Abstract: The Xiaoyangkou geothermal field is located in the southern part of the Haian sag in the Subei Basin, bounded by
the near east-west Benchahe fault and the northeast-oriented Jintan-Rugao fault. The geothermal well RRY1 of Xiaoyangkou is currently the only intermediate-temperature geothermal well in Jiangsu Province, with a peak temperature of 92 ℃ . However, the formation, distribution, and relationship of this geothermal resource with the regional fault system remain unclear. This study integrates regional seismic reflection profiles and drilling data to conduct 3D structural and velocity modeling of the Benchahe fault and Jintan-Rugao fault to explore the structural controls on the formation of the Xiaoyangkou geothermal resources. We collected ambient noise data using a dense array and applied cross-correlation calculations, dispersion curve extraction, and direct inversion tomography to construct a 3D shear wave velocity structure down to 4 km. The 3D structural model shows that the Benchahe and Jintan-Rugao faults trend NEE with a northern dip of 60° to 70°. The 3D velocity model reveals significant lateral heterogeneity in the shear wave velocity structure at depths of 2.0 to 3.5 km, with high-velocity and low-velocity boundaries correlating strongly with the Benchahe and Jintan-Rugao faults, reflecting the influence of normal faulting within the sag on the velocity structure. Additionally, a distinct low-velocity anomaly is observed between the Benchahe fault and Jintan-Rugao fault, manifesting vertically as a decrease in shear wave velocity gradient, which coincides with the position of the Silurian quartz sandstone geothermal reservoir at depths of 2.0 to 2.8 km. Based on the regional 3D fault structure and velocity models, as well as previous studies, this research proposes two geothermal modes:‘conductive’above and‘convective’below the Neogene base are primarily controlled by the regional fault system and deep fluid convection, offering a framework for future exploration of faultrelated geothermal resources in coastal zones. 

Key words: Xiaoyangkou geothermal field, structure of faults, Benchahe fault, three-dimensional structure modeling, ambient
noise shear wave tomography