ARE THE STANDARD VS 30 -KAPPA HOST-TO-TARGET ADJUSTMENTS THE BEST WAY TO GET CONSISTENT HARD- ROCK GROUND MOTION PREDICTION?

Site-specific seismic hazard studies involving detailed account of the site response require the prior estimate of the hazard at the local reference bedrock level. As the actual characteristics of such local bedrock often correspond to "hard-rock" differing significantly from "standard rock" conditions, standard rock PSHA estimates should be adjusted accordingly. The present practice is based on Vs (S-wave velocity) and "κ 0 " (site specific, high-frequency attenuation parameter) values, and generally predicts larger high-frequency motion on hard rock compared to standard rock. However, it also proves to be associated (Biro and Renault, 2012; Al Atik et al., 2014) with a large uncertainty level, related to (i) the measurement of host and target parameters, and (ii) the forward and inverse passage from the response spectrum domain to the Fourier domain to apply the Vs and κ 0 adjustments. Moreover, recent studies (Ktenidou and Abrahamson, 2016) demonstrate that " first generation " V S30 / κ 0 correlation relations are not robust, so that the significant amplification of high frequency content for hard-rock with respect to standard-rock seems questionable. This paper discusses the key aspects of a few, recently proposed, alternative approaches. The calibration of GMPEs directly in the Fourier domain rather than in the response spectrum domain is one example (Bora et al., 2015; Bora et al., 2017). Another example is the derivation of GMPEs, which be valid also for hard-rock conditions (e.g. Laurendeau et al., 2017). In this case the host site response is first removed using theoretical site response analyses (and site velocity profile), or generalized inversions techniques. Finally, when a sufficient amount of records are available at a given site, generic GMPE predictions can be scaled to the site-specific ground motion using empirical site residual (δS2S S). Such alternative approaches present the advantage of a significant simplification with respect to the present practice (with thus a reduced number of uncertainty sources); their generalization calls however for high-quality recordings (including high-quality site metadata) for both host regions and target sites, especially for small to moderate magnitude events. Our answer to the question in the title is thus "No, alternative approaches exist and are promising; though, their routine implementation requires additional work regarding systematic site characterization (host) and high-quality site instrumentation (target)".