海底地震学

在海洋地区进行高质量的长期地震观测是全球地震观测的一个重要组成部分，DSDP／ODP是唯一能够钻穿软的沉积物、在坚硬的岩石里安置地震传感器这一目标的科学计划。介绍了ODP航次在井孔中设立地震台站、并获得一些有趣的结果的成功例子。ODP在日本外海布置了两个井中地震台站，与陆上台站一起来观测板块边界的活动性。此外，还介绍了西太平洋井中宽带台站、海上地震信号和噪音等问题。     

Seismological structure and implications of collision between the Ontong Java Plateau and Solomon Island Arc from ocean bottom seismometer–airgun data

A seismic refraction–reflection experiment using ocean bottom seismometers and a tuned airgun array was conducted around the Solomon Island Arc to investigate the fate of an oceanic plateau adjacent to a subduction zone. Here, the Ontong Java Plateau is converging from north with the Solomon Island Arc as part of the Pacific Plate. According to our two-dimensional P-wave velocity structure modeling, the thickness of the Ontong Java Plateau is about 33 km including a thick (15 km) high-velocity layer (7.2 km/s). The thick crust of the Ontong Java Plateau still persists below the Malaita Accreted Province. We interpreted that the shallow part of the Ontong Java Plateau is accreted in front of the Solomon Island Arc as the Malaita Accreted Province and the North Solomon Trench are not a subduction zone but a deformation front of accreted materials. The subduction of the India–Australia Plate from the south at the San Cristobal Trench is confirmed to a depth of about 20 km below sea level. Seismicity around our survey area shows shallow (about 50 km) hypocenters from the San Cristobal Trench and deep (about 200 km) hypocenters from the other side of the Solomon Island Arc. No earthquakes occurred around the North Solomon Trench. The deep seismicity and our velocity model suggest that the lower part of the Ontong Java Plateau is subducting. After the oceanic plateau closes in on the arc, the upper part of the oceanic plateau is accreted with the arc and the lower part is subducted below the arc. The estimation of crustal bulk composition from the velocity model indicates that the upper portion and the total of the Solomon Island Arc are SiO₂ 58% and 53%, respectively, which is almost same as that of the Izu–Bonin Arc. This means that the Solomon Island Arc can be a contributor to growing continental crust. The bulk composition of the Ontong Java Plateau is SiO₂ 49–50%, which is meaningfully lower than those of continents. The accreted province in front of the arc is growing with the convergence of the two plates, and this accretion of the upper part of the oceanic plateau may be another process of crustal growth, although the proportion of such contribution is not clear.     

Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.     

Earth's melting due to the blanketing effect of the primordial dense atmosphere

When the proto-Earth was growing by the accretion of planetesimals and its mass became greater than about 0.1 M_E, where M_E is the present Earth's mass, an appreciable amount of gas of the surrounding solar nebula was attracted towards the proto-Earth to form an optically thick, dense atmosphere. We have studied the structure of this primordial atmosphere under the assumptions that (1) it is spherically symmetric and in hydrostatic equilibrium, and (2) the net energy outflow (i.e., the luminosity) is constant throughout the atmosphere and is given by GMM/R with M = M/10⁶yr or M/10⁷yr where M and R are the mass and the radius of the proto-Earth, respectively.The results of calculations show that the temperature at the bottom of the atmosphere, namely, at the surface of the proto-Earth increases greatly with the mass of the proto-Earth and it is about 1500°K for M = 0.25 M_E. This high temperature is due to the blanketing effect of the opaque atmosphere. Thus, as long as the primordial solar nebula was existing, the surface temperature of the proto-Earth was kept high enough to melt most of the materials and, hence, the melted iron sedimented towards the center to form the Earth's core.     

Thermal expansion of single-crystal tephroite

The thermal expansion of tephroite (Mn₂SiO₄) at temperatures between 25 and 850°C has been determined by a dilatometric technique. The analysis of data in terms of Grüneisen's theory yields the Grüneisen's parameter γ=1.04, and the pressure derivative of rigidity (?G/?P)=0.7.     

Dissipation of the rare gases contained in the primordial Earth's atmosphere

If the Earth was formed by accumulation of rocky bodies in the presence of the gases of the primordial solar nebula, the Earth at this formation stage was surrounded by a massive primordial atmosphere (of about 1 × 10²⁶ g) composed mainly of H₂ and He. We suppose that the H₂ and He escaped from the Earth, owing to the effects of strong solar wind and EUV radiation, in stages after the solar nebula itself dissipated into the outer space.The primordial atmosphere also contained the rare gases Ne, Ar, Kr and Xe whose amounts were much greater than those contained in the present Earth's atmosphere. Thus, we have studied in this paper the dissipation of these rare gases due to the drag effect of outflowing hydrogen molecules. By means of the two-component gas kinetic theory and under the assumption of spherically symmetric flow, we have found that the outflow velocity of each rare gas relative to that of hydrogen is expressed in terms of only two parameters — the rate of hydrogen mass flow across the spherical surface under consideration and the temperature at this surface. According to this result, the rare gases were dissipated below the levels of their contents in the present atmosphere, when the mass loss rate of hydrogen was much greater than 1 × 10¹⁷ g/yr throughout the stages where the atmospheric mass decreased from 1 × 10²⁶ g to 4 × 10¹⁹ g.     

Dissolution of the primordial rare gases into the molten Earth's material

We have shown in a previous paper that, if the primordial solar nebula existed when the Earth was formed, the Earth was once surrounded by a dense and massive primordial atmosphere, whose temperature and pressure were about 4000 K and 900 atm, respectively, at the bottom. We suppose that this hydrogen-rich atmosphere escaped from the Earth after the solar nebula itself disappeared, both phenomena probably being due to the effect of strong solar wind and radiation.Using the results of our previous and new calculations on the structure of the primordial atmosphere, we have investigated the amount of dissolution of the rare gases, which were contained in the primordial atmosphere, into the molten Earth's material.The amount of the dissolved rare gases is found to be strongly dependent on the grain opacity of the atmosphere, i.e., on the amount of fine grains. However, their isotopic ratios and relative abundance are independent of the opacity and approximately equal to those in the primordial solar nebula, that is, to the present solar values. Especially, the dissolved neon is expected to have remained in the present mantle. Therefore, if a considerable amount of neon with nearly the solar isotopic ratio is discovered in present mantle material, this offers direct evidence for the proposition that the proto-Earth was once surrounded by the primordial atmosphere.