Deformation mechanisms and fluid behavior in a shallow, brittle fault zone during coseismic and interseismic periods: Results from drill core penetrating the Nojima Fault, Japan

Abstract This paper describes the results of petrographical and meso- to microstructural observations of brittle fault rocks in cores obtained by drilling through the Nojima Fault at a drilling depth of 389.52 m. The zonation of deformation and alteration in the central zone of the fault is clearly seen in cores of granite from the hanging wall, in the following order: (i) host rock, which is characterized by some intragranular microcracks and in situ alteration of mafic minerals and feldspars; (ii) weakly deformed and altered rocks, which are characterized by transgranular cracks and the dissolution of mafic minerals, and by the precipitation of zeolites and iron hydroxide materials; (iii) random fabric fault breccia, which is characterized by fragmentation, by anastomosing networks of transgranular cracks, and by the precipitation of zeolites and iron hydroxide materials; and (iv) fault gouge, which is characterized by the precipitation of smectite and localized cataclastic flow. This zonation implies that the fault has been weakened gradually by fluid-related fracturing over time. In the footwall, a gouge layer measuring only 15 mm thick is present just below the surface of the Nojima Fault. These observations are the basis for a model of fluid behavior along the Nojima Fault. The model invokes the percolation of meteoric fluids through cracks in the hanging wall fault zone during interseismic periods, resulting in chemical reactions in the fault gouge layer to form smectite. The low permeability clay-rich gouge layer sealed the footwall. The fault gouge was brecciated during coseismic or postseismic periods, breaking the seal and allowing fluids to readily flow into the footwall, thus causing a slight alteration. Chemical reactions between fluids and the fault breccia and gouge generated new fault gouge, which resealed the footwall, resulting in a low fluid condition in the footwall during interseismic periods.     

Internal structure of the Nojima Fault zone from the Hirabayashi GSJ drill core

Abstract The internal structures of the Nojima Fault, south-west Japan, are examined from mesoscopic observations of continuous core samples from the Hirabayashi Geological Survey of Japan (GSJ) drilling. The drilling penetrated the central part of the Nojima Fault, which ruptured during the 1995 Kobe earthquake (Hyogo-ken Nanbu earthquake) ( M 7.2). It intersected a 0.3 m-thick layer of fault gouge, which is presumed to constitute the fault core (defined as a narrow zone of extremely concentrated deformation) of the Nojima Fault Zone. The rocks obtained from the Hirabayashi GSJ drilling were divided into five types based on the intensities of deformation and alteration: host rock, weakly deformed and altered granodiorite, fault breccia, cataclasite, and fault gouge. Weakly deformed and altered granodiorite is distributed widely in the fault zone. Fault breccia appears mostly just above the fault core. Cataclasite is distributed mainly in a narrow (≈1 m wide) zone in between the fault core and a smaller gouge zone encountered lower down from the drilling. Fault gouge in the fault core is divided into three types based on their color and textures. From their cross-cutting relationships and vein development, the lowest fault gouge in the fault core is judged to be newer than the other two. The fault zone characterized by the deformation and alteration is assumed to be deeper than 426.2 m and its net thickness is > 46.5 m. The fault rocks in the hanging wall (above the fault core) are deformed and altered more intensely than those in the footwall (below the fault core). Furthermore, the intensities of deformation and alteration increase progressively towards the fault core in the hanging wall, but not in the footwall. The difference in the fault rock distribution between the hanging wall and the footwall might be related to the offset of the Nojima Fault and/or the asymmetrical ground motion during earthquakes.     

Temporal changes in the phytoplankton community of the southern basin of Lake Baikal over the last 24,000 years recorded by photosynthetic pigments in a sediment core

Photosynthetic pigments and other indicators of phytoplankton were analyzed in a dated undisturbed sediment core obtained from the southern basin of Lake Baikal to reveal temporal changes in the phytoplankton community in the lake through the last glacial/post-glacial transition. The sedimentation age of the core spans the last 24 ¹⁴C ka. Chlorophyll a, its derivatives, carotenoids and total organic carbon (TOC) started to increase after 15 ¹⁴C ka, and the onset of biogenic silica occurred at 10 ¹⁴C ka. This indicated that the post-glacial growth of diatoms was preceded by that of other phytoplankton groups. In the record of the pigments and TOC, a temporary decrease was observed in the period 11.5–10.5 ¹⁴C ka, corresponding to the Younger Dryas cold period. The similarity found between the depth profiles of pyropheophytin a and steryl chlorin esters formed through predation of phytoplankton by zooplankton and that of TOC suggested the important contribution of fecal pellets to sedimentary organic matter in the lake.     

Process of slip-surface development and formation of slip-surface clay in landslides in Tertiary volcanic rocks, Japan

Once landslide movement has been initiated, the sliding mass tends to move toward a more stable position. However, landslides that have occurred in some Tertiary volcanic rocks in Japan show recurrent movement. A possible reason for this behavior is that the mineral composition and properties of slip-surface materials results in better ‘lubrication’ along the slip surface as movement continues. In order to determine the causes, five landslides in Japanese Tertiary volcanics with different movement histories were studied. Based on geological investigation, the results presented in this paper were obtained by studying the development process and formation of slip surfaces and clays in landslides in these volcanics.

For rocks of similar types and mineralogies the clay content of slip-surface materials can be directly related to the displacement and history of slide movement. This study indicates that the type of slip surface ranges from: (1) striation type (showing only striations on bedrock without clay), to (2) brecciated type (clayey breccia), (3) mylonite type (clay with breccia), and to (4) clay type (clay without breccia). If the slip surface occurs in tuff or tuffaceous materials and is subjected to increasing slide movement, the smectite content in the slip-surface clay increases, while the silicate mineral content decreases. Furthermore, the smectite content is observed to increase within the same landslide from the head to the toe portion of the landslide (direction of groundwater flow). The groundwater along the slip surface (above the impervious zone) contains an increased concentration of Ca⁺⁺ ions. As the concentration of HCO₃⁻ ions exceeds 40 mg/l, an environment is created that promotes the formation of smectite. This process indicates that there is an active ion exchange within the groundwater near the slip surface that forms smectite. Thus, the formation of smectite in the slip surface requires the presence of tuff and a sufficient concentration of HCO₃⁻ ions in the groundwater.

The study results presented here indicate that frictional resistance decreases as clay content increases. The reason for repeated movement of Japanese landslides in Tertiary tuffs or tuffaceous rocks at or in the immediate vicinity of the slip surface is attributed to: (1) an increase in clay materials along the slip surface as the slide movement is repeated and (2) the presence of groundwater that promotes the formation of smectite. As is commonly known, smectite is one of the clay minerals that has lowest frictional resistance.     

Response analysis of flexible MDF systems for multiple-support seismic excitations

Two practical approaches, response spectrum and time-history methods, are developed to evaluate the response of flexible multi-degree-of-freedom (MDF) systems, notably long-span bridges, to multiple-support seismic excitations. For practical convenience, ground motions within a group of adjacent supports on continuous soil or rock are assumed to be uniform and synchronized, while those of different groups are treated as non-uniform and uncorrelated. The response spectrum analysis is extended to include the cross-correlation of modal responses, which prove important when closely spaced modal frequencies exist. An example of the significance of multiple-support excitations is illustrated by application to a suspension bridge. Qualitatively comparable effects can be expected for other bridges of similar type or dimensions.     

Cataclastic lineations

This paper describes a stretching lineation developed on the S-foliations of cataclasites and fault gouges from the Akaishi Tectonic Line (ATL), central Japan. The lineation is called a ‘cataclastic lineation’, and appears to result from homogeneous laminae flow of cryptocrystalline matrix clay minerals and the preferred orientation of fractured porphyroclasts. Slickenlines overprint these textures forming cataclastic lineations. These two kinds of linear structures are developed at large angles to one another.The orthogonal projection of the perpendicular of the intersection of S-foliations, C-surfaces and R₁ shears (B-axis) onto the C-surface, which defines the shear direction, is parallel to the orthogonal projection of cataclastic lineation on the C-surface. In addition, planes cut parallel to the cataclastic lineation and perpendicular to the C-surface show distinctive asymmetric structures indicating left-lateral shear, which coincides with the main fault movement of the ATL as defined by displacement of tectonic units, S-C-surfaces and R₁ shears. In contrast, planes parallel to the slickenlines and perpendicular to the C-surface show no asymmetric fabrics. Thus, cataclastic lineations are considered to indicate the shear direction of the ATL during formation of the cataclasites, whereas the slickenlines indicate shear directions of later, superficial movements along the ATL which have no relationship to the main shear direction.     

Soil erosion and overland flow in Japanese cypress plantations: spatio-temporal variations and a sampling strategy

ABSTRACT

Many studies have focused on soil erosion in unmanaged Japanese cypress plantations because the sparse understory vegetation and litter covering the forest ground enhance soil erosion. In this study, soil erosion, litter, and overland flow measurements were conducted over 14 months to identify the spatio-temporal variation and examine the optimal sample size. Fifteen traps (each 0.25 m wide) were installed in line along the bottom of a 15-m-wide slope. Soil erosion and overland flow had large spatial variations as compared to litter. The temporal coefficient of variation of soil erosion and overland flow was highest during dry seasons, while smaller during wet seasons. The random sampling analysis showed that the rate of decrease in spatio-temporal variation became moderate as the sample size increased beyond six. This result indicated that the optimal sample size was five, the total width of which was equivalent to about 8% of the monitored slope width.