Discovery of an ancient tsunami deposit in coastal sequences of southwest Japan: Verification of a large historic tsunami

Abstract A tsunamigenic sand layer is present in coastal sequences of the Masuda Plain, southwest Japan. The radiometric age of the layer has been estimated at 930 ± 80 years BP. It is proposed that the deposit is the product of a large historic tsunami believed to have occurred in the Japan Sea on 16 June 1026 AD.     

Hydrothermal carbonates in altered wall rocks at the Uwamuki Kuroko deposits, Japan

Magnesite, siderite and dolomite are characteristic alteration minerals occurring in Miocene hanging wall rocks of dacitic composition which host the Kuroko orebodies. These carbonates generally occur in a more stratigraphically upper horizon than chlorite alteration zone surrounding the orebodies. The Mg/(Mg+Fe) ratios of the carbonates decrease from the central alteration zone to marginal zone. The Mg/(Mg+Fe) ratios of carbonates and chlorite positively correlate. The δ¹⁸O and δ¹³C values of magnesite, siderite and dolomite positively correlate with each other and lie between the igneous and marine carbonate values. The petrographic, isotopic and fluid inclusion characteristics and thermochemical modelling calculations indicate that magnesite and dolomite formed in the central zone close to the orebodies due to the interaction of hydrothermal solutions with the biogenic marine carbonates. Calcite formed further from the orebodies from hydrothermal fluids which did not contain a biogenic marine carbon component. The compositional and textural relationships indicate that superimposed alterations (chlorite alteration and carbonate alteration) occurred in hanging wall rocks. The mode of occurrences and the Mg/(Mg+Fe) ratios of magnesite and dolomite occurring in hanging wallrocks are useful in the exploration for concealed volcanogenic massive sulfide-sulfate deposits. Received: 9 September 1997 / Accepted: 23 September 1997     

The detectability of shallow slow earthquakes by the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) in Tonankai district,Japan

In order to understand the characteristics of shallow very low-frequency (VLF) events as revealed by recent ocean-floor observation studies, we perform a trial simulation of earthquake cycles in the Tonankai district by taking the characteristics of the 1944 Tonankai earthquake and assuming that slow earthquakes occur on numerous small asperities. Our simulation results show that the increase of moment release rate of shallower VLF events in the pre-seismic stage of a megathrust earthquake is higher than that of deeper VLF events. This increase may make leveling change due to VLF swarms detectable at Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). We also introduce the time series of hydraulic pressure data at DONET, comparing with the leveling change expected from our numerical simulation. Since leveling change due to shallower VLF swarms is so local as to be incoherent, removal of the moving-averaged data from the data stacked by four nearby observation points in the same node may be useful to detect the short-term local leveling change.     

Hayabusa‐returned sample curation in the Planetary Material Sample Curation Facility of JAXA

Abstract– The Planetary Material Sample Curation Facility of JAXA (PMSCF/JAXA) was established in Sagamihara, Kanagawa, Japan, to curate planetary material samples returned from space in conditions of minimum terrestrial contaminants. The performances for the curation of Hayabusa‐returned samples had been checked with a series of comprehensive tests and rehearsals. After the Hayabusa spacecraft had accomplished a round‐trip flight to asteroid 25143 Itokawa and returned its reentry capsule to the Earth in June 2010, the reentry capsule was brought back to the PMSCF/JAXA and was put to a series of processes to extract recovered samples from Itokawa. The particles recovered from the sample catcher were analyzed by electron microscope, given their ID, grouped into four categories, and preserved in dimples on quartz slide glasses. Some fraction of them has been distributed for initial analyses at NASA, and will be distributed for international announcement of opportunity (AO), but a certain fraction of them will be preserved in vacuum for future analyses.     

Investigation of cutting methods for small samples of Hayabusa and future sample return missions

We report the investigation of cutting methods for Hayabusa samples. The purpose of our study is to explore the possibility of applying multiple analyses to a single particle effectively. We investigated the cutting performance of a blade dicing saw, laser, focused ion beam (FIB), and physical breaking by microindenter. Cutting performance was examined by estimating the aspect ratio of the cut slit, i.e., depth over width of the slit. We also investigated the possible contamination and sample damage by cutting. The result of the investigation shows that we can cut the samples from <50 μm to 500 μm using those methods with aspect ratios from 10 to 20, although they would introduce some contamination or damage to the samples. Our investigations also provide an important basis for the analysis of samples obtained by future sample return missions.     

Crystallization temperature determination of Itokawa particles by plagioclase thermometry with X‐ray diffraction data obtained by a high‐resolution synchrotron Gandolfi camera

The crystallization temperatures of Itokawa surface particles recovered by the space probe Hayabusa were estimated by a plagioclase geothermometer using sodic plagioclase triclinicity. The Δ131‐index required for the thermometer, which is the difference in X‐ray diffraction peak positions between the 131 and 11 reflections of plagioclase, was obtained by a high‐resolution synchrotron Gandolfi camera developed for the third generation synchrotron radiation beamline, BL15XU at SPring‐8. Crystallization temperatures were successfully determined from the Δ131‐indices for four particles. The observed plagioclase crystallization temperatures were in a range from 655 to 660 °C. The temperatures indicate crystallization temperatures of plagioclases in the process of prograde metamorphism before the peak metamorphic stage.     

Sublimation’s impact on temporal change of albedo dichotomy on Iapetus

Iapetus, one of the saturnian moons, has an extreme albedo contrast between the leading and trailing hemispheres. The origin of this albedo dichotomy has led to several hypotheses, however it remains controversial. To clarify the origin of the dichotomy, the key approach is to investigate the detailed distribution of the dark material. Recent studies of impact craters and surface temperature from Cassini spacecraft data implied that sublimation of H₂O ice can occur on Iapetus’ surface. This ice sublimation can change the albedo distribution on the moon with time.In this study, we evaluate the effect of ice sublimation and simulate the temporal change of surface albedo. We assume the dark material and the bright ice on the surface to be uniformly mixed with a certain volume fraction, and the initial albedo distribution to incorporate the dark material deposits on the surface. That is, the albedo at the apex is lowest and concentrically increases in a sinusoidal pattern. This situation simulates that dark materials existed around the Iapetus’ orbit billions of years ago, and the synchronously rotating Iapetus swept the material and then deposited it on its surface. The evolution of the surface albedo during 4.0 Gyr is simulated by estimating the surface temperature from the insolation energy on Iapetus including the effect of Saturn’s eccentricity and Iapetus’ obliquity precession, and evaluating the sublimation rate of H₂O ice from the Iapetus’ surface.As a result, we found that the distribution of the surface albedo changed dramatically after 4.0 Gyr of evolution. The sublimation has three important effects on the resultant surface albedo. First, the albedo in the leading hemisphere has significantly decreased to approach the minimum value. Second, the albedo distribution has been elongated along the equator. Third, the edge of the low albedo region has become clear. Considering the effect of ice sublimation, the current albedo distribution can be reconstructed from the sinusoidal albedo distribution, suggesting the apex-antapex cratering asymmetry as a candidate for the origin of the albedo dichotomy. From the model analysis, we obtained an important aspect that the depth of the turn-over layer where the darkening process proceeded for 4 Gyr should be an order of 10 cm, which is consistent with evaluation from the Cassini radar observations.