Anelasticity of the Mantle Beneath Taiwan Revealed by GPS Tidal Displacements

Abstract

Ocean tide loading (OTL) displacements observed by Global Positioning System (GPS) offer constraints on upper mantle anelasticity, albeit mostly within the asthenosphere. Here, we advance beyond this limitation by probing anelasticity extending into the lower mantle beneath Taiwan. Following removal of body tide errors, we resolve mantle anelasticity signatures through systematic comparison of GPS‐observed three‐component tidal displacements with M2 OTL predictions derived from the FES2014b ocean tide model (validated against tide gauges) and 1‐D Preliminary Reference Earth Model + CRUST1.0 Earth models (Zener rheology). Depth‐dependent anelastic dispersion manifests primarily as spatially coherent up residuals: the known asthenospheric low‐Q zone contributes 0.2–0.5 mm to residuals in western Taiwan, whereas persistent residuals (particularly those exceeding 0.3 mm in northern, central, and eastern Taiwan) point to a deeper low‐Q anomaly (mean Q0 ≈ 85) extending from ∼500 to ∼1,700 km depth. In contrast, the signature of asthenospheric dissipation is observed as phase shifts in the horizontal displacements. These phase shifts produce north residuals of up to 0.2 mm that are used to constrain an asthenospheric effective viscosity of ∼1 × 1016 Pa s. The anelastic exponent in the asthenosphere (0.006–0.010) is significantly lower than that in the sub‐asthenospheric mantle (0.13–0.28). Our methodology—leveraging three‐component GPS‐derived OTL displacements—provides a promising avenue for constraining anelasticity from the asthenosphere into the lower mantle at semidiurnal frequencies beneath Taiwan, despite potential uncertainties in the body‐tide removal procedure and GPS orbit and clock products.