How hydrological factors initiate instability in a model sandy slope   总被引：1，自引：1，他引：0       下载免费PDF全文

Tomomi Terajima  Ei‐ichiro Miyahira  Hiroyuki Miyajima  Hirotaka Ochiai  Katsumi Hattori 《水文研究》2014,28(23):5711-5724

Knowledge of the mechanisms of rain‐induced shallow landslides can improve the prediction of their occurrence and mitigate subsequent sediment disasters. Here, we examine an artificial slope's subsurface hydrology and propose a new slope stability analysis that includes seepage force and the down‐slope transfer of excess shear forces. We measured pore water pressure and volumetric water content immediately prior to a shallow landslide on an artificial sandy slope of 32°: The direction of the subsurface flow shifted from downward to parallel to the slope in the deepest part of the landslide mass, and this shift coincided with the start of soil displacement. A slope stability analysis that was restricted to individual segments of the landslide mass could not explain the initiation of the landslide; however, inclusion of the transfer of excess shear forces from up‐slope to down‐slope segments improved drastically the predictability. The improved stability analysis revealed that an unstable zone expanded down‐slope with an increase in soil water content, showing that the down‐slope soil initially supported the unstable up‐slope soil; destabilization of this down‐slope soil was the eventual trigger of total slope collapse. Initially, the effect of apparent soil cohesion was the most important factor promoting slope stability, but seepage force became the most important factor promoting slope instability closer to the landslide occurrence. These findings indicate that seepage forces, controlled by changes in direction and magnitude of saturated and unsaturated subsurface flows, may be the main cause of shallow landslides in sandy slopes. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Long-term variability of the Kuroshio path south of Japan     

Norihisa Usui  Hiroyuki Tsujino  Hideyuki Nakano  Satoshi Matsumoto 《Journal of Oceanography》2013,69(6):647-670

This study investigates the long-term variability of the Kuroshio path south of Japan. Sensitivity experiments using a data-assimilative model suggest that the duration of the large meander (LM) strongly depends on the Kuroshio transport; specifically, low transport leads to a long duration of the LM. Actually, we find a good correlation between the duration of the past LMs and the Sverdrup transport estimated by a wind-driven linear baroclinic vorticity model. Then we explore favorable conditions for the LM and find a close relationship between the Kuroshio Extension (KE) state and the LM. That is, a precondition for the LM that the Kuroshio path on the Izu Ridge is fixed at a deep channel located around 34°N is achieved during a stable KE state. In addition, westward propagating signals with negative anomalies in the Kuroshio region and high sea-surface height (SSH) state east of Taiwan are key for generation of a small meander southeast of Kyushu that triggers a subsequent LM. The signals related to the above conditions change the upstream Kuroshio transport and velocity, which are consistent with features indicated by the former observational studies. Using reanalysis data, we construct long-time series of indices for the three conditions, which explain well the past LMs. The indices suggest that long-term non-LM states around 1970 and in the 1990s were attributed to a low-SSH state east of Taiwan and an unstable KE state, respectively.  相似文献   

Cliff retreat and talus development at the caldera wall of Mount St. Helens: Computer simulation using a mathematical model     

Hiroyuki Obanawa  Yukinori Matsukura 《Geomorphology》2008,97(3-4):697-711

To simulate the landform evolution at the caldera wall of Mount St. Helens, USA, a mathematical model for talus development was applied to model the topographic change during the 11years from the volcanic eruption, i.e., from formation of the cliff. Simulated results show that the topographic change is predicted to be large for about 10years after the eruption and to decline thereafter. If snow accumulation in the talus slope deposits is negligible, the talus top will not reach the cliff top within 300years after the eruption. Talus growth in Mount St. Helens was much faster than that in the Chichibu Basin, Japan. This may indicate the low strength and/or high weathering rate of the rockwall of Mount St. Helens, resulting in rapid production of debris and rapid retreat of the cliff.  相似文献   

Dating of altered mafic intrusions by applying a zircon fission track thermochronometer to baked country rock,and implications for the timing of volcanic activity during the opening of the Japan Sea          下载免费PDF全文

Hiroyuki Hoshi  Hideki Iwano  Tohru Danhara  Naoyoshi Iwata 《Island Arc》2015,24(2):221-231

Once a mafic intrusive rock has become altered, it is generally difficult to obtain a reliable intrusion age using conventional isotopic dating methods. To overcome this problem, this study used zircon fission track (ZFT) thermochronometry to determine the timing of crystallization of altered mafic intrusions. ZFT dating was carried out on samples of baked granite country rock adjacent to dolerite dikes (5–10 m thick) in the Takato area of central Japan. Three granite samples collected within 8 mm of a dike contact yielded consistent ZFT ages of 17–16 Ma, with confined track lengths indicative of the complete annealing of pre‐existing tracks by reheating due to dike intrusion. An older ZFT age was obtained for one granite sample collected within 20 mm of the contact, but confined track length measurements indicate that this is an incompletely reset age that lies between the ZFT age of the unbaked granitic country rocks (ca. 55 Ma) and the emplacement age of the dike. Petrographic examinations suggest that post‐intrusion hydrothermal activity did not influence the ZFT ages. We conclude that the 17–16 Ma ZFT age represents the emplacement age of the dikes. Our results show that ZFT dating of baked country rock is an effective tool for dating altered mafic intrusions, for which other dating techniques are not applicable. In the eastern part of Southwest Japan, dispersed volcanic activity occurred in the late Early to early Middle Miocene (18–15 Ma), and the volcanic belt extended into the forearc. This pulse of activity was possibly related to the injection of asthenospheric material into the trench‐side mantle wedge beneath the Japan arc. We also present young apatite fission track ages (ca. 4 Ma) that may reflect a Middle Miocene or later thermal event associated with local magmatic activity near the Takato area.  相似文献   

Reaction-induced grain boundary cracking and anisotropic fluid flow during prograde devolatilization reactions within subduction zones     

Atsushi?OkamotoEmail author  Hiroyuki?Shimizu  Jun-ichi?Fukuda  Jun?Muto  Takamoto?Okudaira 《Contributions to Mineralogy and Petrology》2017,172(9):75

Devolatilization reactions during prograde metamorphism are a key control on the fluid distribution within subduction zones. Garnets in Mn-rich quartz schist within the Sanbagawa metamorphic belt of Japan are characterized by skeletal structures containing abundant quartz inclusions. Each quartz inclusion was angular-shaped, and showed random crystallographic orientations, suggesting that these quartz inclusions were trapped via grain boundary cracking during garnet growth. Such skeletal garnet within the quartz schist formed related to decarbonation reactions with a positive total volume change (?V _t > 0), whereas the euhedral garnet within the pelitic schists formed as a result of dehydration reaction with negative ?V _t values. Coupled hydrological–chemical–mechanical processes during metamorphic devolatilization reactions were investigated by a distinct element method (DEM) numerical simulation on a foliated rock that contained reactive minerals and non-reactive matrix minerals. Negative ?V _t reactions cause a decrease in fluid pressure and do not produce fractures within the matrix. In contrast, a fluid pressure increase by positive ?V _t reactions results in hydrofracturing of the matrix. This fracturing preferentially occurs along grain boundaries and causes episodic fluid pulses associated with the development of the fracture network. The precipitation of garnet within grain boundary fractures could explain the formation of the skeletal garnet. Our DEM model also suggests a strong influence of reaction-induced fracturing on anisotropic fluid flow, meaning that dominant fluid flow directions could easily change in response to changes in stress configuration and the magnitude of differential stress during prograde metamorphism within a subduction zone.  相似文献   

Paleomagnetic constraints on Miocene rotation in the central Japan Arc     

Hiroyuki Hoshi  Masakazu Sano 《Island Arc》2013,22(2):197-213

We present new Middle Miocene paleomagnetic data for the central Japan Arc, and discuss their implications for Miocene rotation. To obtain a refined paleodirection, we made magnetic measurements on basaltic to andesitic lavas and intrusive rocks from 12 sites in the Tsugu volcanic rocks (ca 15 Ma) in the northern part of the Shitara area, Japan. Significant secondary magnetizations in samples with strong magnetic intensities are interpreted as lightning‐induced components. Mean directions carried by magnetite and/or titanomagnetite were determined for all sites. An overall mean direction with a northerly declination was obtained from dual‐polarity site means for nine sites. This direction is indistinguishable from the mean direction for coeval parallel dikes in the northern part of the Shitara area, and also indistinguishable from the Miocene reference direction derived from the paleopole for the North China Block in the Asian continent. These comparisons suggest little or no rotation or latitudinal motion in the study area with respect to the North China Block since 15 Ma. We obtained a refined early Middle Miocene paleodirection (D = 9.7°, I = 52.5°, α₉₅ = 4.8°; 30 sites) and paleopole (82.0°N, 230.8°E, A₉₅ = 5.6°) for Shitara by combining data from the Tsugu volcanic rocks and a coeval dike swarm. An anomalous direction found at three sites could be a record of an extraordinary field during a geomagnetic polarity transition or excursion. Paleomagnetic data from Shitara suggest that: (i) the western wing of the Kanto Syntaxis, a prominent cuspate geologic structure in central Honshu, underwent a counterclockwise rotation with respect to the main part of the southwestern Japan Arc between ca 17.5 Ma and 15 Ma; (ii) collision between the Japan and Izu–Bonin (Ogasawara) Arcs began prior to 15 Ma; and (iii) clockwise rotation of the entire southwestern part of the Japan Arc had ceased by 15 Ma.  相似文献   

Anomalous change in atmospheric radon concentration sourced from broad crustal deformation: A case study of the 1995 Kobe earthquake     

Yumi Yasuoka  Yusuke Kawada  Yasutaka Omori  Hiroyuki Nagahama  Tetsuo Ishikawa  Shinji Tokonami  Masahiro Hosoda  Tetsuo Hashimoto  Masaki Shinogi 《Applied Geochemistry》2012

Before the Kobe earthquake, an anomalous increase in atmospheric Rn concentration was observed. By separating the measured concentration of atmospheric Rn into three components according to the distance from the monitoring station, the variation of Rn exhalation rate can be estimated for the respective area using the daily minimum and maximum concentrations. The mean rate of Rn exhalation gradually increased in an area of 20 km around the monitoring station, becoming five times higher than normal in the period between October 1994 and the date of the earthquake. This area had a large co-seismic displacement of up to 30 cm, which roughly corresponds to the crustal strain of 10⁻⁶-order, and it is considered the main source for the atmospheric Rn prior to the Kobe earthquake. Analyses revealed that the pre-seismic change in the atmospheric Rn concentration exhibited an anomalous pattern which would yield information on the spatial distribution of the mechanical response of the ground.  相似文献   

Bioaccumulation of nitroarenes in bivalves at Osaka Bay, Japan     

Uno S  Tanaka H  Miki S  Kokushi E  Ito K  Yamamoto M  Koyama J 《Marine pollution bulletin》2011,63(5-12):477-481

This paper reports for the first time the detection and occurrence of nitroarenes (NPAHs) in marine organisms. Mussels and oysters collected from Osaka Bay, Japan, had total NPAHs concentrations that ranged from 2380 to 24,688 pg/g dry and 2672 to 25,961 pg/g dry, respectively. Relatively higher concentrations were detected in sampling sites located near the central district and suburbs of Osaka City implying that the most probable sources of NPAHs in the two bivalves are exhaust gases and smokes emitted by automobiles and industrial plants. Bivalves had relatively higher residues of 1-nitronaphthalene, 2-nitronaphthalene, 3-nitrophenanthrene, and 9-nitrophenanthrenes. Residues of 2-nitrofluorene, 1-nitropyrene, 4-nitropyrenes, and 6-nitrochrysene were much lower compared to nitronaphthalenes and nitrophenanthrenes. Inter-species differences was only observed for 2-nitronaphthalene with oysters exhibiting significantly higher residues than mussels.  相似文献   

Shaking Maps for Scenario Earthquakes by Applying the Upgraded Version of the Strong Ground Motion Prediction Method ��Recipe��     

Nobuyuki Morikawa  Shigeki Senna  Yuzuru Hayakawa  Hiroyuki Fujiwara 《Pure and Applied Geophysics》2011,168(3-4):645-657

The strong ground motion prediction method ??Recipe?? was published by the Headquarters for Earthquake Research Promotion (HERP) of Japan. HERP has applied this method to prepare shaking maps for scenario earthquakes in specific active faults. Recently, Recipe was updated following its verification by simulations of strong ground motions associated with the Mw?=?6.6 off-shore earthquake west of Fukuoka prefecture in 2005, which occurred in the northwest part of the Kego fault zone located in northern Kyushu, Japan. One of the prominent changes in the upgraded version of Recipe is the inclusion of a procedure to evaluate seismic intensities on the ground surface from waveforms of S-wave velocity of 400?m/s on the engineering bedrock. By applying the upgraded version of Recipe, we have made shaking-maps for earthquakes in the southeast part of the Kego fault zone, which is located directly below the mega-city of Fukuoka. We assume four source models for scenario earthquakes; the locations of the asperities and the hypocenters vary between the models. The results show that in all cases, disastrous seismic intensities can strike a wide area of Fukuoka city. Differences in the distributions of seismic intensities among the four cases can be clearly observed in the area located on the extension of the source fault. Furthermore, we construct a velocity-layer structure model on the engineering bedrock for the central area of Fukuoka city. We assess not only the distribution of seismic intensities but also waveforms on the ground by using an equivalent linear method for the central area of Fukuoka city.  相似文献   

Chemostratigraphic documentation of a complete Miocene intermediate-depth section in the Southern Ocean: Ocean Drilling Program Site 1120, Campbell Plateau off New Zealand     

Atsushi Ando  Boo-Keun Khim  Takanori Nakano  Hiroyuki Takata 《Marine Geology》2011,279(1-4):52-62

An integrated chemostratigraphic (⁸⁷Sr/⁸⁶Sr, δ¹³C and δ¹⁸O) study of benthic foraminifera is presented for a 210 m-thick, intermediate depth (upper/middle bathyal transition), Miocene nannofossil ooze section of Ocean Drilling Program Site 1120, Campbell Plateau off New Zealand. Our results indicate that new ⁸⁷Sr/⁸⁶Sr, δ¹³C and δ¹⁸O profiles are wholly consistent with their respective Miocene reference curves. These observations facilitate identification of a total of five reliable chemostratigraphic datums, which are based on the fundamental structural changes in the ⁸⁷Sr/⁸⁶Sr curve and paired simultaneous δ¹³C and δ¹⁸O events. The resultant age–depth relationship clearly shows that the Miocene (20–5 Ma) biopelagic sedimentation on the Campbell Plateau was essentially continuous at a moderate to high, linear sedimentation rate (17.5 m/m.y. with an exception of the uppermost 13 m). Our findings do not support the shipboard biostratigraphic age model, which assumes that the critical early–middle Miocene transition was interrupted by a major hiatus (<~3 m.y.). Because of its unique bathymetric setting at a paleowater-depth of ~ 600 m, which is among the shallowest of the coeval isotopically studied deep-sea sections in the South Pacific/Southern Ocean, Site 1120 will serve as a reference section for surveying the evolution of intermediate-water paleoceanography in the Southern Hemisphere across the middle Miocene climatic transition.  相似文献