Constraining shear-wave velocity structure using Rayleigh-wave ellipticity: Geothermal site and MaTaiAn landslide dam

The East Longitudinal Valley is in a high seismicity region of Taiwan, characterized by complex subsurface structures and significant deep geothermal potential. Conventional deep geological borehole drilling provides critical constraints on subsurface structures and geothermal resource distribution but is costly and time-consuming. Recently, the degree of polarization–ellipticity (DOP-E) method for Rayleigh waves has been successfully applied to estimate subsurface depth variations beneath ice sheets and to delineate shear-zone depths in landslide environments. In this study, continuous ambient seismic noise records from three seismic stations co-located with geological boreholes (Station code: WL6G, GW2G, and GW3G) in the Wulu geothermal prospect, eastern Taiwan, were analyzed using the DOP-E method. Rayleigh-wave ellipticity was estimated and applied to invert shear-wave velocity (Vs) profiles. The resulting Vs structures were integrated with three-dimensional Magnetotelluric (MT) models to constrain the geometry of potential geothermal reservoirs. Relationships between Vs structures, borehole core interpretations, and well-logging data were further examined. In addition, the failure of a landslide dam in the upstream MaTaiAn Stream on 23 September 2025 caused severe damage, highlighting the importance of internal stratification in understanding dam failure mechanisms. Temporal seismic array data acquired at the MaTaiAn landslide dam were analyzed using the DOP-E approach to derive two-dimensional Vs profiles. Based on insights from the Wulu site, the internal stratigraphic structure of the dam was characterized. Overall, this study demonstrates that ambient seismic noise observations combined with DOP-E analysis provide robust shear-wave velocity constraints, effectively complementing conventional drilling data. The proposed approach is well suited for geothermal exploration and subsurface structural assessment in remote and topographically challenging environments.