Design Response Spectra for Very Hard Rock Based in Swiss Site-Specific Seismic Hazard Model

Abstract eng:
Very hard rock sites (VS30> 2500 m/s) can be found in Switzerland, as suggested at some sites from the site-specific seismic hazard assessment of the PEGASO Refinement Project (PRP). Design spectra for engineering practice for such sites are currently not developed. The main impediment of such development is perhaps the lack of observed strong motion data for such as conditions and for relevant earthquake magnitudes (>5). Consequently, the use of observed data for this task is not possible. A good alternative is to rely on synthetic seismograms. In this paper we developed a generic design response spectrum for very hard rock (VS30=2500-3500m/s) based on synthetic seismograms simulated by the stochastic model of Boore (2003) and constrained by the controlling magnitude and distance events derived from the deaggregation of the computed probabilistic seismic hazard assessment (PSHA) of the PRP. The synthetic earthquake scenarios cover magnitude MW=4.5-7.5, source-site distance 5-100 km and kappa (κ) factor 0.002-0.02 s. The high frequency covered by the proposed normalized design spectra is more or less similar to the Eurocode 8 for hard rock type 2, but amplitude of the spectral plateau is about 8% lower than the Eurocode 8 and also covers a wider high frequency range. The comparison with available data from the NGA-East database for VS30~2000 m/s shows that the proposed spectrum fills the lack of observed data for magnitude larger than 5 and VS30 larger than 2000 m/s and consistently cover the observed high frequency region.

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