Upper-Middle Crustal Structure and Seismogenic Environment in and Around the Baihetan Reservoir: Insights from Ambient Noise Tomography

Abstract Since the filling of the Baihetan Water Reservoir (BWR) in April 2021, a significant number of earthquakes have occurred, with varying levels of seismicity in different regions. Therefore, studying the seismogenic environment and seismic mechanism is crucial for understanding the variability of earthquakes in the BWR area. We utilized continuous waveform data recorded over three years following the filling of BWR, collected from 86 stations covering the BWR area and its surrounding areas, and measured 1545 high-quality Rayleigh-wave phase velocity dispersion curves. We then obtained the 3D S-wave velocity and azimuthal anisotropy models through direct inversion of the dispersion data. The new results provide insights into the seismogenic environment and earthquake mechanism along with their relationship with the fluids in the BWR area. The low VS anomaly in the shallow layers of the Jinsha River and its vicinity is attributed to the influence of lithology and fluid infiltration in the study area. The high VS anomaly along the Zhaotong-Ludian fault zone indicates that it may be the main channel for the intrusion of Emeishan basalt into the upper crust. The moderate and strong earthquakes, as well as the earthquakes in the source area of the Ms 6.5 Ludian earthquake, mostly occur along the boundaries between the high and low VS anomaly. This observation indicates the presence of the previously undetected faults on the northwest side of the Zhaotong-Ludian fault. Earthquakes in a Y-shaped distribution near Qiaojia predominantly occur in a low VS zone, suggesting that the seismicity in the BWR region is markedly influenced by fluid infiltration. The spatiotemporal variability of earthquakes may be influenced by regional lithology and tectonic conditions.