SITE RESPONSES AND SEDIMENT STRUCTURES ALONG THE NW AND NE PROFILES IN BEIJING AREA REVEALED BY MICROTREMOR H/V SPECTRAL RATIO STUDIES

摘要

To estimate the detailed strong ground motion caused by an earthquake for seismic hazard reduction requires high-resolution near-surface geological information. The techniques using microtremor measurements are cost-effective tools that rapidly acquire high-resolution sedimentary layer information over a large area about site response, sedimentary thickness, and velocity structures compared to conventional borehole drilling or active seismic exploration. In summer of 2007, we had conducted microtremor array measurements in the greater Beijing area along two roughly orthogonal profiles in the NW and NE directions centered at the Forbidden City.The profile lengths are 64 km for the NW profile and 76 km for the NE profile, respectively, with spacing interval of 200 meters for both profiles. Microtremors were recorded with the 3-component, short-period seismometers,and processed by the horizontal-to-vertical (H/V) spectral ratio technique (HVSR). The resonance frequencies and their amplification factors were picked from the H/V curves and the sediment thickness were derived from the regressive relationship to the resonance frequency. The results show that site response in Beijing area varies significantly: the resonance frequencies span 0.2-10 Hz and the amplification factors are between 2 to 15. The thickness of sedimentary layer varies from a few meters to over eight hundred meters. The inferred sediment thickness profiles reveal some structural features that may associated with possible blind active faults in Beijing are area, such as the northwesterly oriented Sunhe-Nankou Fault that crosses both profiles, and the northeasterly oriented Shunyi-Liangxiang Fault crossing the NW profile. This project is supported by the Institute of Earthquake Science (Project No.0207690229) and Ministry of Science and Technology of China (Project No.2006DFA21650).