Ground-motion site effect in the Beijing metropolitan area

In the past 500 years, many moderate to strong earthquakes have occurred in or near the Beijing metropolitan area, which is underlain by thick Quaternary and Tertiary sediments. Therefore, the area is susceptible to damage associated with ground-motion site effect, particularly for the large number of high-rise buildings. In order to understand the earthquake site effect potential, particularly in the context of the high-rise buildings, we are undertaking an effort to construct a 3-D shear-velocity model of the Quaternary and Tertiary sediments for the Beijing metropolitan area. In this paper, we present an integrated method of deriving shear-wave velocity profiles at sites across the Beijing metropolitan area using shear-wave velocity and ambient-noise measurements. Our results demonstrate that this method is practical for obtaining shear-wave velocity profiles. We performed 1-D site response analyses at six sites in the Beijing metropolitan area. The results show that the site resonant periods vary between 0.1 s at shallow sites in the west and 4.0 s for deep sites in the east. Moreover, this range of resonant periods coincides with the fundamental periods of many buildings between one to 40 stories. In other words, earthquake site effect could generate a double resonance in a significant number of buildings in the Beijing area. Results also show that the average shear-wave velocity of the top 20 or 30 m, V_s20 or V_s30, does not correlate with site resonance, and is therefore not an appropriate parameter to account for site effect in earthquake engineering.