Abstract: Site effect, which is related to the amplification of seismic waves, is mostly affected by the thickness and softness of sediment layers. The study of site effect in cities is becoming more and more important to the assessment of ground motions, seismic hazard and engineering seismology. However, in highly populated urban cities, traditional seismic surveys cannot be applied extensively due to their destructive consequences and high cost. The ambient noise, including microseisms and microtremor, could be acquired anywhere and anytime, and thus can serve as an effective source for engineering seismology. In order to get the site effect and distribution of sedimentary layers of Guangzhou area, one hundred QS-05A seismographs with frequency bandwidth of 5s-250Hz were deployed in early 2018 for 35 days. The inter-station spacing of these seismometers is approximately 2~5km. Using continuous ambient noise signals, we obtained the resonance frequency and amplification value beneath each station by horizontal-to-vertical spectral ratio(HVSR)method. Then sedimentary layer thicknesses as well as K-values, which are related to the site vulnerability to ground shaking, were calculated. Our results suggest that the resonance frequencies in Guangzhou area are between 1~6.5Hz. The resonance frequencies increase gradually from 1Hz on the north-east side to 6.5Hz on the south-west side of the study area. The sediment thicknesses change from several meters to about 40m, with the maximum thickness at around the estuary of the Pearl River. This distribution is consistent with the topography. The amplifications are mainly between 2~6. The largest amplification is around the Pearl River and the west part of Baiyun District. In general, the K-values are small(<20), less than the dangerous value, suggesting that Guangzhou area is relatively safe in ground shaking. However, there are three small areas beneath Huadu District, Sanshui District and Nanhai District. They all have K-values greater than 20, suggesting those areas are more vulnerable to earthquake destruction, and higher construction standard is needed. The reliability of our results is further supported by its consistency with topography and borehole data in Guangzhou area. Our results provide important information for shallow underground structure in Guangzhou area, and can be referred as guidelines in urban architecture planning and disaster prevention and mitigation.

Key words: Guangzhou area, site effect, ambient noise, sedimentary thickness

摘要： 为获取广州地区的场地效应和沉积层分布情况, 2018年初于广州地区布设了100台流动台站进行背景噪声观测。 文中利用HVSR方法得到了表征场地效应的共振频率和放大倍数2个参数的分布结果, 并利用共振频率-沉积层厚度转换公式得到了广州地区的沉积层分布。 文中综合考虑共振频率和放大系数, 同时得到了广州地区表征场地易破坏程度的K值分布。 研究结果显示: 广州地区的共振频率为1~6.5Hz; 沉积层厚度整体较小, 多为10~25m, 珠江下游最厚, 达40m; 研究区场地整体易破坏程度低, 极少区域超过安全值(K>20), 表明在地震灾害中较为安全。 文中结果与地表地形及公开的钻孔数据相符, 可为城市整体工程扩建和防灾减灾工作提供基本参考。

关键词: 广州地区, 场地效应, 背景噪声, 沉积层厚度