Abstract:

The Weihe Basin, located in the central segment of the North-South Seismic Belt, has experienced multiple historical strong earthquakes. Its thick sedimentary cover can produce pronounced site effects that may amplify seismic damage. At the same time, these sediments preserve key records of the basin’s structure and evolutionary history. However, a detailed shallow three-dimensional(3D)sedimentary model has been lacking for the western basin margin. In 2021, the Geophysical Exploration Center of the China Earthquake Administration deployed a dense seismic array of 288 three-component short-period EPS-2 seismometers across the western Weihe Basin, covering Fufeng, Meixian, and surrounding areas. The array had an average station spacing of ~1.5km and recorded continuously for 60 days.
Using these data, we first applied a short-term average/long-term average(STA/LTA) algorithm to automatically identify and remove nonstationary noise. We then used Konno-Omachi smoothing to obtain stable horizontal-to-vertical spectral ratio(HVSR)curves. The results show widespread multiple peaks across the study area. For each curve, we extracted the amplitude ratios and frequencies of the visible peaks. For the two most consistently observed peaks, f₀ and f₁, both amplitude ratios and resonant frequencies were interpolated by Kriging to produce two-dimensional(2D)distribution maps. In addition, adopting an estimated average shear-wave velocity, we converted peak frequencies to sediment thickness using the standard quarter-wavelength relation.
By integrating existing borehole and geological constraints, we interpret f₀ as the impedance contrast at the interface between the Neogene Youhu Formation and the Quaternary Sanmen Formation, whereas f₁ corresponds to the interface between the Quaternary Sanmen Formation and the overlying Qinchuan Group. Based on these two interfaces, we constructed a layered shallow 3D sedimentary model for the region. Depths derived from HVSR are consistent with borehole data from wells Wei-4 and Wei-5. Comparisons with two published borehole profiles further show that cross-sections extracted from our 3D model reproduce the depth-variation trends observed in the borehole records.
Spatial variations in burial depth and amplitude ratio for f₀ and f₁ are closely related to regional tectonic and sedimentary evolution. The depth to the base of the Sanmen Formation, inferred from f₀, is shallower in the west and deeper in the east, with the transition bounded by the Qishan-Mazhao Fault. The corresponding amplitude ratio(reflecting impedance contrast)is higher in the west and lower in the east. This pattern is consistent with uplift and erosion of the western block since the Pliocene, which produced thinner Sanmen sediments and a stronger impedance contrast, whereas the eastern depression underwent more continuous deposition and weaker environmental variability, yielding thicker Sanmen deposits and a smaller impedance contrast. Peak f₁ reflects the interface between the Sanmen Formation and the Qinchuan Group. From the Sanmen stage to deposition of the Qinchuan Group, paleo-Sanmen Lake in the central Weihe Basin contracted markedly and loess deposition migrated from the basin margins toward the center. The later onset of loess accumulation in the basin center resulted in thinner deposits and a larger vertical impedance contrast, whereas peripheral areas experienced more continuous fluvial-aeolian sedimentation during the Quaternary, leading to a smaller impedance contrast. Pre-existing topography and faulting also influenced thickness variations; for example, along the northern segment of the Weihe Fault, uplifted terrain corresponds to thinner deposits, whereas the southern downthrown block contains thicker accumulations.
In summary, we developed a shallow 3D sedimentary model for the western margin of the Weihe Basin and produced maps of fundamental resonance frequency and site amplification factor. The model agrees with available borehole constraints and regional geological understanding. These results provide a data basis for site-effect assessment and a geophysical reference model for investigating tectonic and environmental evolution. The HVSR approach used here may also serve as a useful complementary method for future studies of shallow sedimentary structure.

Key words: dense array, Weihe Basin, HVSR method, sedimentary structure

摘要：

渭河盆地位于南北地震带中段, 历史上发生多次强震, 区域内较厚的新生代沉积层引起的场地效应对震害具有放大效用。另外, 渭河盆地沉积层的结构也记录了地区沉积演化与构造变形的信息。然而, 渭河盆地西缘目前尚缺少浅层三维沉积结构研究结果。文中利用布设于渭河盆地西缘288台短周期地震仪组成的密集台阵, 基于HVSR法(Horizontal-to-Vertical spectral ratio, 也称谱比法)处理台阵记录的背景噪声数据, 获得波峰频率f₀、 f₁对应的振幅、 频率、 波阻抗界面深度分布图。结合对渭4、 渭5井及2条排钻剖面资料的分析, 发现波峰频率f₀反映了游河组与三门组地层间界面波阻抗和深度信息, 显示区域南、 北两侧山地地区波阻抗较大, 其东部波阻抗较小, 表明第四系沉积层深度变化较大, 整体较厚, 为200~800m, 深度由西到东逐渐增加; 波峰频率f₁反映了三门组和中晚更新世黄土古土壤层间界面波阻抗和深度信息, 显示渭河盆地边部山地波阻抗相对较小, 盆地中部波阻抗较大; 盆地两侧沉积层较厚, 中部沉积层相对较薄。波峰频率f₀、 f₁所反映的波阻抗与深度变化, 指示了渭河盆地西缘新生代沉积相由三门组的湖泊相转变为秦川群的河流相与风成黄土相。文中结果可服务于地震区划、 地震动模拟等防震减灾工作, 也可为研究渭河盆地新生代地层沉积演化过程提供可靠的地球物理模型。

关键词: 密集台阵, 渭河盆地, HVSR法, 沉积层