Crustal Scattering and Intrinsic Attenuation Across the Eastern Margin of the Tibetan Plateau Revealed by High-Frequency Coda Waves

Seismic attenuation provides key constraints on the thermo-mechanical state and small-scale heterogeneity of the crust, but high-frequency observations are strongly affected by the coupling between intrinsic attenuation (Qi) and scattering attenuation (Qsc). This coupling hampers conventional attenuation inversions, particularly in tectonically complex regions such as the eastern margin of the Tibetan Plateau. High-frequency seismic coda wave envelopes provide critical insights into the influence of attenuation structures on energy evolution and serve as an essential data source for scattering studies.  In this study, we combine unsupervised machine learning and physics-based envelope modeling to investigate crustal intrinsic and scattering attenuation across the Longmenshan Fault Zone and adjacent regions. We first apply a Conditional Variational Autoencoder (CVAE) to tens of thousands of high-frequency (2–4 Hz) P- and S-wave envelopes, including their coda, recorded by a regional seismic array. By conditioning on source–receiver distance, the CVAE suppresses geometric effects and extracts latent variables that characterize lateral and vertical variations in envelope shape. Two latent variables are sufficient to describe the dominant envelope features: the first is primarily associated with variations in P-to-S energy ratios and correlates with intrinsic attenuation, while the second reflects changes in envelope width and peak timing, consistent with scattering strength. The spatial distribution of the intrinsic-attenuation-related latent variable reveals a clear contrast between the Tibetan Plateau and the Sichuan Basin, whereas scattering-related variations are mainly controlled by local small-scale heterogeneity and show no systematic dependence on large-scale tectonic units. Guided by these results, we further perform three-dimensional high-frequency envelope modeling using radiative transport theory on ~61,000 three-component seismograms. We constructed two-layer models of intrinsic attenuation and small-scale scattering structures for the crust of Sichuan Basin and Tibetan Plateau regions, respectively. The sedimentary layer of the Sichuan Basin displays strong scattering and intrinsic attenuation, suggesting a porous, potentially fluid-rich structure, which aligns with the presence of abundant oil and gas resources. The relatively weak scattering and intrinsic attenuation in the Sichuan Basin's crust indicate its nature as an ancient, stable geological block. The lower crust of the Tibetan Plateau shows stronger intrinsic attenuation than the upper crust but significantly weaker scattering, suggesting the presence of a high-temperature, viscous flow structure in the region. The upper crust of the Tibetan Plateau exhibits significantly stronger scattering and intrinsic attenuation compared to that of the Sichuan Basin, reflecting the extensively faulted and fractured structure due to ongoing tectonic collisions.