Toshitaka Kamai

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.

Broader perspective on ecosystem sustainability: Consequences for decision making

Although the concept of ecosystem sustainability has a long-term focus, it is often viewed from a static system perspective. Because most ecosystems are dynamic, we explore sustainability assessments from three additional perspectives: resilient systems; systems where tipping points occur; and systems subject to episodic resetting. Whereas foundations of ecosystem resilience originated in ecology, recent discussions have focused on geophysical attributes, and it is recognized that dynamic system components may not return to their former state following perturbations. Tipping points emerge when chronic changes (typically anthropogenic, but sometimes natural) push ecosystems to thresholds that cause collapse of process and function and may become permanent. Ecosystem resetting occurs when episodic natural disasters breach thresholds with little or no warning, resulting in long-term changes to environmental attributes or ecosystem function. An example of sustainability assessment of ecosystem goods and services along the Gulf Coast (USA) demonstrates the need to include both the resilient and dynamic nature of biogeomorphic components. Mountain road development in northwest Yunnan, China, makes rivers and related habitat vulnerable to tipping points. Ecosystems reset by natural disasters are also presented, emphasizing the need to understand the magnitude frequency and interrelationships among major disturbances, as shown by (i) the 2011 Great East Japan Earthquake and resulting tsunami, including how unsustainable urban development exacerbates geodisaster propagation, and (ii) repeated major earthquakes and associated geomorphic and vegetation disturbances in Papua New Guinea. Although all of these ecosystem perturbations and shifts are individually recognized, they are not embraced in contemporary sustainable decision making.

Detection of a landslide movement as geometric misregistration in image matching of SPOT HRV data of two different dates

A landslide movement was detected as geometric misregistration in image matching between two SPOT HRV panchromatic image data of different acquisition dates. The test site is an active landslide in Itaya area, Yamagata Prefecture, northern Japan. The average rate of the landslide movement is approximately 1 cm per day based upon differential GPS measurements. Pixel level and subpixel level image matching was applied to an HRV image pair obtained on 1 November 1987 and 29 October 1993. As a result, the landslide movement was successfully detected as misregistration vectors between the two HRV scenes. The displacement is approximately 20 m to 30 m over the 6-year period between 1987 and 1993 and is consistent with the landslide movement inferred from the GPS measurements.

Effect of pore-water chemistry on undrained shear behaviour of saturated loess

Pore-water chemistry can have a fundamental influence on the shear behaviour of soil. To study the effects of salt concentration and desalinization on undrained shear behaviour, a series of ring shear tests was conducted on Chinese loess saturated with various NaCl solutions of differing concentration, under the same initial void ratio and consolidating stress conditions. Results showed that the peak and steady-state shear strengths increase as the NaCl concentration increases only up to a certain level. When this level is exceeded, any further increase of NaCl concentration results in lower shear strengths. After desalinization the peak and steady-state shear strengths recover to those of the original sample, showing that the salinization of loess is reversible. A pore pressure ratio of 0.6 can be used as a criterion to assess whether liquefaction can be induced in saturated loess. These findings may provide a useful explanation for progressive or seasonal activity of some irrigation-induced loess landslides in the Chinese Loess Plateau.

Evaluating landslide damage during the 2004 Chuetsu earthquake, Niigata Japan

A series of earthquakes, the strongest with a magnitude of 6.6 on the Richter scale (6.8 on the Japan Meteorological Agency (JMA) scale), jolted Niigata Prefecture, Japan, late in the afternoon of 23 October 2004, killing about 40 people and injuring about 3000, largely as the result of building collapse. More than 100,000 residents were forced to evacuate their homes, and 2200 residents of Yamakoshi village and other areas have not yet returned. The three strongest earthquakes (M>6.2 on the JMA scale) occurred within less than 40 min with epicenters spread across Ojiya city and Hirokami and Yamakoshi villages in Niigata Prefecture. These earthquakes were characterized by shallow focal depths (9–14 km) that generated strong levels of ground motion, resulting in extensive damage throughout the region.

Preliminary Study on Contribution of Predominant Frequency Components of Strong Motion for Earthquake-Induced Landslide

In Kamikamato Area, Iwaki City, a landslide was induced by the large scale aftershock (the 2011 Hamadori Fukushima Prefecture Earthquake (2011/04/11; Mj7.0)), while it was not induced by the 2011 off the Pacific coast of Tohoku Earthquake (2011/03/11; Mw9.0). In this study, we found the similarity of site amplification factor between the landslide area and strong motion station, namely, “Iwaki-Yumoto IC”. At Iwaki-Yumoto IC site, where the waveforms were not recorded during the large scale aftershock, the estimated waveforms were adopted based on the effect of the existing study. Moreover, we examined the effect of frequency components of the seismic waveforms on the dynamic landslide comparing the estimated waveforms.

On the Initiation and Movement of Hanokidaira Landslide from the 2011 Tohoku Earthquake, Japan

A flow type landslide was triggered in Hanokidaira area by The 2011 off the Pacific coast of Tohoku Earthquake, killing more than ten people. The slope consists of pyroclastic flow deposits that were formed at different times, with a layer of paleosol that was outcropped. Above the paleosol there is a layer of pumice and scoria, which is very rich in natural moisture content (~145 %). From field observation, we inferred that the sliding surface originated on the boundary between the paleosol layer and the layer of pumice and scoria. To examine the possible trigger and movement mechanism of these landslides, we monitored the aftershocks on landslide area, and inferred the possible seismic response of the landslide area during the main shock. We took samples from both layers, and performed undrained static and cyclic shear tests on them in fully saturated or in natural water content states. The results showed that high pore-water pressure could be generated after failure with increase of shear displacement, resulting in great loss in the shear strength. We also performed seismic simulation tests on these samples by using the inferred seismic waveform, and the test results showed that the amplified vertical motion may have played a key role in the initiation of this landslide.

Strong Motion Estimation at the Kanaga Landslide, Nasukarasuyama City, Induced by the 2011 off the Pacific Coast of Tohoku Earthquake (MW9.0) Based on Empirical Site Amplification and Phase Effects

Many landslides were triggered by the 2011 off the Pacific coast of Tohoku Earthquake (M W 9.0). Among them, a large-scale landslide occurring in Kanaga, Nasukarasuyama City, Tochigi Prefecture, destroyed several houses and killed two people. To clarify the triggering mechanism of this landslide, it is great importance to estimate the seismic waveform at Kanaga Landslide site with higher accuracy, taking into account the site effects. In this study, seismic waveform at the Kanaga Landslide was estimated based on site-effect substitution method, while the site effects were evaluated based on aftershock records. Difference of site effects between the Kanaga Landslide and the nearby strong motion observation stations was presented, and discussed.

The Internal Structure and Stability of Some Large Landslide Dams Induced by the 2008 Wenchuan (Mw7.9) Earthquake, China

The internal structure of landslide dams plays a key role in their stability; however, it has not been much studied, probably due to the difficulty in obtaining information on internal structure in most cases. Here, by using a surface-wave technique we examined the shear wave velocity structure (Vs) of three landslide dams induced by the 2008 Wenchuan earthquake (Mw7.9). The Vs profile of the dams revealed that the internal structure of the dams depends on the geology and the movement of the landslides. Based on the internal structure and the real performance of the dam, the stability of these landslide dams had been discussed.

Landslides in Urban Residential Slopes Induced by the 2011 Off the Pacific Coast of Tohoku Earthquake

The preliminary report on landslides in residential areas induced by the 2011 off the Pacific Coast of Tohoku earthquake (M9.0) was conducted. Many urban landslides occurred in gentle residential slopes constructed as artificial valley fills (embankments) during this earthquake, particularly in Sendai, Shiroishi, Fukushima, Iwaki, and Tokai, from southern Tohoku to northern Kanto province. The landslides are mainly described based on the field investigations and are classified into five types. Some of the landslides that initiated in 2011 overlap with landslides induced by the earthquake in 1978. The repeated landslides at the same sites indicate that recent urban development and planning did not learn from the past disasters. Although countermeasures were constructed for landslides induced by the 1978 earthquake, serious landslides were found in same place in 2011. The disasters in 2011 have far-reaching ramifications for urban planning in Japan. We need a new design concept for urban development to minimize artificial geomorphological changes. The conceptual model of a “Counter line city” should both minimize risk of disasters and create a favorable natural environment in new urban regions along the Tohoku coast.