contents from the TSFs. Seismic loads are also considered in the design of rock storage facilities. Estimates of seismic hazard for the site have been provided by GNS Science in 2007 and 2017 (Ref. 2). The 2017 update incorporated the latest knowledge of the Kerepehi Fault System (Ref. 3), the Hikurangi subduction zone and updated estimates of background seismicity. The Kerepehi Fault System is comprised of several faults and is 21 km from the site at its closest point. It is the closest known active fault to the site and is the Controlling Maximum Earthquake (CME) as defined by the NZDSG. The largest rupture scenario for this fault system is when all segments of the fault rupture together. This scenario represents a rupture length of about 81 km, a magnitude (Mw) of 7.3, resulting in normal (dip-slip) fault displacement of about 3.6 m. This scenario has a recurrence interval of approximately 10,000 years. Very large earthquakes, up to magnitude (Mw) 9, are associated with the Hikurangi subduction zone and contribute to the seismic hazard at the site. The subduction zone is approximately 200km distant from the site and so ground motions are attenuated and are not as significant as those from nearby smaller background earthquakes or the Kerepehi Fault. The GNS study 2017 considered new research recently published updating the understanding of the Kerepehi Fault (Ref. 3). A rigorous approach was adopted by GNS for determining estimates of seismic hazard that accounted for epistemic uncertainty (systematic uncertainty due to the method of assessment) as required by the NZDSG for High Potential Impact Classification (PIC) dams. As the Kerepehi Fault is comprised of several segments, GNS modelled different representations of the Kerepehi Fault to address uncertainty in the knowledge of how different segments could combine. There was negligible difference in results for the different representations that were considered. GNS consider the best estimate model is that representing the most up-to-date information regarding the Kerepehi Fault, and therefore, the recommended spectra are those produced using this best estimate model. The GNS 2017 study provided both probabilistic and deterministic (i.e., scenario based) estimates of horizontal peak ground accelerations (PGAs) and acceleration response spectra (weighted and unweighted). Probabilistic estimates were determined for return periods of 150, 2500 and 10,000 years. The 2017 probabilistic estimates of spectra are lower than the 2007 estimates. This is principally because there has been a reduction in the rate of seismicity associated with the local distributed earthquake source in the national seismic hazard model (Ref. 2). This source contributes the most to seismic hazard at the site.
RkJQdWJsaXNoZXIy MjE2NDg3