Seite - 22 - in Book of Full Papers - Symposium Hydro Engineering

earthquake occurring about 25 km away. If the structure is a concrete dam or supports safety-critical electromechanical equipment sensitive to short period (< 0.2 to 0.3 seconds) shaking, then kappa largely controls the spectral accelerations in this range and the ground motion almost doubles or triples at certain periods. It is therefore imperative that dam owners and their engineers know the values of Vs30 and kappa for their hard-rock dam sites in order to reduce the uncertainty in the ground motion estimates. Figure 1 Response Spectra and Shear Wave Velocity Variation with Kappa At BC Hydro, relative site amplifications based on site-specific properties are calculated and applied to probabilistic seismic hazard based on a reference rock with a VS30 of 760 m/s and a kappa of 0.040 seconds. Initial results show that empirical amplifications from field data are generally consistent with analytical amplifications using a hard-rock kappa of 0.015 seconds. The results also show that computed ground motions generally decrease with higher VS30 at all frequencies but tend to be significantly underestimated at high frequencies (> 5 Hz) if kappa and its uncertainty are not appropriately considered in the computation of the amplifications. This finding has significant loading implications for high frequency response dams and other safety-critical structures. Also shown in Figure 1 is an approximate relationship between Kappa and Vs30 from available global data (Al Atik et al, 2014). Due to the poor correlation, it is preferable to obtain site-specific kappa values rather than rely on its dependency on Vs. To measure kappa, one needs to deploy an energy source powerful enough to excite the top 2-5 km of the earth’s crust (which is impractical even if possible) or install strong motion accelerographs (SMAs) and patiently wait for an earthquake to occur. Kappa can then be estimated from the slope of the high frequency (5-20 Hz) portion of the Fourier amplitude spectrum (where it is linear in log-linear space) of an earthquake recording if the event hypocenter is within 50 km or so. Data from the SMAs, if remotely accessible, can also be used to timely inform decision-making on earthquake emergency response planning, further underscoring the importance and urgency of installing SMAs on ‘significant’ consequence dam sites. 22