Region-specific linear site amplification model for peaty organic soil sites in Hokkaido, Japan

We develop a site amplification model for a regional geologic condition that includes surficial deposits of peaty organic soils in Hokkaido, Japan. We use ground motion data from national Japanese networks along with local data from the study region. We apply a non-reference site approach to infer site effects from misfits of reference ground motion models to data from 10 subduction zone earthquakes. Application of this approach requires removal of source-specific biases and careful consideration of source-to-site path effects. These considerations are essential to avoid mapping source- or path-related model misfits into estimates of site response. We consider three subduction ground motion models as reference models. By paying special attention to the conditions for which the path models are effective, and making adjustments for between-island path misfits (Hokkaido to Honshu and vice versa), we identify regional site effects that are insensitive to the ground motion models used in their derivation. Observed site responses are characterized by strong resonances at first-mode site frequencies, and as a result the regional model is conditioned on peak frequency from horizontal-to-vertical spectral ratios.