Abstract
An OBS (ocean-bottom seismograph) airgun refraction survey was conducted near the extinct spreading center in the 18–21 m.y. old Shikoku Basin, North Philippine Sea. The P-wave velocity structure of the ocean crust (layers IIA and IIB) is in accord with that of the average ocean crust. P to S converted waves were remarkably well recorded by the OBS. Utilizing these phases, P-wave velocity, V p, S-wave velocity, V s, velocity ratio V p V s , and Poisson's ratio, σ, were determined as follows: V p = 1.8 km/sec, V s = 0.43 km/sec, V p V s = 4.2 , σ = 0.47 for layer I (sediment); V p = 3.7 km/sec, V s = 1.8 km/sec, V p V s = 2.0 , σ = 0.33 for layer IIA; and V p = 4.8 km/sec for layer IIB. The refraction of P to S converted wave, V s = 3.2 km/sec, might be identified as arising from layer IIC, and that of V s = 3.8 km/sec as arising from layer III.
Published Version
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