Central Fault-Zone Structure in the Northern Longmen Shan Revealed by a Dense Linear Array

ABSTRACT The Longmen Shan (LMS) fault zone, located in the eastern margin of the Tibetan Plateau, serves as the principal fault controlling the regional seismicity. Because of the harsh natural environment, previous studies in this region have primarily focused on large-scale crustal and mantle structures, with limited attention to shallow, near-surface features. However, fine structures of the northern LMS central fault zone (Guangyuan area) remain unclear. To investigate the near-surface characteristics, we used ambient noise data recorded by 54 short-period three-component seismometers deployed across the northern LMS fault zone, between 16 March and 6 April 2023, based on surface-wave attenuation tomography and horizontal-to-vertical spectral ratio (HVSR) methods. Surface-wave attenuation measurements reveal a low Q-value in the LMS central fault zone, characterized by widespread attenuation and significant heterogeneity. Differences between Rayleigh- and Love-wave attenuation allow us to depict an imbricate thrust fault plane, on the hanging wall of the fault. The HVSR results show conspicuous variation in resonant frequency and amplitude data at stations above the fault zone, with a correspondingly high seismic vulnerability index (K-value). By integrating the attenuation and HVSR findings, we infer that the LMS central fault zone exhibits severe near-surface fractures with a widespread distribution. In addition, the nearby Pai-lung River water may be infiltrating deeper strata through these fractures, potentially changing site stability. Understanding these fault-zone characteristics is crucial for regional development, construction, as well as seismic hazard preparation and mitigation.