Masahiro Chigira

Long-term gravitational deformation of rocks by mass rock creep

Abstract Subsurface rocks beneath slopes are deformed to various degrees in various ways by mass rock creep (MRC), when they are subjected to a gravitationally unstable state for a long period. MRC forms folds, faults and numerous kinds of fractures; such deformation is one of the main factors for deteriorating rock masses on slopes and also precursors of many landslides. MRC deformations, which have often been ascribed to tectonic origins, can be distinguished from tectonic ones by their deformational features, landforms and relationships between slope morphology and subsurface structures. MRC folds are flexural-slip folds that have the following mesoscopic to microscopic characteristics: their axial surfaces are not smooth but are jagged and many visible openings or open fractures develop in association with the folds. The jagged axial surfaces become smoother at depth and openings decrease in width with increasing overburden at the deformation site. Macroscopic types of MRC folds change according to the relationship between foliations and slopes; buckling folds form in consequent slopes, drag folds form in a deeper part of obsequent slopes and bending folds form in slopes with steep-dipping foliations (consequent or obsequent) by downslope bowing of foliations. Shear fractures and tension fractures are the main deformational structures formed by MRC in massive rocks. Among shear fractures, those of listric origin are remarkable. Tension fractures, in general, occur either in networks, dividing rocks into fragments that slip and rotate irregularly in relation to each other, or in parallel arrays of steeply dipping fractures. An MRC fault has a shear zone consisting of a pulverized zone with gouge and a phyllitic or brecciated zone. In a densely foliated rock, the phyllitic zone is formed by microscopic slip along foliations; in a sparsely foliated rock, the brecciated zone is formed by random crushing except for the faults made through the shear fractures in massive rocks. Landforms due to MRC are gentle slopes bounded by horseshoe knicklines upslope, convex slopes, ridge-top depressions, multiple ridges, uphill-facing scarps and ridge-crossing depressions.

Geological causes and geomorphological precursors of the Tsaoling landslide triggered by the 1999 Chi-Chi earthquake, Taiwan

The Tsaoling landslide, one of the largest landslide areas in Taiwan, has been affected by catastrophic events triggered by rain or earthquakes six times since 1862. These landslides, including that caused by the 1999 earthquake, have essentially not been reactivated old slides, but were sequential new ones that developed upslope, retrogressively. The landslide area is underlain by Pliocene sandstone and shale to form a dip slope with a bedding plane, dipping uniformly at 14°. The slip surface of the 1999 landslide was smooth and planar, parallel to the bedding plane with a slightly stepped profile; it formed within thinly alternated beds of fine sandstone and shale with ripple lamination or in a shale bed. The shale is weathered by slaking and probably by sulfuric acid, which is inferred to be one of the major causes of the intermittent retrogressive development of the landslides. The weathering was likely accelerated by the removal of overlying beds during earlier landslides in 1941 and 1942. The top margin of the 1999 landslide, in plan view, coincided with a V-shaped scarplet, which can be clearly recognized on aerial photographs taken before the landslide. This geomorphological feature indicates that this landslide had already moved slightly before its 1999 occurrence, providing precursory evidences.

Geological and geomorphological precursors of the Chiu-fen-erh-shan landslide triggered by the Chi-chi earthquake in central Taiwan

Abstract Special features were correlated to the geological causes of the Chiu-fen-erh-shan landslide, a gigantic rockslide on a dip slope, induced by the Chi-chi earthquake (ML=7.3) in central Taiwan in 1999. An aerial photo interpretation and the succeeding geological mapping of the failure were employed in this study. A linear depression, a steep step, and a low drainage density in the landslide area were detected from the aerial photos taken in 1998. The gravitational creep was believed to result in the features of the linear depression and the low drainage density. The steep step represented a buckling feature found in the field. The landslide area is composed of stratified sandstone and shale, with dip angles ranging 20–36°. The slip surface developed along a pre-existing bedding fault that resulted from flexural slip folding. Before the Chi-chi earthquake, the rock on the upslope side buckled and was retained by a thick-bedded sandstone downslope. The earthquake shock seriously damaged the sandstone support and led to the catastrophic landslide. This type of landslide is likely to occur on the moderately dipping slope of stratified rocks that were previously deformed by flexural slip folding.

Mechanism and effect of chemical weathering of sedimentary rocks

The mineralogy, geochemistry and physical and mechanical properties of rocks from four weathering profiles of Miocene to Pleistocene mudstones and sandstones in Japan showed that chemical weathering of sedimentary rocks is characterized by sequential reaction between percolating groundwater and rock-forming minerals. Pyrite, a common mineral contained in sedimentary rocks, is especially important in these sequential reactions. Pyrite is oxidized by oxygen coming from the ground surface and sulfuric acid is generated at the base of the oxidized zone. The sulfuric acid, in turn, dissolves rock-forming materials to make a dissolved zone. If the fluxes of oxygen and water are in the same direction, sulfuric acid generated at the oxidation front migrates farther and forms a dissolved zone. If these fluxes are in opposite directions, the oxidized and dissolved zones are not differentiated. Rocks in the dissolved zone are caused to deteriorate by the acid leaching and are acidic if buffering minerals, such as calcite and zeolite, are absent. In the oxidized zone, sandstone is strengthened because of cementation by iron oxide or hydroxide, while mudstone is weakened because it has greater clay fractions and larger specific surface areas than sandstone.

Deep-seated rockslide-avalanches preceded by mass rock creep of sedimentary rocks in the Akaishi Mountains, central Japan

Abstract Seven large rockslide-avalanches of sedimentary rocks are known in Japan. Almost all of these major landslides were preceded by gravitational deformation of rocks (mass rock creep, MRC), as inferred from geologic and geomorphic investigations of five rockslide-avalanches in the Akaishi Mountains. The MRC that preceded the rockslide-avalanches in the Akaishi Mountains was characterized by a slow but steady downslope bowing of steeply dipping foliations, which accompanied intensive deformation and fragmentation of the rock mass by shearing along foliations. This deformation and fragmentation of the rock mass provides a basic cause for the occurrence of rockslide-avalanches, which occur when a creeping rock mass has lost its support from the lower part of a slope by stream erosion. This type of MRC continues for a long time after a part of the creeping area has slid and results in longterm generation of debris by the landslide sear of the rockslide-avalanche.

Micro-sheeting of granite and its relationship with landsliding specifically after the heavy rainstorm in June 1999, Hiroshima Prefecture, Japan

Certain types of granite in mountainous areas are microscopically sheeted to a depth of 50 m due to unloading under the stress field that reflects slope morphology. Micro-sheets generally strike parallel to major slope surfaces and gently dip downslope, forming cataclinal overdip slopes. The cataclinal overdip slope accelerates creep movement of micro-sheeted granite, which in turn loosens and disintegrates granite via the widening or neoformation of cracks, probably in combination with stress release, temperature change, and changes in water content near the ground surface. The surface portion of micro-sheeted granite is thus loosened with a well-defined basal front, which finally slides in response to heavy rain. Innumerable landslides of this type occurred in Hiroshima Prefecture, western Japan, following the heavy rainstorm of 29 June 1999. Following such landslides, the weathering of micro-sheeted granite exposed on the landslide scar recommences, setting the stage for future landslide.

A mechanism of chemical weathering of mudstone in a mountainous area

Abstract The chemical weathering of marine mudstone in humid mountainous areas of Japan is dominated by the oxidation of pyrite, a common mineral in marine sedimentary rocks. This process occurs at the oxidation front, that is the base of the oxidized zone, and sulfuric acid is formed. Chlorite is transformed into smectite due to the oxidizing and acidic conditions. As sulfuric acid migrates downward from the oxidation front, minerals are dissolved and components are leached out from the underlying dissolved zone, whose lower boundary is the dissolution front. The oxidation and dissolution fronts migrate downward as weathering progresses in mountainous areas.

Weathering mechanisms and their effects on the landsliding of ignimbrite subject to vapor-phase crystallization in the Shirakawa pyroclastic flow, northern Japan

Abstract Ignimbrite, which is consolidated by vapor-phase crystallization, is weathered in humid regions to form a special type of weathering profile that consists of a hydrated zone, an exfoliated zone, and a disintegrated zone from the depth to the ground surface, with each zone having a basal front. The ignimbrite is hydrated first and loses a significant amount of phosphorous at the hydration front, where rock-forming tridymite is dissolved and cristobalite is precipitated. The ignimbrite further loses its alkali and alkali earth components at the top of the hydrated zone by reacting with reactive water from the exfoliated zone, then the leached layers are exfoliated and become part of the exfoliated zone, and then they soften significantly. At the top of the exfoliated zone, rock is disintegrated so completely that rock texture disappears. Water from rainstorms infiltrates down to the exfoliation front, but penetrates only slightly further downward, thus saturating the weathered rock in the exfoliated and disintegrated zones and leading to a landslide with a slip surface within the exfoliated zone.

Landslides and debris flows strike Kyushu, Japan

On 20 July 2003, several damaging landslides and debris flows in southern Kyushu, Japan, attracted international attention and resulted in one of the most major natural disasters of recent years. Large amounts of rain fell on 19 and 20 July as a Baiu front passing over the Sea of Japan met a high-pressure zone moving up from the southeast over the Pacific Ocean. Altogether, 21 lives were lost due to the sediment disasters, and more than 240 homes were either damaged or destroyed by landslides, debris flows, and flooding. Nevertheless, such natural disasters occur frequently in Japan. In summer 1993, 121 people were killed by landslides and debris flows within an area of unwelded pyroclastic flow deposits (known as Shirasu) in the Kagoshima Bay area of Kyushu. Thus, local residents generally acknowledge their potential exposure to these hazards, but risk and vulnerability issues may be clouded by inadequate warnings and scientific knowledge, socio-economic factors, and the general feeling that local and national governments are overly protective.

Chemical weathering mechanisms and their effects on engineering properties of soft sandstone and conglomerate cemented by zeolite in a mountainous area

Abstract Sandstone and conglomerate cemented by zeolite crystals in a humid mountainous area in Japan are weathered, and also weakened or strengthened, by chemical interaction with percolating water that was originally in equilibrium with the atmosphere. Near the ground surface, carbonic acid generated by the dissolution of carbon dioxide from the atmosphere and biological activity in the soil dissolves zeolite, and thereby weakens the rocks. At the oxidation front, the base of the oxidized zone, oxygen from the atmosphere reacts with pyrite in the rock to form sulfuric acid and iron oxide and/or hydroxide. The iron oxide and/or hydroxide precipitates on zeolite surfaces, and strengthens the rocks by cementing the grains to each other. Beneath the oxidation front and extending to the dissolution front, sulfuric acid migrates downward, dissolves zeolite, and thus weakens the rocks. This weathering mechanism and the resulting changes in mechanical properties of rocks generally are to be expected in marine sandstone and conglomerate with small amounts of clay in the matrix because the most common cementing mineral is carbonate which is also easily dissolved by acid, and further because pyrite is a common rock-forming mineral in marine sedimentary rocks. The effects of chemical weathering on the mechanical properties of sandstone and conglomerate are different from those of mudstone, because the latter contains much larger amounts of clay minerals. The cementation of constituent grains by iron oxide and/or hydroxide is inhibited by clay minerals, because clay minerals have high specific surface areas, and also because large amounts of iron can be incorporated in the minerals.