Ground-Motion Site Effects in Beijing Metropolitan Area

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Beijing Metropolitan Area had experienced many moderate to strong earthquakes in the past 500 years, the largest being the 1679 Sanhe-Pinggu (M 8.0) and 1976 Tangshan (M 7.8) earthquakes. More than 2,000 high rise buildings have been erected in recent years in the Area, which is underlain by thick Quaternary and Tertiary sediments. During the 1985 Michoacán earthquake (M 8.0), Mexico City experienced significant damage, particularly to six- to 15-story buildings, from site effects (i.e., site resonances) due to underlying lake sediments. Thus, the Beijing Metropolitan Area faces significant seismic risk, particularly for high rise buildings, caused by potential site effects.

In this study, we determined shear wave velocity profiles at sites across the Beijing Metropolitan Area utilizing borehole data, ambient noise measurements, and broadband seismic data. Shear-wave velocities for shallow sediments were determined directly from borehole data, and the average shear-wave velocities of the entire sediment column were determined from the predominant frequencies of ambient noise. For the deeper sediments, shear-wave velocities were calculated from the shallow and average velocities. We also determined the other sediment dynamic properties, such as damping ratios and unit weights. We then conducted 1-D site response analyses.

The results show that the predominant (i.e., first mode) site response periods vary from 0.1s for the shallow sites in the west to 3.6s for deep sites in the east. This predominant period range coincides with the predominant resonance periods of buildings with one to 36 stories. In other words, this study showed that site effect could be a significant potential problem for the area. In current Chinese engineering practice, the average shear-wave velocity of top 20 meters,Vs20, has been used as a key parameter to account for site effect. However, as shown in this study, Vs20 has no direct relationship with site effect. Thus, the use of Vs20 in earthquake engineering in the Beijing Metropolitan Area may not be appropriate.

Presenting Author: Yanju Peng

Authors