Short-period dense linear array ambient noise tomography of shallow crustal structure across the Haichenghe fault zone, China

The Haicheng region, Liaoning, China, likely hosts a conjugate fault system comprising the NW-trending Haichenghe fault and NE-trending secondary faults. On February 4, 1975, at 19:36 CST, an earthquake of M S 7.3 and intensity (MMI) IX hit the city of Haicheng, Liaoning, China. Although deep seismic profiling was previously conducted along the Haichenghe fault, the limited horizontal resolution in the shallow part prevented the recognition of kilometer-scale anomalies. The velocity structure characteristics of the Haichenghe fault and its NE-trending conjugate faults remain unclear. Using the extended range phase shift method, the high-resolution S-wave velocity structures are obtained by deploying a long, dense linear array of 55 short-period seismometers across the fault and NE-trending conjugate faults. The array length was 32 km and inter-station spacing was approximately 600 m, facilitating the collection of approximately 22 days of continuous waveform data. Employing the Extended Range Phase Shift (ERPS) method enabled the extraction of broadband 0.2–5 s Rayleigh wave phase velocity dispersion curves. The broadband dispersion data were used for inversion of the high-resolution S-wave velocity structure to a depth of 8 km from the surface. The velocity structure characteristics and seismicity of the Haichenghe fault and NE-trending conjugate faults were analyzed and compared with nearby fault gas measurements. Results show (1) shallow S-wave velocities show a low-high-low horizontal distribution, corresponding to basin-uplift-basin topography; (2) significant velocity contrasts occur across the Haichenghe fault: its SW segment (0–17 km) exhibits high velocities consistent with Paleoproterozoic crystalline basement (Pt 1 ), while the NE segment (17–32 km) shows low velocities related to Yanshanian intrusions (γ 5 ) and Quaternary sediments. NE-trending conjugate faults display sharp velocity gradients marking fracture locations, with all faults being near-vertical to ∼8 km depth; (3) seismicity at 1–6 km depth mainly clusters in high-velocity zones; at 6–8 km depth, it concentrates beneath the Haichenghe fault in low-velocity areas and along NE-trending faults; (4) the seismic activity characteristics and fault zone width of the Haicheng he fault reflected by velocity imaging results are basically consistent with those obtained by the fault gas measurement method.