Report - PEER - University of California, Berkeley

As a basis for comparison, the earthquake specific attenuation model developedby Somerville and Abrahamson (Somerville, personal comm.) will be compared with themodel developed in this study. The Somerville and Abrahamson model will be denotedas S&A. This model divides sites into rock/shallow soil (rock) and deep stiff soils (soil).Deep soft clay sites are excluded. Figure 13 shows a comparison of the results at adistance of 20 km. Note that the spectra for soil sites in S&A generally match the spectrafor Site D (deep stiff soils). However, the spectra for rock sites in S&A generally matchthe spectra for Site C (shallow and intermediate depth soils and weathered/soft rock).This result reflects the fact that for the joint database of rock and shallow soil sites, 83%of the sites are shallow soil or weathered rock sites, and only 17% of these sites actuallybelong to the Site B classification (competent rock sites). Note that the spectrum for SiteB falls significantly below that for Site C (approximately 30% lower on average).A significant difference in response spectra was observed between the proposedsite categories (see Figures 9 and 10). Again, Site B (California rock) data plotsignificantly below that for Site C (weathered rock/shallow stiff soil), which illustratesthat a further subdivision from the 'rock' vs. 'soil' classification is warranted. Moresignificant, however, is the reduction of uncertainty that results from the proposedclassification system. Table 4 compares the standard deviations from the S&Arelationships with those from the relationships proposed in this report. The decrease inthe standard deviation for Site B compared with S&A rock sites is between 30% and40%. A similar reduction is observed for soil sites (S&A Soil vs. Site D). Standarddeviations for Site C, however, remain high and are only marginally lower than standard18