Abstract
A detailed shear wave velocity structure in the Pacific lithosphere for an ocean-floor age greater than 100 m.y. has been obtained in high resolution by means of several isotropic and anisotropic surface wave inversions. The vertically heterogeneous velocity structure of the vertically polarized shear wave has the following 3 characteristics: (1) has a small-scale low velocity zone near a depth of 30 km; (2) reaches a maximum value at a depth of 70 km; (3) obtains a minimum value at a depth of 130 km in the large-scale low velocity zone. The velocity increases with depth between 130-220 km. The velocity structure of the horizontally polarized shear wave does not possess Characteristics 1 and 2. Characteristics 2 and 3 give strong constraints on the rigorous estimation of the plate thickness. If the bottom of the heterogeneous lithosphere is tentatively defined as a mid-point between the depths indicating Characteristics 2 and 3, the plate thickness is estimated to be 100 km for the region where the age is greater than 100 m.y. A relationship between Characteristic 1 and the double-planed seismic zone is also discussed.
