Trace-element characteristics of east–west mantle geochemical hemispheres

Recent statistical analyses on the isotopic compositions of oceanic, arc, and continental basalts have revealed that the Earth's mantle is broadly divided into eastern and western hemispheres. The present study aimed to characterize the isotopically defined east–west geochemical hemispheres using trace-element concentrations. Basalt data with Rb, Sr, Nd, Sm, Pb, Th, and U in addition to the isotopic ratios ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb were selected mostly from the GEOROC and PetDB databases. A total of 4787 samples were used to investigate the global geochemical variations. The results show that the wide trace-element variations are broadly explained by the melting of melt-metasomatized and fluid-metasomatized mantle sources. The larger amount of the fluid component derived from subducted plates in the eastern hemisphere than that in the western hemisphere is inferred from the basalts. These characteristics support the hypothesis that focused subduction towards the supercontinent created the mantle geochemical hemispheres.     

Co-ignimbrite ash-fall deposits of the 1991 eruptions of Fugen-dake, Unzen Volcano, Japan

Fugen-dake, the main peak of Unzen Volcano, began a new eruption sequence on November 17, 1990. On May 20, 1991, a new lava dome appeared near the eastern edge of the Fugen-dake summit. Small-scale, 10⁴–10⁶ m³ in volume, Merapi-type block and ash flows were frequently generated from the growing lava dome during May–June, 1991. These pyroclastic flows were accompanied by co-ignimbrite ash plumes that deposited ash-fall deposits downwind of the volcano. Three examples of co-ignimbrite ash-fall deposits from Unzen pyroclastic flows are described. The volume of fall deposits was estimated to be about 30% by volume of the collapsed portions of the dome that formed pyroclastic flows. This proportion is smaller than that described for other larger co-ignimbrite ash-fall deposits from other volcanoes. Grain size distributions of the Unzen co-ignimbrite ash-fall deposits are bi-modal or tri-modal. Most ashes are finer than 4 phi and two modes were observed at around 4–7 phi and 9 phi. They are composed mainly of groundmass fragments. Fractions of another mode at around 2 phi are rich in crystals derived from dome lava. Some of the fine ash component fell as accretionary lapilli from the co-ignimbrite ash cloud indicating either moisture or electrostatic aggregation. We believe that the co-ignimbrite ash of Unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope. These ashes were entrained into the convective plumes generated off the tops of the moving flows.     

Frequency-dependence of velocity and attenuation of elastic waves in granite under uniaxial compression

Velocity as well as attenuation factorQ ^–1 ofP-wave in a dry granitic rock sample under uniaxial compressions were measured in the range of frequency between 100 kHz and 710 kHz by using the pulse transmission technique. Above the stress of 0.5 _f , where _f is the fracture stress, theP-wave velocity decreases with increasing axial stress, whereasQ ^–1 increases. Particularly, the change ofQ ^–1 is greater for high frequency than for low frequency. At a given stress level, the higher the frequency, the higher theP-wave velocity and the largerQ ^–1. This result means that the velocity decrease with increasing stress is smaller for higher frequency. Because of this frequency-dependence of velocity decrease, theP-wave in the rock under dilatant state shows dispersion. The body wave dispersion is more remarkable at higher stress, and is not found in a homogeneous material with no cracks. Thus the disperison is attributed to the generation of cracks. When the frequency-dependence ofQ ^–1 is approximated asf ⁿ in the present frequency range, the exponentn takes a value from 0.63 to 0.77.     

Stratigraphic records of tsunamis along the Japan Sea,southwest Hokkaido,northern Japan

The stratigraphy of tsunami deposits along the Japan Sea, southwest Hokkaido, northern Japan, reveals tsunami recurrences in this particular area. Sandy tsunami deposits are preserved in small valley plains, whereas gravelly deposits of possible tsunami origin are identified in surficial soils covering a Holocene marine terrace and a slope talus. At least five horizons of tsunami events can be defined in the Okushiri Island, the youngest of which immediately overlies the Ko‐d tephra layer (1640 AD) and was likely formed by the historical Oshima‐Ohshima tsunami in 1741 AD. The four older tsunami deposits, dated using accelerator mass spectrometry ¹⁴C, were formed at around the 12th century, 1.5–1.6, 2.4–2.6, and 2.8–3.1 ka, respectively. Tsunami sand beds of the 1741 AD and circa 12th century events are recognized in the Hiyama District of Hokkaido Island, but the older tsunami deposits are missing. The deposits of these two tsunamis are found together at the same sites and distributed in regions where wave heights of the 1993 tsunami (Hokkaido Nansei‐oki earthquake, Mw = 7.7) were less than 3 m. Thus, the 12th century tsunami waves were possibly generated near the south of Okushiri Island, whereas the 1993 tsunami was generated towards the north of the island. The estimated recurrence intervals of paleotsunamis, 200–1100 years with an average of 500 years, likely represents the recurrence interval of large earthquakes which would have occurred along several active faults offshore of southwest Hokkaido.     

Splitting of the subtropical gyre in the western North Pacific

Examined here is a hypothetical idea of the splitting of the subtropical gyre in the western North Pacific on the basis of two independent sources of data,i.e., the long-term mean geopotential-anomaly data compiled by the Japanese Oceanographic Data Center and the synoptic hydrographic (STD) data taken by the Hakuho Maru in the source region of the Kuroshio and the Subtropical Countercurrent in the period February and March 1974. Both of the synoptic and the long-term mean dynamic-topographic maps reveal three major ridges, which indicate that the western subtropical gyre is split into three subgyres. Each subgyre is made up of the pair of currents, the Kuroshio and the Kuroshio Countercurrent, the Subtropical Countercurrent and a westward flow lying just south of the Countercurrent (18°N–21°N), and the northern part of the North Fquatorial Current and an eastward flow at around 18°N. The subgyres are more or less composed of a train of anticyclonic eddies with meridional scales of between 300 and 600 km, so that the volume transport of the subgyres varies by a factor of two or more from section to section. The upper-water characteristics also support the splitting of the subtropical gyre; the water characteristics are fairly uniform within each subgyre, but markedly different between them. The northern rim of each subgyre appears as a sharp density front accompanied by an eastward flow. The bifurcations of the sharp density fronts across the western boundary current indicate that the major part of the surface waters in the North Equatorial Countercurrent is not brought into the Kuroshio. The western boundary current appears as a continuous feature of high speed, but the waters transported change discontinuously at some places.     

Estimation of crustal structure from observed group velocities ofPL-waves in Japan

Forward modelling of the crustal structure of the eastern Honshu Island, Japan, was made based on the group velocities ofPL-waves in the period range of 20–30 s. The observed values of group velocity were obtained by appling the multiple filter technique to the seismograms for earthquakes with the epicentral distance ranging from 500 to 1000 km. The theoretical values were calculated using Oliver and Major's method to find the best fit dispersion curve in the least-squares sense. The obtained structural model has considerably high crustal velocities compared to other previous models. It was shown that thePL-wave group velocity in the period range of interest was most sensitive to seismic velocities of the center of the crust. Numerical experiments confirmed the applicability of the approximation methods employed to obtain both observed and theoretical group velocities.     

Estimation of crustal structure in central and west Japan from waveform modeling of regionalPL waves

An inverse method of modeling the regionalPL waveform with the predominant period of about 20 s was developed to estimate the averageS-velocity structure of the upper crust. Applicability of the waveform modeling was confirmed by the results of the numerical experiments: thePL waveform is most sensitive to theS velocity in the upper crust, whereas it is not affected significantly by errors involved in the focal mechanism solution and focal depth determination when thePL wave is well developed. The method was applied to the observed seismograms recorded in central Japan from the earthquakes with epicentral distances 300–500 km. As a result, distinct regional differences were found in the upper crustalS velocity; in particular, between the southern Shikoku district, west Japan, and the southern Chubu district, central Japan, and between the mountainous and the coastal areas in the southern Chubu district. These differences are in agreement with the general features ofP-velocity structures obtained by explosion experiments and by analyses of natural earthquakes. Our method is effective to the extent that the crustal structure along the propagation path can be assumed a horizontally layered structure; it is not applicable when the sensitivity of thePL waveform to the error in the focal mechanism solution is exceptionally high.     

Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in ²³⁸U–²³⁴U–²³⁰Th–²³²Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The ²³⁸U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high ²³⁸U peak at 23.8 m in depth, while the ²³²Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk ²³⁸U into authigenic and terrigenous ²³⁸U fractions was attempted, based on the measured ²³²Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic ²³⁸U was higher in interglacials and lower in glacials. This depth profile of authigenic ²³⁸U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic ²³⁸U contents were associated with the environmental change around Lake Hovsgol.     

Interrelationships Between Topography,Quaternary Vertical Displacement,Active Faults and Short-term Horizontal Crustal Strain in Honshu,Japan

Regional and local characteristics of active fault patterns and elevation variation throughout Honshu, Japan are characterized in terms of their fractal dimensions; this allows variation in these complex variables to be compared directly to the scalar properties of net Quaternary vertical displacement, elevation and 10- and 110-year horizontal strains. The comparisons reveal that, throughout Honshu as a whole, there is significant correlation (r=0.75) between Quaternary vertical displacement, elevation, and its fractal properties. There is poor correlation, however, of elevation and its fractal properties to horizontal crustal strain, and also between Quaternary vertical displacement and horizontal crustal strain. A slight negative correlation is observed between the fractal properties of the active fault system and horizontal crustal strain measured over 10- and 110-year time periods (–0.43 and –0.26, respectively). The correlation between the 10-year (1985–1994) and 110-year (1883–1994) area strains, 0.48, reveals the occurrence of considerable change in the distribution of regional strain over these short time frames. Local computations of the correlation between data sets made for overlapping 160 km length windows of data spaced every 20 km along analysis lines reveal internal fluctuations in the correlation between variables. The local correlation between Quaternary vertical displacement and elevation is highest through central Japan and the Kinki Triangle. There is weak negative correlation between area strain and fractal dimensions of the active fault network. The local correlation between the fractal dimensions of active faults and horizontal area strain over the recent 10-year time period averages about –0.6 through central Japan in an area that extends across the Kinki Triangle through the northern part of central Honshu and northeast across the Itoigawa Shizuoka Tectonic Line. In general, regions of greatest complexity in the active fault network are associated with persistent negative area or compressional strain. Sparsely faulted areas in general coincide with areas of positive or roughly zero area strain. The presence of negative correlation through central Japan and the Kinki Triangle area in the recent 10-year period results from a decrease of area strain within an increasingly complex active fault system that reaches maximum negative values concentrated in the Kinki Triangle during the 1985–1994 time period.