Page - 21 - in Book of Full Papers - Symposium Hydro Engineering

COMMISSION INTERNATIONALE DES GRANDS BARRAGES ------- VINGT-SIXIÈME CONGRÈS DES GRANDS BARRAGES Autriche, juillet 2018 ------- SEISMIC HAZARD ANALYSIS OF HARD-ROCK DAM SITES Kofi O. ADDO Principal Engineer, British Columbia Hydro & Power Authority, Burnaby Member, ICOLD Committee on Seismic Aspects of Dam Design CANADA EXTENDED ABSTRACT Current ground motion models for western North America (WNA) characterize site effects based on scaling with respect to VS30, the time-averaged shear wave velocity (VS) in the uppermost 30 metres. The upper limit on this VS30- scaling in these models is in the range of 1100 to 1500 m/s and reflects the paucity of earthquake recordings from rock sites with VS30 > 800 m/s. A review of earthquake recordings on hard-rock shows that shear wave velocity alone does not fully explain observed trends and that another characteristic of the rock, kappa (k), a measure of shallow crustal damping, is required to better capture or reproduce the observations in the data. In general, kappa decreases as VS30 increases but there is no explicit kappa scaling in available WNA ground motion models and hence kappa is not directly included in seismic hazard analysis Most hard-rock sites in British Columbia are glaciated and have VS30 values higher than 800 m/s, sometimes as much as three times higher. Hence, until such time that the development of these models incorporates data from high VS30 sites, seismic hazard analysis for hard-rock sites has to be performed in two stages. First, a seismic hazard analysis is conducted for a reference rock with a known VS30 (usually < 800 m/s) and a known inherent kappa. Secondly, relative amplifications based on the site-specific VS30 and site-specific kappa are calculated and applied to the computed reference hazard to derive the site-specific seismic hazard. Figure 1 shows different response spectra (with kappa varying from .05 to .01 seconds) for a structure on a hard-rock site that needs to safely resist a Mw6.5 21