三河—平谷地区地脉动H/V谱比法探测:场地响应、浅层沉积结构及其反映的断层活动

1679年三河—平谷8级地震，造成了包括北京在内的华北广大地区严重的人员和经济损失，但是该地区和地震灾害密切相关的沉积结构和地震场地响应特征研究，相对比较缺乏.本文利用1~4 km台间距的高密度单台地脉动观测、几个小孔径台阵观测和钻孔测井数据，研究了三河—平谷地区的浅层沉积的场地响应和沉积层三维起伏特征，以及构造运动对沉积厚度的影响.结果表明，研究区沉积层的场地共振频率和沉积厚度有明显的分区特征：东北部场地共振频率主要在1.0~7.0 Hz之间，具有相对较薄的沉积厚度，沉积界面起伏也相对平稳；西南部场地共振频率大部分区域小于1.0 Hz，可低至0.3 Hz，相应的沉积厚度起伏显著，其中在大厂凹陷厚度可达300~600 m.沉积起伏和隐伏断裂带的分布有较好的对应关系，反映了夏垫断裂、南苑—通县断裂第四纪以来较为活跃的正断活动.北东走向的夏垫等隐伏断裂在东北部山前的正断活动相对不发育，以及东北部和西南部沉积厚度的显著差异，都可能受到北西走向的二十里长山断裂带活动的影响.按照Vs30场地分类，研究区内沉积较厚的凹陷地区主要为E类软土，其他区域为D类中硬土.研究结果与区域地质调查、钻孔数据和浅层地震勘探剖面结果基本相符，说明地脉动探测方法能够高效、低成本的获取区域沉积厚度和场地作用，为地震小区划和抗震设防提供有意义的参考.

Microtremer H/V spectral ratio investigation in the Sanhe-Pinggu area: site responses,shallow sedimentary structure,and fault activity revealed

In 1679, the Sanhe-Pinggu M8 earthquake caused serious human and economic losses in a broad area of North China, including Beijing. Despite many studies on this historical event, less was concerned with the soil structure and its seismic site responses closely related to seismic hazard. In this work, the site responses and three-dimensional relief characteristics of shallow sediments in the Sanhe-Pinggu area, as well as the influence of tectonic movement on the sedimentary thickness, were studied by dense microtremor observations with intervals of 1~4 kilometers, several small aperture arrays, and borehole data. The results show that the soil resonant frequencies and the shallow sedimentary thickness in this area are obviously zoning. In the northeast, the resonant frequencies are mainly in 1.0~7.0 Hz, with relatively thin sedimentary thickness and smooth fluctuation of the sedimentary interface. While for most areas in the southwest, the resonant frequencies are less than 1.0 Hz with the lowest down to 0.3 Hz. Their sedimentary thickness fluctuations are significant, up to 300~600 m in the Dachang depression. The sedimentary fluctuations are well consistent with the locations of hidden faults, and the very thick sediments may imply significant Quaternary normal faulting on the Xiadian and Nanyuan-Tongxian faults. The activity of the NW Ershilichangshan fault probably controls the obvious sedimentary differences between the northeast and southwest subzones, and also obstructs recent normal faulting in the mountain front for NE-trending faults such as the Xiadian fault. According to Vs30 site classification, the soils in areas with thick sediments are more likely to be type-E and others are type-D. Our results are largely consistent with those from geological surveys, boreholes and seismic exploration profiles, suggesting that microtremor investigations can efficiently determine the sedimentary thickness and the site response at low cost, and provide valuable information for seismic microzoning and seismic hazard prevention.