关键词: 密集台阵, 渭河盆地, HVSR法, 沉积层

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