EGL Ref: 8983 8 October 2021 Page 11 File: TSF3 Stability Analysis Appendix Summary.docx.doc 6.3. Safety Evaluation Earthquake - 10,000 year return period earthquake event and aftershock The probabilistic 10,000 year return period uniform hazard spectrum is shown in Figure B6 and the associated deaggradation plots are shown in Figure B7, Figure B8, and Figure B9. The probabilistic 10,000 year return period earthquake event is one of two cases considered for the Safety Evaluation Earthquake criteria. The mean magnitude of the deaagregated rupture sources for PGA (Peak Ground Acceleration) is provided by GNS (Ref. 1), however, not for SA(0.5s) and SA(1.0s). These have been visually estimated as summarised in Table B12 by EGL for use in estimating coseismic displacement. The aftershock spectrum is assessed using the same ground motion prediction equations as for the main rupture. The mean magnitude for the main rupture is selected as Mw7.3, based on a representative period of 0.5s, for the process of calculating the aftershock spectrum. Indicative natural periods and degraded natural periods (1.3T) for the embankment are in Table B10 which informed the selection of 0.5s. Normal faulting and a distance to rupture of 21km was applied in the GMPE and the epsilon values (standard deviations above the median estimate in a log normal distribution) were adjusted to match the 10,000 year spectra and are reported in Table B11. The standard deviations are between 1.15 and 2.01 which represent between 87 to 98%ile motions from a M7.3 at 21km. The rupture parameters apart from epsilon are the same as applied for the Kerepehi Fault System rupture, however, this does not imply that the rupture would be on this fault system as the 10,000 year uniform hazard spectra is made up of many sources, including potential rupture on unknown faults and the subduction zone. The aftershock (uniform hazard) spectrum following a 10,000 year return period rupture was estimated using the same parameters as the main earthquake, however, using one magnitude lower i.e. Mw 6.3. TABLE B10: ESTIMATES OF EMBANKMENT FUNDMENTAL PERIOD Section RL155 Emb. Height Crest to Toe RL155 Emb. height above rock beneath crest Time average shear wave velocity, VsH (m/s) Fundamental period on downstream slope of embankment T = 2.6H/VsH Fundamental period of embankment beneath crest T = 4.0H/VsH Height to Toe Description 1 46 m Collection Pond S7 pond base to embankment crest. 46m Vs46 = 429 m/s T= 0.28s 1.3T = 0.364 T= 0.43s 1.3T = 0.56s 2 45 m Toe of Perimeter Bund to embankment crest. 47m Vs47 = 431 m/s T= 0.28s 1.3T = 0.364 T= 0.44s 1.3T = 0.57s
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