Primary melt from Sannome-gata volcano,NE Japan arc: constraints on generation conditions of rear-arc magmas

The conditions under which rear-arc magmas are generated were estimated using primary basalts from the Sannome-gata volcano, located in the rear of the NE Japan arc. Scoriae from the volcano occur with abundant crustal and mantle xenoliths, suggesting that the magma ascended rapidly from the upper mantle. The scoriae show significant variations in their whole-rock compositions (7.9–11.1 wt% MgO). High-MgO scoriae (MgO > ~9.5 wt%) have mostly homogeneous ⁸⁷Sr/⁸⁶Sr ratios (0.70318–0.70320), whereas low-MgO scoriae (MgO < ~9 wt%) have higher ⁸⁷Sr/⁸⁶Sr ratios (>0.70327); ratios tend to increase with decreasing MgO content. The high-MgO scoriae are aphyric, containing ~5 vol% olivine microphenocrysts with Mg# [100 × Mg/(Mg + Fe²⁺)] of up to 90. In contrast, the low-MgO scoriae have crustal xenocrysts of plagioclase, alkali feldspar, and quartz, and the mineralogic modes correlate negatively with whole-rock MgO content. On the basis of these observations, it is inferred that the high-MgO scoriae represent primary or near-primary melts, while the low-MgO scoriae underwent considerable interaction with the crust. Using thermodynamic analysis of the observed petrological features of the high-MgO scoriae, the eruption temperature of the magmas was constrained to 1,160–1,220 °C. Given that the source mantle was depleted MORB-source mantle, the primary magma was plausibly generated by ~7 % melting of a garnet-bearing spinel peridotite; taking this into consideration, and considering the constraints of multi-component thermodynamics, we estimated that the primary Sannome-gata magma was generated in the source mantle with 0.5–0.6 wt% H₂O at 1,220–1,230 °C and at ~1.8 GPa, and that the H₂O content of the primary magma was 6–7 wt%. The rear-arc Sannome-gata magma was generated by a lower degree of melting of the mantle at greater depths and lower temperatures than the frontal-arc magma from the Iwate volcano, which was also estimated to be generated by ~15 % melting of the source mantle with 0.6–0.7 wt% H₂O at ~1,250 °C and at ~1.3 GPa.     

Nd and Sr isotopic variations in acidic rocks formed under a peculiar tectonic environment in Miocene Southwest Japan

Initial Nd and Sr isotopic ratios were obtained for middle Miocene igneous rocks as well as for related rocks from the Outer Zone of Southwest Japan to investigate the petrogenesis of acidic magmas and their relation to a peculiar tectonic environment bearing on the back-arc spreading of the Japan Sea. On the _Nd- _Sr diagram, data points for the acidic rocks fall in the – _Nd, + _Sr quadrant occupying different positions from those for sedimentary and old crustal rocks, and seem to define several subparallel lines which extend towards the lower-righthand sedimentary field. The S-type acidic rocks occupy an intermediate position between I-type rocks and sedimentary ones, a fact suggesting mixing of an igneous component and a sedimentary one. The linear mixing trend observed on the _Nd- _Sr diagram can be attained in the restricted case that the igneous component has similar Sr/Nd concentration ratios to that of the sedimentary one, which implies an intermediate to acidic composition for the igneous component. Inconsistency between the elemental and isotopic variations observed may be reconciled by considering that mixing, probably in the relatively deep part of the crust, might have occured prior to chemical differentiation processes. The episodic igneous activity and the high heat energy required to melt such materials involving sedimentary rocks may be explained by a model in which a hot mantle region probably corresponding to the rising part of the mantle convection supplied the heating energy to the Outer Zone of Southwest Japan when passing beneath Southwest Japan in the course of movement of the hot rising part from the Shikoku basin areas to the Japan Sea area.     

Compilation method for 1 km grid data of monthly mean air temperature for quantitative assessments of climate change impacts

A new method is proposed to compile 1 km grid data of monthly mean air temperature by dynamically downscaling general circulation model (GCM) data with a regional climate model (RCM). The downscaling method used is a technique referred to as the pseudoglobal warming method to reduce GCM bias. For the grid data, RCM data were corrected with data from an existing meteorological network. The correction model for the RCM bias was developed by stepwise multiple regression analysis using the difference in the monthly mean air temperatures between the observation and RCM output as a dependent variable and the geographical factors as independent variables. Our method corrected the RCM bias from 1.69°C to 0.58°C for the month of August in the 1990s (1990–1999).     

Response analysis of the Higashi–Kobe Bridge and surrounding soil in the 1995 Hyogoken–Nanbu Earthquake

This paper presents results of observation and analysis of the response of one of the longest cable-stayed bridges in the world to the Hyogoken–Nanbu (Kobe) Earthquake of 17 January 1995. It is determined that interaction of the foundations of the bridge towers with the supporting soil plays a decisive role in the overall structural behaviour. The key factor governing the changes of the soil properties at this site is pore-water pressure buildup, which results in liquefaction of the saturated surface soil layers under large dynamic loads. Models of the soil and structure are created and initially validated by accurately simulating the system response to a small earthquake. Soil parameters reflecting the pore-water pressure buildup in the strong earthquake are determined by an advanced non-linear effective stress analysis, combining the Ramberg–Osgood model of stress–strain dependence with a pore pressure model based on shear work concept. They are utilized to investigate and simulate the interaction of the foundation and the supporting soil using the program SASSI with the flexible volume substructuring approach. The results show a good agreement with the observations and have useful implications to the scientific and engineering practice. © 1998 John Wiley & Sons, Ltd.     

Variability of surface characteristics and energy flux patterns of sunn hemp (Crotalaria juncea L.) under well-watered conditions

There is not much information in the literature about the energy partitioning and micrometeorological features of sunn hemp. Therefore, in this study, the variations in the energy-balance components and plant characteristics such as aerodynamic and surface conductance, crop coefficient, albedo, short- and long wave down- and upward radiation have been measured and estimated for the time period from August to October 2004 over an irrigated sand field at the Arid Land Research Center in Tottori, Japan. The Bowen ratio energy-balance method was used to calculate the partitioning of heat fluxes of sunn hemp. The Bowen ratio values at the first growing stages in August were found to be higher than the Bowen ratio values at the latest growing stages in September and October because of the heavy rain and high soil-water content. The daytime averaged Bowen ratio was 0.19. During the measurement period, the daytime average net radiation, and soil, latent and sensible heat fluxes were approximately 231, 28, 164, and 39 W m^–2, respectively. The net radiation and soil heat flux showed decreasing trends from the beginning to the end of the experiment period due to the atmospheric and crop growth conditions. The daytime averages of aerodynamic and surface conductance for sunn hemp were around 31 and 17 mm s^–1, respectively. Also, the daytime average albedo of sunn hemp was around 19%. Finally, the high precipitation amount due to typhoons, high soil-water content, low available energy and low vapor-pressure deficit lead to decreasing trend of the energy fluxes during the generative phase of sunn hemp.     

Simulations of seasonal variations of sulfur compounds in the remote marine atmosphere

A photochemical box model is used to simulate seasonal variations in concentrations of sulfur compounds at latitude 40° S. It is assumed that the hydroxyl radical (OH) addition reaction to sulfur in the dimethyl sulfide (DMS) molecule is the predominant pathway for methanesulfonic acid (MSA) production, and that the rate constant increases as the air temperature decreases. Concentration of the nitrate radical (NO₃) is a function of the DMS flux, because the reaction of DMS with NO₃ is the most important loss mechanism of NO₃. While the diurnally averaged concentration of OH in winter is a factor of about 8 smaller than in summer, due to the weak photolysis process, the diurnally averaged concentration of NO₃ in winter is a factor of about 4–5 larger than in summer, due to the decrease of DMS flux. Therefore, at middle and high latitudes in winter, atmospheric DMS is mainly oxidized by the reaction with NO₃. The calculated ratio of the MSA to SO₂ production rates is smaller in winter than in summer, and the MSA to non-sea-salt sulfate (nssSO₄ ^2-) molar ratio varies seasonally. This result agrees with data on the seasonal variation of the MSA/nssSO₄ ^2- molar ratio obtained at middle and high latitudes. The calculations indicate that during winter the reaction of DMS with NO₃ is likely to be a more important sink of NO_x (NO+NO₂) than the reaction of NO₂ with OH, and to serve as a significant pathway of the HNO₃ production. If dimethyl sulfoxide (DMSO) is produced through the OH addition reaction and is heterogeneously oxidized in aqueous solutions, half of the nssSO₄ ^2- produced in summer may be through the oxidation process of DMSO. It is necessary to further investigate the oxidation products by the reaction of DMS with OH, and the possibility of the reaction of DMS with NO₃ during winter.     

Seasonal variations of spectra of wind speed and air temperature in the mesoscale frequency range

Seasonal variations of the spectra of wind speed and air temperature in the mesoscale frequency range from 1.3 × 10^-4 to 1.5 × 10^-3 Hz (10 min to 2 h periods) have been studied through observations over land for one year. Spectrographs [time series of isopleths of spectral densities, f · S(f) vs f] of wind speed and air temperature contain occasional peaks that are attributed to short-lived mesoscale atmospheric activity with narrow frequency bands. Significant spectral peaks of wind speed were found in 19% of the total observations in winter, and in 15–16% in the other seasons; for air temperature, they occured in 12% of observations in autumn, and in 16–19% in the other seasons. The peaks most often occurred in the period range from 30 min to 1 h; most had durations less than 24 h. Mesoscale fluctuations of wind speed and air temperature were highly correlated, and in most cases, phase differences were 90–180 ° with air temperature leading wind speed. Significant spectral peaks of wind speed often occurred during northerly seasonal cold winds in winter, and accompanied tropical and/or mid-latitude cyclones in the other seasons. When the peaks occurred, wind speed was usually relatively high and the atmospheric surface layer was unstable.     

Interactive behavior of soil–pile-superstructure system in transient state to liquefaction by means of large shake table tests

Shaking table tests were conducted by means of a large-scale laminar box with 4 m in length, 2 m in width and 2 m in height in order to investigate behavior of a soil-pile-superstructure system in liquefiable ground. A model two-storey structure, supported by a pile group, was set in a saturated sand deposit, and subjected to a sinusoidal base motion with increasing amplitude. Discussions are focused on the transient behavior until soil liquefaction occurs. Main interests are characteristics of springs used in a sway-rocking model and a multi-freedom lumped mass (MFLM) model that are frequently used in soil–pile interaction analysis. The spring constant in the sway-rocking model is represented by restoring force characteristics at the pile head, and that in the MFLM system is represented by an interaction spring connecting the pile to the free field. The transient state prior to soil liquefaction is shown to be important in the design of a pile because dynamic earth pressure shows peak response in this state. The reduction of the stiffness due to excess porewater generation and strain dependent nonlinear behavior is evaluated.     

MHD Dynamo Simulation Using the GeoFEM Platform: Comparison with a Spectral Method

— We investigate differences of results of a MHD dynamo simulation using the finite-element method (FEM) with that by the spherical harmonics expansion. The simulation was performed with a total of 7.8×10⁴ elements in the simulation domain. The results show that the magnetic energy is generated approximately four times of the kinetic energy in both cases. The outline of present results has consistence with that by the spectral method. However, small-scale patterns can be observed in the spectral method case. These discrepancies are caused by limitation of the spatial resolution in both of the present simulations.