3D high-density ambient noise imaging of the Nankou-Sunhe buried active fault in Beijing

Abstract

The Nankou-Sunhe fault (NSF) is a vital buried active fault within the Beijing Plain. Investigating the detailed structure of the NSF and the sedimentary structures on both sides is pivotal for urban engineering projects to mitigate geological hazards. Three-dimensional (3D) seismic exploration can provide comprehensive and precise information about subsurface structures, aiding in the identification and characterisation of the spatial geometry, extensional direction, and features of sedimentary structures on both sides of the fault. In this study, we conducted a 3D high-density ambient noise seismic survey covering approximately 40 km2 around the NSF. Utilising the Multichannel Analysis of Passive Surface waves (MAPS), we obtained fundamental mode Rayleigh wave phase velocity diagrams in the 0.5–4 Hz range, subsequently inverting them to reveal a 3D shear (S)-wave velocity model extending to a depth of 1 km. The model shows lower velocities in the southwestern region, with shallow S-wave velocities of approximately 400–600 m/s and deep-seated velocities ranging from 1800 to 2000 m/s. The northeastern region is characterised by higher velocities, with shallow S-wave velocities of approximately 900–1200 m/s and deeper velocities reaching 3000–3400 m/s. The NSF is a high angle (50°-70°) normal fault striking northeast and dipping towards the southwest; the dip angle exhibits local variation. An S-wave velocity of 1 km/s was used to estimate sediment thickness on both sides of the fault. The MaChikou Sag (MCKS) on the southwest side of the NSF is approximately 400–600 m thick, while the northeastern JingXi High (JXH) is less than 200 m thick. The NSF was positively correlated with the bedrock surface (1 km/s isovelocity). Moreover, this study ascertained that by employing MAPS with a 200 m station spacing within a complex and noisy urban environment, one can extract accurate fundamental-mode Rayleigh waves, allowing for the subsequent construction of a 3D high-resolution S-wave velocity model.