Landslide and debris-flow hazard analysis and prediction using GIS in Minamata–Hougawachi area, Japan

On July 20, 2003, following a short duration of heavy rainfall, a debris-flow disaster occurred in the Minamata–Hougawachi area, Kumamoto Prefecture, Japan. This disaster was triggered by a landslide. In order to assess the landslide and debris-flow hazard potential of this mountainous region, the study of historic landslides is critical. The objective of the study is to couple 3D slope-stability analysis models and 2D numerical simulation of debris flow within a geographical information systems in order to identity the potential landslide-hazard area. Based on field observations, the failure mechanism of the past landslide is analyzed and the mechanical parameters for 3D slope-stability analysis are calculated from the historic landslide. Then, to locate potential new landslides, the studied area is divided into slope units. Based on 3D slope-stability analysis models and on Monte Carlo simulation, the spots of potential landslides are identified. Finally, we propose a depth-averaged 2D numerical model, in which the debris and water mixture is assumed to be a uniform continuous, incompressible, unsteady Newtonian fluid. The method accurately models the historic debris flow. According to the 2D numerical simulation, the results of the debris-flow model, including the potentially inundated areas, are analyzed, and potentially affected houses, river and road are mapped.     

Nankai Trough and Zenisu Ridge: a deep-sea submersible survey

Eight submersible dives between 3000 and 4200 m water depth were made off southern Japan in the eastern Nankai subduction zone. Benthic communities associated with chemosynthetic processes were discovered along the 800 m wide active tectonic zone, at the toe of the accretionary prism. A benthic community was also discovered along a zone of active compression, at the foot of Zenisu Ridge, 30 km south of Nankai Trough. Temperature measurements within the sediments below the benthic communities confirm that upward motion of interstitial water occurs there. Studies of water samples indicate advection of methane and light hydrocarbons. Specimens of the benthic community have been shown to have included in their shells carbonate resulting from methane consumption. Thus the benthic communities are related to overpressure-driven fluid advection along tectonic zones with active surface deformation. A 300 m high active scarp at the toe of the accretionary prism is related to relative motion in a 280° direction which is close to the 305° average direction of subduction in this area. The dives establish further that compressive deformation is presently occurring at the foot of Zenisu Ridge. The previous interpretation of the Zenisu Ridge as a zone of recent north-south intraplate shortening, 40 km south of the Nankai Trench, is confirmed. We conclude that tectonic evolution might well lead to future detachment of the Zenisu Ridge and overthrusting of this large piece of oceanic crust over the continental margin. Such a process might be an efficient one to emplace ophiolites over continents.     

Latitudinal and downcore (0–750 ka) changes in n-alkane chain lengths in the eastern equatorial Pacific

The n-alkane C₃₁/(C₂₉ + C₃₁) ratios from surface sediments in the eastern equatorial Pacific (EEP) exhibit higher values to the north and lower values to the south across the southern edge (2–4°N) of the Intertropical Convergence Zone (ITCZ). Since plants tend to synthesize longer chain length n-alkanes in response to elevated temperature and/or aridity, the higher C₃₁/(C₂₉ + C₃₁) ratios at northern sites suggest a higher contribution of vegetation under hot and/or dry conditions. This is consistent with the observation that northern sites receive higher levels of plant waxes transported by northeasterly trade winds from northern South America, where hot and dry conditions prevail. Furthermore, from a sediment core covering the past 750 ka (core HY04; 4°N, 95°W) we found that C₃₁/(C₂₉ + C₃₁) ratios exhibit a long-term decrease from MIS (marine oxygen isotope stage) 17 to 13. During this period, the zonal SST (sea-surface temperature) gradient in the equatorial Pacific increased, suggesting an increase in Walker circulation. Such intensified Walker circulation may have enhanced moisture advection from the equatorial Atlantic warm pool to the adjacent northern South America, causing arid regions in northern South America to contract, which may explain long-term decrease in n-alkane chain lengths.     

Accreting white dwarfs as supersoft X‐ray sources

I review various phenomena associated with mass‐accreting white dwarfs (WDs) in the view of supersoft X‐ray sources. When the mass‐accretion rate is low (Ṁ_acc < a few × 10^–7 M⊙yr^–1), hydrogen nuclear burning is unstable and nova outbursts occur. A nova is a transient supersoft X‐ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X‐ray turn on/off time is a good indicator of the WD mass. At an intermediate mass‐accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass‐accretion rate, the WD undergoes “accretion wind evolution” in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions. Two SSS, namely RX J0513‐6951 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light‐curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X‐rays. Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Kalman filter finite element method applied to dynamic ground motion

The purpose of this paper is to investigate the estimation of dynamic elastic behavior of the ground using the Kalman filter finite element method. In the present paper, as the state equation, the balance of stress equation, the strain–displacement equation and the stress–strain equation are used. For temporal discretization, the Newmark ¼ method is employed, and for the spatial discretization the Galerkin method is applied. The Kalman filter finite element method is a combination of the Kalman filter and the finite element method. The present method is adaptable to estimations not only in time but also in space, as we have confirmed by its application to the Futatsuishi quarry site. The input data are the measured velocity, acceleration, etc., which may include mechanical noise. It has been shown in numerical studies that the estimated velocity, acceleration, etc., at any other spatial and temporal point can be obtained by removing the noise included in the observation. Copyright © 2008 John Wiley & Sons, Ltd.     

Crustal movements in the tohoku district, Japan, during the period 1900–1975, and their tectonic implications

Vertical crustal movements in the Tohoku district in the past 75 years are discussed with reference to their tectonic implications.

For this purpose, the author first compiled a map of accumulated vertical movement in the past 75 years, by making proper correction for closure. In addition, a new presentation technique utilizing a time-space domain representation of elevation changes is applied to the data. These contour maps provide an informative summary of the vertical crustal movement history in this district. From them, we see that the northeastern part of the district has subsided continuously and aseismically, with an area of significant subsidence inland at a distance about 300 km or more from the trench axis.

In order to explain the mechanism of the remarkable extension of the subsiding area, the finite element method is applied to model the elastic strain field in the district. It is shown that a simple model of uniform dragging at the interface of a sinking slab does not provide a good interpretation of the inland distribution of subsidence. A modification of the conventional model is proposed which hypothesizes vertical movement of the subducting lithospheric slab under the Tohoku district.     

Fault system of the 2004 Mid Niigata Prefecture Earthquake and its aftershocks

Data recorded by a seismic network deployed the day after the 2004 Mid Niigata Prefecture Earthquake (M6.8) in central Japan are used to determine the major source faults responsible for the mainshock and major aftershocks. Using this high-resolution seismic data, three major source faults are identified: two parallel faults dipping steeply to the west located 5 km apart, and the other dipping eastward and oriented perpendicular to the west-dipping faults. The analysis also reveals that the lateral variation in seismic velocity observed at the surface extends to a depth of 15 km, encompassing the source area of the mainshock. This strong heterogeneity of the crust, related to the complex geological and tectonic evolution of the area, is considered to be responsible for the prominent aftershock activity following the 2004 Niigata event.     

K-Ar dating studies of Ashigawa and Tokuwa granodiorite bodies and plutonic geochronology in the South Fossa Magna, central Japan

Abstract K-Ar age studies in the Ashigawa and the southern part of the Tokuwa granodioritic bodies, which consist of the southern part of the Kofu plutonic complex, revealed that they formed between 12 and 9 Ma. Quite a narrow range of ages obtained from the Ashigawa southernmost part of the Tokuwa pluton implies that they cooled rapidly. The southern part of the Tokuwa pluton, as a whole, shows a systematic age distribution with a decrease in age to the north. Compilation of currently available plutonic ages in the South Fossa Magna suggests that the plutonic activities occurred three times in this region. Episodic activity like this could be argued in relation to the tectonic development of this region.     

Lake Suwa — Eutrophication and its partial recent recovery

Lake Suwa, located in the center of the largest Japanese island of Honshu was a typical hypertrophic lake with a dense scum Microcystis species in summer during the 1970's. However, due to the introduction of a sewage treatment plant and the awareness of environmental pollution by local residents, a decrease in the concentration of nutrients in lake water since 1981 has been observed (from 1600 g l^–1 and 160 g l^–1 in 1977 to 990 g l^–1 and 110 g l^–1 in 1984 for total nitrogen and phosphorus respectively).