Ambient Noise Multimode Surface Wave Imaging of the Giant Yigong Landslide Deposit, Tibet

Multimode surface wave dispersion can be used to constrain near-surface shear wave velocity (v_S) structures. In this study, we applied multimode Rayleigh wave inversion to the Zhamunong Gully deposit of the giant Yigong (Tibet) landslide using high-frequency ambient noise data acquired by a dense linear nodal array. We used the frequency–Bessel (F–J) transform to extract the Rayleigh wave dispersion curves for the fundamental and first higher modes. We then performed joint inversion of the multimode dispersion data to obtain a two-dimensional subsurface v_S model of the deposit. The inversion indicates the presence of an extensive shallow low-velocity layer, which may correspond to loose landslide–debris-flow deposits related to the 2000 event and subsequent reworking. At the northwestern end of the velocity profile, a local velocity reversal was observed at depths of ~30–50 m, possibly indicating a locally compacted or coarse-grained interlayer within the landslide deposit. Comparing the v_S profile with that obtained using fundamental-mode inversion alone indicates that the inclusion of the first higher mode resolved the shallow velocity-reversal more clearly. At depths greater than ~100 m, the cross-section contained an undulating velocity-gradient boundary, across which v_S generally increased. This boundary was tentatively interpreted as a bedrock-related velocity boundary, the relief of which may have influenced the preserved thickness and lateral distribution of the overlying landslide–debris-flow material. This study demonstrates the applicability of ambient noise F–J multimode surface wave imaging for obtaining useful v_S constraints on the internal architecture of complex giant landslide deposits.