Given the somewhat arbitrary nature of the site-effect distinction, it must be carefully defined in any given context. With respect to PSHA, we define the site effect as the response, relative to an attenuation relationship, averaged over all damaging earthquakes in the region. A diligent effort has been made to identify any attributes that predispose a site to greater or lower levels of shaking. A map based on the average shear-wave velocity in the upper 30 m, however, is found to delineate significantly different amplification factors. A correlation of amplification with basin depth is also found to be significant, implying up to a factor of two difference between the shallowest and deepest parts of the Los Angeles basin.
Questions remain, however, as to whether basin depth is a proxy for some other site attribute.
Our best hope for reducing such uncertainties is via waveform modeling based on first principals of physics. Finally, questions remain with respect to the overall reliability of attenuation relationships at large magnitudes and short distances.