A few recent studies showed that variability of site response
in the Ground Motion Prediction Equations (GMPEs) could be reduced by replacing VS30 with the site fundamental
period or the combination of site fundamental period
and VS30. For this, the key justification is that the fundamental
period considers depth of the soil column and stiffness ,
whereas VS30 accounts only the near-surface stiffness.
I propose that this topic is discussed in-depth and practical modeling solutions are developed which are easy to use and can be quicly applied to a large area or region.
Vs-30 represents significant contribution for settlement, liquefaction and ground motion amplification, within the soil layers of 30m (100 feet, as a thumb rule) depth. This is applicable for normal and routine buildings and structures, However, for large size rafts, of the order of 100m or more, it may be wiser to look at the response governed by Vs-100 (for 100m depth) to include local site effects. Thus, Vs-30 parameter is to be used cautiously, with adequate judgement.
I agree with you that for large size rafts one should consider Vs100. My question is more focused on estimation of local Soil amplification, which is practically being done through Vs30, which is one of the component in recent Ground Motion Prediction Equations (GMPEs)- some people refer as seismic attenuation equation. Since, use of site fundamental period or the combination of site fundamental period and VS30 is scientifically more robust, can we develop such GMPEs for different parts of the world and easy to use software applications, which are applicable to a large area and quick to implement ?
Dear Sushil,
Deep geology with the depth of several hundred meters, or a few km, can also cause amplification of surface ground motion, both at short and longer vibration periods:
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If a certain region has a uniform deep geological surrounding, in that case the GMPEs that were developed based on strong motion from that region only and that are based only on VS30, could perhaps be successfully modified by replacing VS30 with the site fundamental period, but for most GMPEs one should be very careful with any such analysis.
Hope this would be helpful.
Borko