Normal mode coupling due to azimuthal anisotropy in the transition zone: an example from Taiwan Island

SUMMARY We investigate the effect of azimuthal anisotropy on coupling between spheroidal and toroidal modes at frequencies below 4 mHz after large earthquakes of M w > 7.8, based on the seismic records from Broad‐band Array in Taiwan for Seismology stations. Strong coupling of 0 S 25 – 0 T 25 is widely observed at Taiwan Island, but Earth's rotation cannot cause coupling between 0 S 25 and 0 T 25 according to coupling selection rules. Different from the Coriolis coupling, the anisotropic coupling of 0 S 25 – 0 T 25 becomes notable only when stations close to nodal lines of 0 S 25 , and the coupling attenuates so rapidly that it is notable on the spectra of 18–24 hr vertical‐component records. Modelling indicates that local azimuthal anisotropy is the most possible cause for the strong coupling. The abnormally strong coupling of 0 S 20 – 0 T 21 observed at Taiwan Island also shows similar characteristic of nodal nature and fast attenuation, suggesting local azimuthal anisotropy as the dominant factor for the coupling rather than the Earth's rotation, isotropy and transverse isotropy. Our estimation of sensitivity kernels indicates that the coupled modes 0 S 20 – 0 T 21 and 0 S 25 – 0 T 25 show peak sensitivity to azimuthal anisotropy at depth of 400–650 km, suggesting that the strong coupling may be explained by the presence of azimuthal anisotropy in the upper‐mantle transition zone beneath Taiwan Island. The east‐dipping Eurasian slab and the north‐dipping Philippine Sea Plate beneath the Taiwan orogen are responsible for the formation of the anisotropy.