Unveiling sedimentary architecture and concealed faults in the Handan region through HVSR and high-mode surface wave analysis based on a dense array

Detailed imaging of shallow architectural structures in sedimentary basins is critical for seismic hazard assessment, especially in regions with complex tectonic environments. The Handan area, located at the junction of the Taihang Mountain Piedmont Fault and the Yongnian-Cixian Fault, features significant variations in Quaternary sediment thickness and frequent seismicity. However, thick sedimentary cover often obscures basement faults, necessitating high-resolution geophysical methods for structural characterization. In this study, we utilize continuous waveform data from a dense array of 500 short-period seismometers (approx. 1 km spacing) deployed in the Handan region from August to September 2024. We present a joint investigative approach combining the Horizontal-to-Vertical Spectral Ratio (HVSR) method and higher-mode surface wave analysis. First, the HVSR method was employed to extract fundamental resonance frequencies (f0), which reveal a strong spatial correlation with local topographic and tectonic features. Second, we implemented the subarray-based frequency-Bessel (F-J) transform method to extract higher-mode Rayleigh wave dispersion curves from ambient noise cross-correlation functions. The inclusion of higher modes significantly enhances the imaging resolution and provides stronger constraints on the velocity structure compared to fundamental-mode methods. By inverting these dispersion curves, we obtained a high-resolution 3D S-wave velocity (Vs) model extending to a depth of 1.5 km. The results reveal pronounced velocity contrasts across major fault zones, particularly the Taihang Piedmont Fault. Furthermore, by integrating the f0 data with the Vs model, we derived a precise regional sedimentary thickness map. The estimated thickness ranges from tens of meters in the western mountainous areas to over 800 meters in the eastern basins, aligning well with existing borehole data and geological frameworks. These findings provide quantifiable constraints for earthquake hazard assessment, urban planning, and the identification of concealed faults in the North China Plain.