Response spectrum predictions for potential near-field and far-field earthquakes affecting Hong Kong: Soil sites
This paper considers the modelling of design-level earthquake response spectra for soil sites in Hong Kong. It forms part of a series of connected studies aimed at quantifying the detailed characteristics of seismic activity and design ground shaking levels in Hong Kong and its surrounding region. This series of papers should appeal to a broad international readership interested in adopting a methodology to determine seismic demand for infrastructure in megacities located within a low or moderate seismic region. The peak velocity and displacement demands (soil RSVmax and soil RSDmax, respectively) have been obtained from soil response analyses for numerous representative soil column models. The comparison of the soil RSVmax between individual soil columns displays interesting similarities for a given earthquake scenario. A unique upper bound RSVmax value has been identified for each probability of exceedence (PE). The predicted soil RSVmax was found to be relatively insensitive to the lengthening of return period. Another significant trend is the excellent linear correlation between the peak displacement demand (soil RSDmax) and the site natural period (Ts). A simple relationship has accordingly been developed to determine the soil RSDmax from the given soil RSVmax and Ts. Maximum soil amplification factors (spectral ratios) in the order of 5-6 have been observed in the present study, for Hong Kong soil sites resonating under the action of bedrock shaking induced by distant, large magnitude earthquake events associated with a range of PE. Previous studies have grossly underestimated this factor, due to inaccurate representation of the bedrock motions generated by far-field earthquakes and due to underestimation of the soil-rock impedance contrast. Furthermore, codified estimates of site amplification may significantly underestimate the effect of resonance, due to distortion introduced by various forms of idealisation and normalisation when dealing with the soil and bedrock response spectra.
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