Tidal corrections are very important for oceanographic study of altimetry because tidal signals contaminate the low‐frequency part of raw altimetric signals. Accurate tidal predictions are also necessary for geophysical research, such as with the superconducting gravimeter (SCG), which needs to distinguish very weak signals from the overwhelming tidal signals. The aim of this paper is to derive accurate global ocean tide models from the TOPEX/POSEIDON sea surface height data of 5 cm accuracy, more accurate than any other previous altimeter data. TOPEX geophysical data records of cycles 9–94 were processed and analyzed. We describe response analysis of tidal waves at crossover points and application of hydrodynamical interpolation. The tidal solutions with 0.5° grid system are obtained for eight major constituents (i.e., M2, S2, N2, K2, K1, O1, P1, and Q1) and compared with the Schwiderski (1980a, b, c) model, the Cartwright and Ray (1991) model, and the eight‐constituent version of the Egbert et al. (1994) model or TPXO.2. The accuracy of the models is checked using island and bottom tide gauge data in open ocean and using SCG data in Antarctica and Japan.

A linear magnetic pattern, offset along fracture zones was found in the Shikoku Basin, which lies between the Izu-Bonin and northern Kyushu-Palau Ridges.This lineation, nearly parallel to the trend of the Kyushu-PalauRidge, seems to indicate that this basin originated from a type of seafloor spreading.A possible spreading center was suggested on the basis of symmetry of magnetic anomaly profiles and distribution of sediment cover in the basin. The anomaly profiles were tentatively correlated to the synthetic profile calculated from the reversal history of the earth's magnetic field during a period in Early to Middle Miocene, which gives a good fit of age to the result of the DSDP hole 297.

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The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow‐spreading mid‐ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the average length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30′N, the opening of the trough started about 25 Ma with an average half‐spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active mid‐ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active mid‐ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma.

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