Utilizing joint active-passive surface source wave exploration to study the shallow tectonic characteristics of the Anqiu-Juxian fault in the Tanlu fault zone

Liu Yunze; Fan Xiaoping; Fu Wei; Huang Qilong; Ren Shiyu; Zhang Tiantian; Hu Tiantian

doi:10.11939/jass.20240111

Abstract：The Tanlu fault zone is the most active fault zone in eastern China,exhibiting multi-phase,complex,and segmented activity characteristics.Based on its geometric structure and activity,the Tanlu fault zone can be divided into four sections from south to north:the Jiashan-Guangji,Weifang-Jiashan,Xialiaohe-Laizhouwan,andHegang-Tielingsections.Amongthese,the Weifang-Jiashan section displays the strongest tectonic activity,within which the F5 fault(Anqiu-Juxian fault)is identified as the most recent Holocene active fault.Studying its shallow tectonic characteristics is critical for understanding fault evolution and assessing seismic hazards. Domestic and international scholars have reached a consensus on the developmental scale,strike,and activity characteristics of the F5 fault.However,rapid and effective methods for detecting fault zone width and internal medium structure remain lacking.While core drilling offers high precision,its high cost and lengthy duration limit widespread application.Reflec-tion wave exploration can locate fault points by analyzing co-phase axis deformation and dif-fracted wave development,but it fails to resolve detailed internal medium structures.Advances in observation technology and data processing now make high-resolution surface wave explora-tion a promising solution. Rayleigh wave exploration,an emerging geophysical method,enables rapid and cost-effective shear wave velocity measurements,making it a research hotspot.Rayleigh wave methods are categorized into active-source and passive-source techniques based on seismic excitation modes.The active-source method yields high-frequency signals but has limited depth resolution,whereas the passive-source method provides low-frequency signals for deeper exploration.Combining both approaches enhances effective exploration depth while maintain-ing shallow-structure accuracy. This study employs a joint detection method for active and passive surface waves,acquir-ing data using identical array configurations for both sources.Dispersion spectra are superim-posed to improve dispersion curve quality and expand the observable frequency range.A genet-ic algorithm inverts the 2D S-wave velocity structure. Results reveal significant lateral heterogeneity in the crustal medium,with a segmented S-wave velocity structure.Fault points are identified at distances 110 m(F5-1-1),160m(F5-1),and 640 m(F5-2).The F5-1 and F5-2 faults form opposing boundary faults(dip angles 70° and 65°,respectively),creating a graben structure with a 330 m-wide fracture zone.The western boundary is not a single fault but a composite zone comprising F5-1 and its secondary fault F5-1-1.The fracture zone exhibits overall low S-wave velocities but contains localized high-velocity anomalies,likely reflecting F5 fault activity. Comparisons with shallow seismic exploration and borehole profiles confirm that the joint method accurately locates main fault surfaces,while also providing clearer delineation of frac-ture zone width and internal material structure.This approach is particularly effective for shal-low faults or high-interference environments like urban areas.