Report - PEER - University of California, Berkeley

Bozorgnia and Campbell (2004) find that the ratio of vertical-to-horizontal spectralordinates is strongly dependent on period and on source-to-site distance, as indicatedin Figure 1, where the simplified ratio they propose is compared with the V/H ratio ofthe Type 1 spectrum in EC8. The comparison suggests that in the relevant periodrange the EC8 vertical spectrum is likely to be conservative except for sites close tothe source of earthquakes.Figure 1. Vertical-to-horizontal spectral ratios for NEHRP Class D site fromEC8 Type 1 spectrum (dashed line) and from Bozorgnia and Campbell (2004).The curves in Figure 1 support the definition of the vertical spectrum independentlyfrom the horizontal spectrum because their shapes are clearly different (hence thevariation of the V/H ratio with period) and their amplitudes display very differentvariation with distance from the earthquake source.2.2.3 Site Amplification EffectsDuring recent years there has been a tendency to move from two or three site classesto four or five, as embodied now in the NEHRP guidelines. The most significantadvance was the definition in UBC 1997 (based on NEHRP) of spectral amplificationfactors at short and intermediate periods simultaneously in terms of site class and thespectral accelerations at rock sites, thus modeling the non-linear response of soils(Dobry et al., 2000). The NEHRP site classes, like those adopted in EC8 (e.g., Rey etal., 2002), are based on the average shear-wave velocity over the uppermost 30 m, adepth related to the cost of drilling boreholes rather than of any geophysicalsignificance: it has been pointed out that for long-period ground motions, thewavelengths are such that the shaking is not strongly influenced by the properties ofthe uppermost 30 m (e.g., Gregor and Bolt, 1997). This has led to some groundmotionprediction equations including the depth to basement rock as an explanatoryvariable, which “boosts long period motions” (Spudich, 1999).473