Takahiko Furuya

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.

Mean slope-angle frequency distribution and size frequency distribution of landslide masses in Higashikubiki area, Japan

Abstract Landslide mass size frequency distributions and mean slope-angle frequency distributions were calculated for slump, slide, and creep type landslides in the Higashikubiki area. Mean slope-angle frequency distributions closely approximated Weibull distributions. Size frequency distributions show power-law dependencies. Both can be explained by modeling landslides as linked uniform blocks in tensile force. Power coefficients for size frequency distributions were 2.01–2.32 (approximation to power functions) or 2.10–2.24 (approximation to Pareto distributions).

On some informations about the landslides on the mid-stream in the Yoshino river and Iya river

The writer got a chance for the observation of landslides on the mid-stream in the Yoshino river and Iya river, and had some informations. Those informations are summerized as follows.1) Geology of this area is classified two groups which consist of pelitic schist and psammitic schist. There are many landslides in Pelitic schist area, but there are few in psammitic schist area.2) Surface water and ground water of pelitic schst area have higher electric conductivity in comparison with psammitic schst area, and heve only a little tendency to alkalinity.

Preliminary Report of the Crushing Type Landslide

In Japan, without volcanic region, a great many active landslide areas coincide with the distribution of the Tertiary formations, faulted and fractured zones and metamorphic belts.These landslide areas are in general divided geologically into Tertiary type and Crushing type. Tertiary type landslide are mostly connected with mountainlands. Accordingly, on the landslide areas, the geological and geomorphological studies are extremely important.From this point of view, the writer applied technics of the geomorphology on the distribution of the Crushing type landslide which was shown in the topographic map of Kawaguchi 1: 50, 000. The results of this investigation are summurized as follows;1) The landslide areas abound in the fringe of residual erosion surface.2) The localities of the landslide areas abound in the 500m and 800m heights, and these heights are concerned with base level of erosion.3) The inclinations of the landslide areas are about 25° in general.4) The geology of the landslide areas consist of the metamorphic rocks rnd weakly metamorphosed rocks of the Palaeozoic formations. The localities of the landslides areas abound chiefly in crystalline schist of the Minawa formation, Koboke formation and Kawaguchi formation of the Sambagawa metamorphic belts, and are only a little on the weakly metamorphosed rocks so-called Mikabu greenrocks which compose of meta-gabblo, meta-diabase, basalt, and basaltic agglomerate.5) The landslide areas having closed on the fault line which is shown in the geological maps of the sheet Kochi-42 are little 20 percents.