Shozaburo Nagumo

Sub-Moho seismic profile in the Mariana Basin — Ocean bottom seismograph long-range explosion experiment

Abstract Ocean bottom seismograph (OBS) long-range explosion experiments were carried out in the Mariana Basin in 1973 and 1976. Seven large shots (8.5–1.5 ton) as well as several tens of small shots were fired. The maximum range of observation was about 1900 km. As many as 25 OBS stations were deployed in an array of about 800 km. It is found that the sub-Moho P-wave velocity structure is of stratified nature, being composed of alternating high- and low-velocity layers. High-velocity layers with apparent velocities of 8.1, 8.2, 8.4, 8.6 and 8.7 km/s are identified. Low-velocity layers, sandwiched between the high-velocity layers of 8.4, 8.6 and 8.7 km/s, are very prominent. The sub-Moho high-velocity lid with an apparent velocity of 8.4 km/s is very thin. Thinning of this lid, thickening of the low-velocity layer, and the presence under it of another high-velocity layer (8.6 km/s) appear to characterize the uppermost mantle structure beneath the Mariana Basin.

Crustal structure of the Bonin Trough

Abstract A seismic refraction study using OBSH and explosives was carried out in the Bonin Trough. The P-wave velocity model along the trough axis consists of 4 layers; the velocities, Vp, of these layers are from top to bottom 2.0, 3.6, 6.3 and 7.8 km s−1 respectively. The thickness of the crust is 13 km at the northern part of the trough and becomes thicker toward the south. The negative free-air anomaly pattern in the trough can be explained by thickening of the crust.

Seismic velocity structure near the extinct spreading center in the Shikoku Basin, North Philippine Sea

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 Poissons 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.

Large Poisson's ratio and low S-wave velocity within the Japan trench inner wall toe

Abstract P to S and S to P converted waves arising from the interface between inner wall rock mass and the subducting oceanic plate were observed by an ocean bottom seismograph (OBS). Using these data, mean S-wave velocity, V s , and Poissons ratio, σ, were determined within the Japan trench inner wall toe. The Poissons ratio is as large as σ = 0.41 , and the average S-wave velocity is as low as V s = 0.91 km / sec . Occurrence of efficient P to S and S to P conversion at this interface implies the existence of a high acoustic impedance. The low shear velocity of the inner trench wall material implies a low rigidity and hence this zone can be easily deformed.

The P-wave structure of the lithosphere-asthenosphere in the Western Pacific: a comparison of ocean versus continent

Abstract In this paper, we present the lithosphere-asthenosphere P-wave velocity structures in the Western Pacific as determined by ocean-bottom seismometer array observations to illustrate the contrasts and similarities between continental and oceanic regions. These structures are compared with representative velocity structures beneath continental shield regions and the tectonically active regions. The subcrustal lithosphere in the western Northwest Pacific Basin possesses a layered P-wave structure (apparent surface velocities are 8.0, 8.4 and 8.6 km/s) and extends to a depth of about 149 km. The lithosphere is underlain by a low-velocity layer which appears to be the major transition zone from the lithosphere to asthenosphere. This velocity structure is similar to that of the continental shield regions. In the East Mariana Basin, the uppermost mantle layer below the Moho is about 30 km thick with a thin high-velocity lid (8.3 km/s) at its base. These layers form the subcrustal lithosphere, below which several alternating low- and high-velocity layers together form a thick asthenosphere. The upper mantle structure beneath the East Mariana Basin seems to show an intermediate feature between continental shield regions and tectonically active regions. The differences between the upper mantle structure of the continent and ocean could be regarded as aspects of an evolution of upper mantle. The western Northwest Pacific Basin could be regarded as being almost matured, while the East Mariana Basin could be regarded as being rejuvenated.