Recent and historical tsunami deposits from Lake Tokotan,eastern Hokkaido,Japan, inferred from nondestructive,grain size,and radioactive cesium analyses

Geological evidence of recent tsunamis from sediment samples collected from Lake Tokotan, a coastal lagoon in eastern Hokkaido, northern Japan, was detected using computed tomography (CT) and soft X-ray images, grain size, and radionuclide profiles. Initial field observations revealed that sediments had no discernable sedimentary structures at the top of the core. However, results of CT imaging, soft X-ray, and grain size analyses show evidence for three invisible sand layers that are intercalated with mud layers. These sand layers exhibit trends of landward fining and thinning. Furthermore, the distribution of sand layers was limited to the center and seaward parts of the lake. Vertical profiles of cesium and lead concentrations in combination with recent eyewitness accounts indicated that these sand layers are correlated with the 1973 Nemuro-oki, 1960 Chilean, and 1952 Tokachi-oki tsunami events. The deeper part of the sediment cores includes three volcanic ash layers and three prehistoric coarse sand layers. The prehistoric layers are correlated with unusually large tsunamis that were geologically identified in previous studies from eastern Hokkaido. These findings suggest that nondestructive techniques, in combination with radionuclide analysis, allow for detection of frequent but faint tsunami deposits. This technique allows for an improved understanding of the history of tsunami inundation in Lake Tokotan and of other locations for which stratigraphic evidence for faint tsunamis layers is not readily apparent from field assessments.

     

structure of south-western Honshu, Japan and the nature of the Mohorovicić discontinuity

Summary. Two record sections have been prepared for a series of explosions near Lake Biwa, south-western Honshu, Japan. The record sections cover both the array stations and the network of microearthquake stations that are distributed throughout the area. This enabled us to detect for the first time in the Japanese islands the phase reflected at the Mohorovicić discontinuity. Until now the weakness and obscurity of the phase has made it difficult to detect. We were able to overcome this problem by processing the seismograms to enhance the phase. Our work leads us to propose that the surface of the Mohorović discontinuity may be divided into pieces by a number of deep faults, while a thin layer of ultrabasic rock in the lower crust may account for the weakness of the signal.     

Relationship of Mn-carbonates in varved lake-sediments to catchment vegetation in Big Watab Lake, MN, USA

Mn-carbonates are documented in the late-glacial varved sediments from Big Watab Lake, Minnesota, USA. The Mn-carbonate is authigenic and forms rims around contemporaneous epilimnetic calcite. Although such carbonates are found in minor amounts throughout the entire late-glacial sequence, significant quantities of Mn-carbonate are associated mainly with laminated intervals.Because of the suspected difference in isotopic fractionation between different carbonate minerals, the stable-isotopic compositions of bulk carbonate samples are used as a proxy for relative amounts of the Mn-carbonate in the sediment. High ¹⁸O and low ¹³C values are associated with abundant Mn-carbonates. Low ¹⁸O and high ¹³C values are associated with only minor concentrations of Mn-carbonates.The oxygen-18 record is correlated with fluctuations in the vegetation assemblage based on pollen spectra using a multiple regression model with backward elimination. The proposed link between the sedimentary archive and local vegetation is the mediation of advective mixing in the lake by forest composition. In this model, periods of forest closure resulted in a well-stratified water column that was anoxic at the sediment/water interface, permitting the formation of authigenic Mn-carbonates. Openings of Artemisia in the forest allowed wind shear to mix oxygen to depth, causing bioturbation of the laminations and preventing the formation of Mn-carbonate.     

http://www.sciencedirect.com/science/article/pii/S167498711000037X

The tried and tested multianvil apparatus has been widely used for high-pressure and high-temperature experimental studies in Earth science. As a result, many important results have been obtained for a better understanding of the components, structure and evolution of the Earth. Due to the strength limitation of materials, the attainable multianvil pressure is generally limited to about 30 GPa (corresponding to about 900 km of the depth in the Earth) when tungsten carbide cubes are adopted as second-stage anvils. Compared with tungsten carbide, the sintered diamond is a much harder material. The sintered diamond cubes were introduced as second-stage anvils in a 6–8 type multianvil apparatus in the 1980s, which largely enhanced the capacity of pressure generation in a large volume press. With the development of material synthesis and processing techniques, a large sintered diamond cube (14 mm) is now available. Recently, maximum attainable pressures reaching higher than 90 GPa (corresponding to about 2700 km of the depth in the Earth) have been generated at room temperature by adopting 14-mm sintered diamond anvils. Using this technique, a few researches have been carried out by the quenched method or combined with synchrotron radiation in situ observation. In this paper we review the properties of sintered diamond and the evolution of pressure generation using sintered diamond anvils. As-yet unsolved problems and perspectives for uses in Earth Science are also discussed.     

Characteristics of evapotranspiration from a permafrost black spruce forest in interior Alaska

Here, the year 2011 characteristics of evapotranspiration and the energy budget of a black spruce forest underlain by permafrost in interior Alaska were explored. Energy balance was nearly closed during summer, and the mean value of the daily energy balance ratio (the ratio of turbulent energy fluxes to available energy) from June to August was 1.00, though a large energy balance deficit was observed in the spring. Such a deficit was explained partly by the energy consumed by snowmelt. Ground heat flux played an important role in the energy balance, explaining 26.5% of net radiation during summer. The mean daily evapotranspiration of this forest during summer was 1.37 mm day^?1 – considered typical for boreal forests. The annual evapotranspiration and sublimation yielded 207.3 mm year^?1, a value much smaller than the annual precipitation. Sublimation accounted for 8.8% (18.2 mm year^?1) of the annual evapotranspiration and sublimation; thus, the sublimation is not negligible in the annual water balance in boreal forests. The daytime average decoupling coefficient was very small, and the mean value was 0.05 during summer. Thus, evapotranspiration from this forest was mostly explained by the component from the dryness of the air, resulting from the aerodynamically rough surface of this forest.     

Dynamic analysis of ground with rigorous use of strain dependency and its application to seismic microzonation of alluvial plane

Seismic microzonation is one of the most important measures to mitigate earthquake hazards in urban areas. Because the ground motion varies significantly with the subsurface geology, it is needed for microzonation to account as much as possible for the local soil conditions. Noteworthy is that nonlinear deformation properties of soil play essential roles in amplification of strong ground motion. It is desired furthermore to focus on the expected damage extent in addition to the calculated maximum acceleration and/or velocity. The present study first developed a computer code for one-dimensional response analysis of ground that reasonably takes into account nonlinear dynamic soil properties. Second, correlations between the calculated ground motion and damage extent were obtained by examining seismic damages during the past earthquakes. By combining these two issues, seismic microzonation was carried out, and detailed damage distribution was assessed. The product of this study covers not only the damage caused by ground shaking but also liquefaction problem and lifeline damage.     

Structural changes of synthetic opal by heat treatment

The structural changes of synthetic opal by heat treatment up to 1,400 °C were investigated using scanning electron microscopy, X-ray diffraction, and Fourier transform infrared and Raman spectroscopies. The results indicate that the dehydration and condensation of silanol in opal are very important factors in the structural evolution of heat-treated synthetic opal. Synthetic opal releases water molecules and silanols by heat treatment up to 400 °C, where the dehydration of silanol may lead to the condensation of a new Si–O–Si network comprising a four-membered ring structure of SiO₄ tetrahedra, even at 400 °C. Above 600 °C, water molecules are lost and the opal surface and internal silanol molecules are completely dehydrated by heat effect, and the medium-temperature range structure of opal may begin to thermally reconstruct to six-membered rings of SiO₄ tetrahedra. Above 1,000 °C, the opal structure almost approaches that of silica glass with an average structure of six-membered rings. Above 1,200 °C, the opal changes to low-cristobalite; however, minor evidence of low-tridymite stacking was evident after heat treatment at 1,400 °C.