Crustal Deformation of the Haiyuan Fault Zone Inferred from Dense Seismic Array Observations

The Haiyuan Fault, a major strike-slip structure in the northeastern Tibetan Plateau margin, is bounded by the first-order Tibetan Plateau and South China blocks, plus the second-order Ordos and Alxa blocks. Seismic anisotropy serves as a robust proxy for probing deep crustal deformation, geodynamic processes, and subsurface seismic structures. We conducted receiver function analyses on teleseismic data from two dense profiles and five broadband stations across the study area; crustal thickness (42–56 km) and Vp/Vs ratios (1.60–1.88) were quantified by the H-κ domain search algorithm, while common conversion point (CCP) imaging delineated the Moho discontinuity across the Haiyuan Arc Fault Zone. Crustal thickening reflects shortening driven by Tibetan-Eurasian collision, with the Haiyuan tectonic evolution linked to high-temperature/pressure regimes induced by Indo-Asian convergence. CCP images reveal a distinct Moho offset and ambiguous continuity beneath the fault zone, confirming it as a Moho-penetrating transcrustal structure associated with intense crustal extrusion from the plateau interior. We characterized multi-scale crustal anisotropy via shear-wave splitting (SWS) analysis of local earthquake data. SWS parameters exhibit clear zoning controlled by the Haiyuan Fault: fast polarization orientations are NNE–NE north of the fault and WNW–EW south of it. Within ~10 km of the fault, fast orientations align with the fault strike (WNW), indicating the fault’s stress influence range spans dozens of kilometers. Enhanced normalized time-delays near the fault signal stronger anisotropy along this strike-slip belt. Upper crustal anisotropy likely arises from crack-induced fabric, whereas middle-lower crustal anisotropy reflects deformation-controlled fabric. Spatial anisotropy patterns imply the combined effects of stress, faulting, and local tectonics. Notably, SWS results suggest the Haiyuan Fault constitutes the actual crustal boundary of the northeastern Tibetan Plateau, ~200 km north of the previously reported plateau block boundary.