慢地震与深部不连续颤动和慢滑事件同时发生

我们对日本西南部南海俯冲板块界面过渡带发生的超低频地震进行报道。由矩震级为3.1～3.5的超低频地震引起的地震波显示其卓越长周期约为20s。超低频地震活动伴随深源低频颤动和慢滑事件同时发生,并随之移动。慢地震一直被认为会增加向上倾斜的大型逆冲地震断裂带上的应力,而这3种现象的同时发生提高了人们对慢地震的探测和特性鉴定水平。     

A hybrid approach for deriving horizontal site amplification factors considering both the similarity of HVSRE and the vertical amplification correction function

Under hypothetical and idealized conditions, the horizontal site amplification factor (HSAF) is defined as the ratio of the horizontal Fourier amplitude spectrum (FAS) at a point on the Earth's surface with respect to the horizontal FAS at the identical location but on the outcropping seismological bedrock. The HSAF reflects the profile of local sedimentary soils and weathered rock formations, indicating site effects. In most cases, such an idealized HSAF is difficult to measure directly. Thus, quantitatively estimating a HSAF value is a crucial task in strong-motion prediction over the last century. Fortunately, many strong-motion stations have been constructed throughout Japan, facilitating the characterization of HSAFs down to seismological bedrock at more than 1600 observational sites by the generalized inversion technique (GIT). First, this study reported the similarity distribution of the HSAF and the horizontal-to-vertical spectral ratio of earthquakes (HVSR_E). Subsequently, we proposed a hybrid method from a novel aspect for estimating HSAF in terms of the observed similarity distribution and the vertical amplification correction function (VACF) proposed previously. Compared with the direct use of HVSR_E for substituting HSAF, the hybrid method proposed herein demonstrated an improvement of greater than 30% in terms of the residuals between estimated HSAFs and those separated from observations.     

Further characterization of carbonaceous materials in Hayabusa‐returned samples to understand their origin

Carbonaceous materials in the sample catcher of the Hayabusa spacecraft were assigned as category 3 particles. We investigated the category 3 particles with a suite of in situ microanalytical methods. Possible contaminants collected from the cleanrooms of the spacecraft assembly and extraterrestrial sample curation center (ESCuC) were also analyzed in the same manner as category 3 particles for comparison. Our data were integrated with those of the preliminary examination team for category 3 particles. Possible origins for the category 3 particles include contamination before and after the operation of the Hayabusa spacecraft.     

Compression mechanism and amorphization of portlandite, Ca(OH)2: structural refinement under pressure

In order to investigate compression mechanism and the pressure-induced amorphization of portlandite, Ca(OH)₂, the crystal structure has been refined up to 9.7?GPa using Rietveld analysis. Angular-dispersive synchrotron X-ray powder diffraction experiments were performed using a diamond anvil cell and an imaging plate at BL-18C in the Photon Factory at KEK, Japan. Compression behavior is highly anisotropic and the c axis is approximately 2.5 times as compressible as the a axis (β_a=0.004, β_c=0.011?GPa^?1). Because the refined fractional coordinate, z, of the O atom increases linearly with pressure, compression along the c axis is due to the shortening of the interlayer spacing. The compression mechanism shows no change up to the amorphization pressure and is basically the same as that of brucite, Mg(OH)₂, observed below 10 GPa. The octahedral regularity of CaO₆ approaches a regular configuration with pressure. The interlayer O…O distance is expected to be about 2.75 Å at the amorphization pressure and should affect hydrogen bonding.     

Multiple temporal scale variability during the twentieth century in global carbon dynamics simulated by a coupled climate–terrestrial carbon cycle model

A coupled climate–carbon cycle model composed of a process-based terrestrial carbon cycle model, Sim-CYCLE, and the CCSR/NIES/FRCGC atmospheric general circulation model was developed. We examined the multiple temporal scale functions of terrestrial ecosystem carbon dynamics induced by human activities and natural processes and evaluated their contribution to fluctuations in the global carbon budget during the twentieth century. Global annual net primary production (NPP) and heterotrophic respiration (HR) increased gradually by 6.7 and 4.7%, respectively, from the 1900s to the 1990s. The difference between NPP and HR was the net carbon uptake by natural ecosystems, which was 0.6 Pg C year^?1 in the 1980s, whereas the carbon emission induced by human land-use changes was 0.5 Pg C year^?1, largely offsetting the natural terrestrial carbon sequestration. Our results indicate that monthly to interannual variation in atmospheric CO₂ growth rate anomalies show 2- and 6-month time lags behind anomalies in temperature and the NiNO₃ index, respectively. The simulated anomaly amplitude in monthly net carbon flux from terrestrial ecosystems to the atmosphere was much larger than in the prescribed air-to-sea carbon flux. Fluctuations in the global atmospheric CO₂ time series were dominated by the activity of terrestrial vegetation. These results suggest that terrestrial ecosystems have acted as a net neutral reservoir for atmospheric CO₂ concentrations during the twentieth century on an interdecadal timescale, but as the dominant driver for atmospheric CO₂ fluctuations on a monthly to interannual timescale.     

Dating late Quaternary events by the combined U‐Pb LA‐ICP‐MS and (U‐Th)/He dating of zircon: A case study on Omachi Tephra suite (central Japan)

Dating of young (<1 Ma) geological events has long been a challenge for geochronologists. Combining (U‐Th)/He with U‐Pb or U‐Th‐disequilibrium dating methods offers a unique dating tool that can address this important period. We present a new methodology that combines U‐Pb LA‐ICP‐MS and (U‐Th)/He dating of zircon and use it to date two Pleistocene marker tephras (A1Pm and DPm) from the Omachi Tephra suite (Japan). A1Pm and DPm yield U‐Pb ages in the range of 350–850 and ~140–630 ka, respectively, documenting protracted periods of zircon crystallisation (100's of k.y.) prior to eruption. (U‐Th)/He ages constrain the eruption ages of the A1Pm and DPm tephras to 375 ± 13 and 97.1 ± 7.3 ka, respectively, and are in agreement with published estimates. This study demonstrates the potential of combined zircon U‐Pb LA‐ICP‐MS and (U‐Th)/He dating to constrain magmatic and eruption events in the critical ~100 ka–1 Ma interval.     

Quasi-periodic dust events in the summertime south polar region of Mars

A Hovmöller diagram analysis of the dust optical depth measured by the Mars Global Surveyor Thermal Emission Spectrometer shows the occurrence of quasi-periodic westwardly-propagating disturbances with timescales of 10-20 sols during summer in the south polar region of Mars. Dust clouds emerge repeatedly around the region with a latitude of around 70-80°S and a longitude of 240-300°E, move westward at speeds of 3-6 m s⁻¹, reach the region with a longitude of 60-120°E, and finally disappear. This longitude range coincides with elevated terrains in the south polar region, and in this region an increase of dust optical depth encircling the south pole is also observed. This implies that the quasi-periodic dust events will contribute to the enhancement of the atmospheric dust loading in this region. These dust events might be related to baroclinic instability caused by the thermal contrast across the CO₂ cap edge, or the horizontal advection or vertical convection with radiative-dynamical feedback. The westward movement of the dust clouds suggests steady westward winds blowing in the near-surface layer, where the quasi-periodic dust lifting is expected to occur. Such a westward cap-edge flow will be created by the Coriolis force acting on the flow from the ice side to the regolith side.     

Phase relations in the carbon-saturated C–Mg–Fe–Si–O system and C and Si solubility in liquid Fe at high pressure and temperature: implications for planetary interiors

The phase and melting relations of the C-saturated C–Mg–Fe–Si–O system were investigated at high pressure and temperature to understand the role of carbon in the structure of the Earth, terrestrial planets, and carbon-enriched extraterrestrial planets. The phase relations were studied using two types of experiments at 4 GPa: analyses of recovered samples and in situ X-ray diffractions. Our experiments revealed that the composition of metallic iron melts changes from a C-rich composition with up to about 5 wt.% C under oxidizing conditions (ΔIW = ?1.7 to ?1.2, where ΔIW is the deviation of the oxygen fugacity (fO₂) from an iron-wüstite (IW) buffer) to a C-depleted composition with 21 wt.% Si under reducing conditions (ΔIW < ?3.3) at 4 GPa and 1,873 K. SiC grains also coexisted with the Fe–Si melt under the most reducing conditions. The solubility of C in liquid Fe increased with increasing fO₂, whereas the solubility of Si decreased with increasing fO₂. The carbon-bearing phases were graphite, Fe₃C, SiC, and Fe alloy melt (Fe–C or Fe–Si–C melts) under the redox conditions applied at 4 GPa, but carbonate was not observed under our experimental conditions. The phase relations observed in this study can be applicable to the Earth and other planets. In hypothetical reducing carbon planets (ΔIW < ?6.2), graphite/diamond and/or SiC exist in the mantle, whereas the core would be an Fe–Si alloy containing very small amount of C even in the carbon-enriched planets. The mutually exclusive nature of C and Si may be important also for considering the light elements of the Earth’s core.     

Raman spectra and X-ray diffraction of tuite at various temperatures

High-temperature Raman spectra and thermal expansion of tuite, γ-Ca₃(PO₄)₂, have been investigated. The effect of temperature on the Raman spectra of synthetic tuite was studied in the range from 80 to 973 K at atmospheric pressure. The Raman frequencies of all observed bands for tuite continuously decrease with increasing temperature. The quantitative analysis of temperature dependence of Raman bands indicates that the changes in Raman frequencies for stretching modes (ν₃ and ν₁) are faster than those for bending modes (ν₄ and ν₂) of PO₄ in the present temperature range, which may be attributed to the structural evolution of PO₄ tetrahedron in tuite at high temperature. The thermal expansion of tuite was examined by means of in situ X-ray diffraction measurements in the temperature range from 298 to 923 K. Unit cell parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 3.67 (3), 1.18 (1), and 1.32 (3) × 10^?5 K^?1 for V, a, and c, respectively. Thermal expansion of tuite shows an axial anisotropy with a larger expansion coefficient along the c-axis. The isothermal and isobaric mode Grüneisen parameters and intrinsic anharmonicity of tuite have been calculated by using present high-temperature Raman spectra and thermal expansion coefficient combined with previous results of the isothermal bulk modulus and high-pressure Raman spectra.     

Application of chemical tracers to an estimate of benthic denitrification in the Okhotsk Sea

To estimate benthic denitrification in a marginal sea, we assessed the usefulness of \({\text{N}}_{2}^{*}\) , a new tracer to measure the excess nitrogen gas (N₂) using dissolved N₂ and argon (Ar) with N* in the intermediate layer (26.6–27.4σ _θ ) of the Okhotsk Sea. The examined parameters capable of affecting \({\text{N}}_{2}^{*}\) are denitrification, air injection and rapid cooling. We investigated the relative proportions of these effects on \({\text{N}}_{2}^{*}\) using multiple linear regression analysis. The best model included two examined parameters of denitrification and air injection based on the Akaike information criterion as a measure of the model fit to data. More than 80 % of \({\text{N}}_{2}^{*}\) was derived from the denitrification, followed by air injection. Denitrification over the Okhotsk Sea shelf region was estimated to be 5.6 ± 2.4 μmol kg^?1. The distribution of \({\text{N}}_{2}^{*}\) was correlated with potential temperature (θ) between 26.6 and 27.4σ _θ (r = ?0.55). Therefore, we concluded that \({\text{N}}_{2}^{*}\) and N* can act complementarily as a quasi-conservative tracer of benthic denitrification in the Okhotsk Sea. Our findings suggest that \({\text{N}}_{2}^{*}\) in combination with N* is a useful chemical tracer to estimate benthic denitrification in a marginal sea.