Geological structure of the marginal plateau off Tottori,Southern-West of Honshu,Japan

A geological and geophysical survey off Tottori, southern-west of Honshu, Japan, was carried out by the Hydrographic Department of Japan during May and June (26 days) of 1974.In the survey area, there is marginal plateau which lies under the slope of the continental shelf adjacent to the Sanin district. By the closely-spaced surveying the marginal plateau was divided into two different zones based on differences of genesis and structure, which are identified in this paper as marginal plateaus A (west side) and marginal plateau B (east side). The tectonic boundary between marginal plateaus A and B, previously believed to exist near Tottori City, was not confirmed. An erosional plane, beneath the horizontally bedded upper layer was found to be 75 m lower than the submerged surface of marginal plateau A. Marginal plateau B is hypothesized to have been generated by the sliding of the upper layer. This is suggested by the presence of prominent slumping directed toward the bottom of the Oki Trough. This result implies that the submergence of the Oki Trough area and formation of the marginal plateau are closely related.     

Investigation of the variation of the Kuroshio and daily mean sea levels

In the past 30 years, I have been engaged in the study of the Kuroshio, especially of its time variations. In the course of the study I have tried to introduce various new methods such as GEK, mean sea level observation at island tide stations, multiple ship survey and temperature measurement by submarine cable and so on. In this report, some aspects of these studies are described and the usefulness of tide stations for the investigation of oceanic current is emphasized.     

Deep CTD Casts in the Challenger Deep,Mariana Trench

On 1 December 1992, CTD (conductivity-temperature-depth profiler) casts were made at three stations in a north-south section of the Challenger Deep to examine temperature and salinity profiles. The station in the Challenger Deep was located at 11°22.78′ N and 142°34.95′ E, and the CTD cast was made down to 11197 db or 10877 m, 7 m above the bottom by reeling out titanium cable of 10980 m length. The southern station was located at 11° 14.19′ N and 142°34.79′ E, 16.1 km from the central station, where water depth is 9012 m. CTD was lowered to 7014 db or 6872 m. The northern station was located at 11°31.47′ N and 142° 35.30′ E, 15.9 km from the central station, and CTD was lowered to 8536 db or 8336 m, 10 m above the bottom. Below the thermocline, potential temperature decreased monotonously down to 7300–7500 db beyond a sill depth between 5500 m and 6000 m, or between 5597 db and 6112 db, of the trench. Potential temperature increased from 7500 db to the bottom at a constant rate of 0.9 m°C/1000 db. Salinity increased down to 6020–6320 db, and then stayed almost constant down to around 9000 db. From 9500 db to the bottom, salinity increased up to 34.703 psu at 11197 db. Potential density referred to 8000 db increased monotonously down to about 6200 db, and it was almost constant from 6500 db to 9500 db. Potential density increased from 9500 db in accordance with the salinity increase. Geostrophic flows were calculated from the CTD data at three stations. Below an adopted reference level of 3000 db, the flow was westward in the north of Challenger Deep and eastward in the south, which suggests a cyclonic circulation over the Challenger Deep. Sound speed in Challenger Deep was estimated from the CTD data, and a relation among readout depth of the sonic depth recorder, true depth, and pressure was examined.     

Distribution of moon jellyfish Aurelia aurita in relation to summer hypoxia in Hiroshima Bay,Seto Inland Sea

Biological and physical surveys were conducted in order to investigate the relationship between environmental conditions and the distribution of moon jellyfish Aurelia aurita in Hiroshima Bay, western Seto Inland Sea, Japan. Moon jellyfish and ichthyoplankton were collected at 13 stations in Hiroshima Bay during monthly surveys from July to September in 2006 and 2007. Surface temperature in 2006 was significantly lower during the August and September cruises and surface salinity was lower during all cruises than in 2007. Moon jellyfish was the most dominant gelatinous plankton collected, accounting for 89.7% in wet weight. Mean moon jellyfish abundance in 2006 was higher than that in 2007 from July through September, with significant inter-year differences for July and September. Variability in precipitation and nutritional input from the Ohta River, northernmost part of Hiroshima Bay, were suggested as possible factors affecting the inter-annual variability in moon jellyfish abundance in the coastal areas of northern Hiroshima Bay. Moon jellyfish were more abundant in the coastal areas of northern Hiroshima Bay, where the dissolved oxygen (DO) concentration was lower, while low in the central part of the bay. Japanese anchovy Engraulis japonicus eggs were most dominant (58.1% in number) among the ichthyoplankton and were abundant in the central area of Hiroshima Bay. Explanatory analysis was conducted to detect possible effects of environmental conditions on the abundance of moon jellyfish and Japanese anchovy eggs during the summer months in Hiroshima Bay. Of the environmental conditions tested (temperature, salinity and DO of surface and bottom layers at each sampling station), bottom DO had the most significant effect on the moon jellyfish abundance: there was a negative correlation between the bottom DO and the moon jellyfish abundance in Hiroshima Bay during summer.     

Chromian spinel lamellae in olivine from the Iwanai-dake peridotite mass,Hokkaido, Japan

Lamellar inclusions of chromian spinel (Cr/Cr + Al> 0.7), clinopyroxene and chromian spinel-clinopyroxene symplectite occur in olivines from alpine-type peridotites which have equilibrated at relatively low temperature (<700°C). They occur most commonly in dunite with very magnesian olivine (Fo₉₃ to Fo₉₅) and discrete grains of Cr-rich spinel. Olivine which initially equilibrated with magnesian and Cr-rich liquid had contained small amounts of calcium and trivalent chromium in the octahedral site, and lamellar chromian spinel and diopsidic clinopyroxene exsolved during the annealing process. The ordinary depletion of chromium or absence of chromian spinel lamellae in olivines in igneous rocks may be partly due to the effective exclusion of chromium from olivine upon cooling.     

Radiocarbon dates of elevated shorelines,Indonesia and Malaysia. Part 1

Four new radiocarbon dates of elevated strandlines in tectonically active areas of eastern Indonesia and East Malaysia indicate average rates of uplift that range between 4.5 and 9 mm annually during the past 24,000 yr. These values are at least three times higher than former estimates from eastern Indonesia. Another radiocarbon date from the south arm of Sulawesi—also tectonically mobile—indicates a rate of uplift of 1.4–2.5 mm per year which corresponds with earlier determinations. This particular case, however, suggests that the sample was probably located close to a north-south axis about which southern Sulawesi was tilted during the Quaternary.     

Magma mixing model for the genesis of middle crust in the Izu–Bonin–Mariana arc: evidence from plutonic rocks in the Mineoka-Setogawa ophiolitic mélange,central Japan

ABSTRACT

A Paleogene accretionary complex, the Mineoka–Setogawa belt is distributed adjacent to the northern portion of the collision zone between Honshu and Izu–Bonin–Mariana (IBM) arcs in central Japan, comprising a mélange of ophiolitic fragments of various sizes. The Eocene-Oligocene plutonic rocks in this belt (gabbro, diorite, and tonalite) have been interpreted as fragments brought from the deep crust beneath the IBM arc through tectonic collisions. The geochemical characteristics of the gabbro and associated basaltic dike are similar to those of the Eocene IBM tholeiitic basalt; thus, the gabbro was likely formed via the crystallization of the Eocene tholeiitic basaltic magmas, which was produced by the partial meltings of a depleted mantle wedge. A comparison with experimental results and geochemical modeling indicates that the tonalite was generated by 10–30% dehydration melting of the gabbro. Actually, Eocene–Oligocene felsic veins, which are coeval with the plutonic rocks, occur in the Mineoka–Setogawa gabbro. Plagioclase crystals in the diorite comprise Ca-rich and -poor parts in a single crystal. Their compositional characteristics are consistent with those of plagioclase in the gabbro and tonalite, respectively. The textures and chemical composition of plagioclase indicate that the diorite was formed by the mixing between mafic and silicic magmas. The whole-rock composition of the diorite also indicates the evidence for the mixing between basaltic magmas which were fractionated to variable degrees and homogeneous silicic magma. The mixing model proposed from the first direct observations of the IBM middle crust exposed on the Mineoka–Setogawa belt is applied to the genesis of the Eocene to present intermediate rocks in the IBM arc. If the continental crust were created at intra-oceanic arc settings such as the IBM arc, the magma mixing model would be one of the most likely mechanisms for the genesis of the continental crust.     

A study on the duration and amplitude characteristics of earthquake ground motions

The duration, total power, rms amplitude, and peak ground acceleration (PGA) are examined using the data from the KiK-net digital borehole accelerograph arrays in Japan during three large earthquakes. We investigate correlations between the duration, PGA on the surface and hypocentral distance, instrumental seismic intensity, and the local site conditions. Also considered in this paper, are correlation between the duration and PGA. This present study suggests that PGA is inversely proportional to the duration.Comparing the duration, total power, rms amplitude, and PGA on the surface with those at the bottom of borehole, the duration and maximum amplitude characteristics of earthquake ground motions are investigated with emphasis on site amplification due to the local site conditions. The difference of amplification factors is not only site-dependent but also event-dependent. The comparison of the site amplification factors and the average shear-wave velocity in the upper 30-m showed a good agreement.This paper indicated that the duration and amplitude characteristics suffer a reciprocal effect due to the local site conditions and the site dependency of maximum amplitude PGA is greater than that of the duration. In view of the results, the duration is not less important than the maximum amplitude and frequency content in earthquake engineering. Moreover, this study provided meaningful information of input motion available to earthquake-resistant design.