Topography‐Incorporated Adjoint‐State Surface Wave Traveltime Tomography for Azimuthally Anisotropic Media

Abstract Ambient noise surface wave traveltime tomography has been increasingly used to investigate shallow crustal structures. In relatively small‐scale studies, factors such as topography and the uneven distribution of ambient noise sources may have significant influence on the tomographic results. In addition, anisotropy can also affect surface wave propagation, but is usually neglected. In this study, we adopt an anisotropic eikonal equation to model Rayleigh wave phase traveltime in weak anisotropic media with topographic variation. An inversion scheme is developed to invert for both shear wave velocity (Vs) and anisotropy using adjoint‐state method. This tomography method is applied in the Huidong region, located near the southwestern segment of the Lianhuashan Fault Zone, China. Rayleigh wave phase delays caused by the uneven distribution of ambient noise sources are observed. This effect is corrected through traveltime correction which is determined by inverting for the azimuthal amplitude density of ambient noise. The tomographic results reveal low‐Vs anomalies and ENE‐oriented fast directions that are consistent with the strike of the Danshui Fault. In addition, a hidden fault is inferred from the NW‐oriented fast directions and low‐Vs anomalies.