Upper mantle velocity structure in the Ryukyu and Taiwan-Luzon Island arc regions of the northwest Pacific has been determined to a depth of about 255 km from the analysis of P and S wave travel times data of 106 earthquakes with focal depths varying from 35 to 255 km. Wave velocities were obtained at the depths of foci of earthquakes, in the inclined seismic zones in these regions, by using KAILA's (1969) analytical method. By a linear fitting of velocity variation with depth, the following results were obtained. In the Ryukyu arc region, P velocity increases from 8.01 km/s at 40 km depth to 8.43 km/s at 255 km depth, and S velocity increases from 4.32 km/s at 40 km depth to 4.53 km/s at 255 km depth. In the Taiwan-Luzon region also, P velocity increases from 8.00 km/s at 40 km depth to 8.38 km/s at 240 km depth, and S velocity increases from 4.41 km/s at 40 km depth to 4.50 km/s at 180 km depth. Thus, the P velocity functions in the two regions are quite similar and reveal velocities which are about 5 % higher (on an average) than those in the Shikoku and Kyushu Islands of the southwest Japan region. However, the S velocities in the Taiwan-Luzon region are 2 to 3 % higher than those in the Ryukyu and the southwest Japan regions. There is no evidence for the presence of a significant low velocity Iayer in the inclined seismic zones beneath the Ryukyu and the Taiwan-Luzon arcs.

View full abstract

An airgun-OBS (ocean bottom seismometer) refraction profiling was performed at the continental slope off the Joban coast, Honshu, Japan. Three identical profiles were shot at 200-300 m water depth to see the effects of airgun signal change mainly due to airgun shot depth and reproducibility of record sections. It was found that an airgun towing depth of more than 10 m for a 4.9 liter chamber with about 90 kg/cm² pressure shot yields better signal penetration than do shallower depths. Picked arrivals from the three profiles matched within 0.1 s scatter.
P wavespeed structure down to about 8 km depth was obtained from this experiment. Constraints from the data of two OBSs require a laterally varying model. Our model consists of layers with 1.7, 2.9, 4.3, 4.7, and 5.5 km/s wavespeed. The top four layers have about 1.2 km thicknesses. The layer distinction is not clear below 4.3 km/s layer, suggesting gradual increase. However, the wavespeed does not increase as high as 6 km/s, which is characteristic of the upper crust beneath northern Japan. This may be due to the shortness of the refraction profiles.

View full abstract

The melting relation of the Fe₂SiO₄(Fa)-FeSiO₃(Fs) system has been studied at pressures of 5, 6, and 9 GPa. This pseudo-binary system shows an eutectic melting between fayalite and ferrosilite, and ferrosilite is observed to melt incongruently into coesite plus melt at all pressures studied. The eutectic point of this system is determined as Fa₇₄Fs₂₆ and 1, 335±15°C at 6 GPa, and Fa₅₅Fs₄₅ and 1, 530±20°C at 9 GPa, where spinel phase emerges in place of fayalite as a liquidus phase of the Fe₂SiO₄-rich portion of the system. The peritectic point, where the liquidus surface of ferrosilite intersects with that of coesite, is determined as Fa₃₅Fs₆₅ and 1, 435±35°C at 6 GPa, and Fa₁₅Fs₈₅ and 1, 610±30°C at 9 GPa. On the basis of the melting curves for iron silicate minerals, the liquidus relation of the FeO-SiO₂ system is extrapolated to higher pressures. The result demonstrates the large influence of phase transitions in the constituent minerals on the melting relation.

View full abstract

The first higher mode of oceanic Rayleigh waves from a deep earthquake which occurred at the New Hebrides Islands on October 7, 1966, has been investigated with respect to group velocities, wave forms, and particle orbits, the waves propagating through the normal oceanic basin in the Pacific. Observed and theoretical particle orbits of the wavetrains of the first higher mode showed predominantly retrograde elliptic motions for periods around 30 s, though a pure progressive motion was partly observed. Group velocities of the first higher mode showed 4.32 km/s for periods from 20 to 50 s, and are close to those of the fundamental mode of Love waves, though Love waves contain longer period components than the first higher mode of Rayleigh waves. The observed group velocity 4.32 km/s of the first higher mode for periods mentioned above significantly exceeds a theoretical value of 4.28 km/s calculated for the model PC-MAX given for the highest-velocity-area in the Pacific. Those observed high group velocities and simulated seismograms of the first higher mode suggest that the shear velocity of 4.35 km/s is dominant in the low velocity zone under the normal oceanic basin, particularly in regions of the ocean-floor age of more than 90 m.y.

View full abstract

The origin of the detailed crustal structure obtained by explosion seismology in the Izu Peninsula is explained by a numerical simulation of viscous fluid flow. At the place where the two blocks of different crustal structure are in contact, a certain layer of one block is inserted by viscous flow along the interface of the two layers in the other block. The inserted layer extends to about 5 km in 40 m.y. after the contaction, and the similar structure as observed in the Izu Peninsula appears if the crustal viscosity is assumed as 10²⁰ poise.

View full abstract