Defining a Standard Rock Site: Propositions Based on the KiK-net Database

Site-over-reference techniques are commonly used to characterize site effects. In these techniques, the choice of reference site significantly affects the amplification estimates. Hence, a clear definition of a standard reference rock site is necessary. The present article tests various standard rock site definitions using the KiK-net Japanese database. Two parameters, the mean shear-wave velocity over the first 30 m ( V S 30) and the fundamental resonance frequency ( f ), are used to define a standard rock site. The variability of the shear-wave velocity profiles and of the estimated rock site response for five different categories, defined according to V S 30 and f values, are derived and compared. It is shown that the criterion commonly used in building codes establishing a lower limit of V S 30 (of about 800 m/sec for rock or soft rock categories) groups a wide variety of shear-wave velocity profiles and a corresponding wide variety in site amplification. A more restrictive, twin criterion, combining (lower) limits on the V S 30 and f values, significantly reduces the variability in terms of amplification and the amplified frequency band of the rock response. This criterion is thus proposed as a definition of a standard rock site, and its practical applicability is discussed.

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