The 23 October 2004 Chuetsu Earthquakes and Related Disasters, Niigata, Japan III
Presiding: R Sidle, Disaster Prevention Research Institute, Geohazards Division, Kyoto University; Y Iio, Disaster Prevention Research Institute, Research Center for Earthquake Prediction, Kyoto University
S54A-01 15:30h
Landslide Distribution, Damage and Land Use Interactions During the 2004 Chuetsu Earthquake
A series of earthquakes struck Niigata Prefecture, Japan, on 23 October 2004 killing about 40 people and injuring about 3000. These earthquakes were characterized by a shallow focal depth (13 km) that generated strong levels of ground motion, resulting in extensive damage and thousands of landslides throughout the region. Most landslides on natural slopes occurred in the regional geological structure consisting of sandy siltstone and thin-bedded alternations of sandstone and siltstone. Earthquakes exacerbate such potential instabilities by the ground motion induced and the enhancement of pore water pressure in wet regoliths. The three strongest earthquakes occurred within a period of less than 40 minutes, and had sequential magnitudes (JMA) of 6.8, 6.3, and 6.5. The highest density of landslides (12/km2) was mapped within a 2.9 km radius of the M6.5 epicenter near Yamakoshi village; about 4 times higher density compared to the other epicenters located to the east and west. This higher density may be a consequence of the cumulative shaking effects associated with the two earlier earthquakes of M6.8 and 6.5, in addition to the topographic and geologic factors controlling the stability of the region. Roads, residential fills, agricultural terraces on hillslopes, and other earthworks increased the susceptibility of sites to slope failure. Numerous earthquake-induced failures in terraces and adjacent hillslopes around rice paddy fields occurred near Yamakoshi village. A housing development in Nagaoka city constructed on an old earthflow suffered from severe damage to fill slopes during the earthquake. Nearly saturated conditions in these deep fills together with poor drainage systems contributed to the landslide damages. Clearly, land use activities in rural and urban areas exacerbated the extent of earthquake-triggered landslides.
S54A-02 15:45h
Topographic feature of the disasters along the railway by the Niigata Chuetsu Earthquake in 2004
The damage of the railway relation by the Niigata Chuetsu Earthquake which occurred on October 23, 2004 was serious. The main damage is derailment of a Joetsu Shinkansen Line, damage on a viaduct or a tunnel, failure of slopes or fills, etc. In order to grasp the actual state of failures around slopes or fills along the railway line, we carried out interpretation using the high resolution digital aerial photograph taken immediately after the occurrence of an earthquake. Next, a topographic feature of the failure was analyzed quantitatively by using 1m-DEM (Digital Elevation Model) made from the LIDAR(LIght Detection And Ranging) system. This report presents an overview of the topographic analysis result for Joetsu Line in which the numbers and size of failure were especially large. The topographical feature of the study area consists of mountainous area called the Uonuma hill and a terrace which develops into along Shinano River and Uono River. This area is characterized by the folding structure which has the axis of the direction of north-northeast-south-southwest. The outline of an analysis result is as follows. (1) Most cases of disaster were surface failure. (2) Many failures occurred in the terrace scarp along the river terrace of Shinano River and Uono River. (3) Landslide topography is widely distributed over this area, and many failures occurred in it. (4) The main aspect of the slope where failure occurred is the same as the direction of dip of stratum (northwest - west). (5) Many failures occurred on the convex or ridge type slope which is generally said to be easy to collapse in case of an earthquake, but also occurred on the concave or valley type slope. The influence of the rainfall before the earthquake can be considered as this reason.
S54A-03 16:00h
Urban Landslides Induced by the 2004 Niigata-Chuetsu Earthquake
Landslides triggered by the Chuetsu earthquake occurred in artificial slopes of some new developments in suburban Nagaoka, the largest city in the affected area. The landslides occurred in hilly terrain of the eastern part of Nagaoka between the alluvial plain and Tertiary folded mountains of Yamakoshi. Although the extent of landslides in urban Nagaoka was small compared with landslides on natural slopes (especially near Yamakoshi), they represent an important case study for urban landslide disasters. Slope instabilities in urban residential areas were classified as: A) landslides in steep embankments; B) landslides in gently sloping artificial valley fills; C) re-activation of old landslides; and D) liquefaction in deep artificial valley fills. All these failures occurred in relatively uniform suburban landscapes, which were significantly modified from the original landforms. Recent destructive earthquakes in Japan caused similar types of slope failures in urban regions, suggesting that lessons from past earthquakes were not implemented. The greatest damage due to type-A failures occurred in the 25-yr old Takamachi residential area, where about 70 of 522 homes were judged to be uninhabitable. Before development, this area was an isolated hill (90 m elevation) with an adjacent terrace (60 m elevation) consisting of gravel, sand, and silt of the lower to middle Pleistocene deposits. Development earthworks removed the hill crest and created a wide plateau (70 m elevation); excavated soil was placed on the perimeter as an embankment. During the earthquake, the embankment slope collapsed, including retaining walls, perimeter road, and homes. The most serious damage occurred in five places around the margin of the plateau corresponding to shallow valley fills (5 to 8 m thick). Earthquake response analyses using an equivalent linear model indicated the amplification of seismic waves at the surface of embankment slopes, and the peak earthquake acceleration exceeded 1 G in the case of embankment thicknesses <8 m. The natural frequency at the shoulder of embankment slopes is inversely proportion to fill thickness; less than the predominant frequency of the earthquake (3 Hz). Shallow embankments were unstable compared to deeper embankments because their natural frequency was close to the predominant frequency of the earthquake. Thus, we expect widespread embankment failures in megacities during the next giant earthquake shaking with low predominant frequency originating offshore of the Pacific Ocean.
S54A-04 16:15h
Damage to Tunnels Caused by the October 23, 2004, Chuetsu Earthquake, Niigata, Japan
A huge number of landslides induced by the Chuetsu earthquake caused a total of 233 segments of national and prefectural routes in Higashiyama mountain area to be closed to traffic, and totally 61 village areas were completely isolated. Since railway and road facilities follow closely the motions of soil, damage to these facilities must be discussed in terms of soil deformations that they experienced. As examples, cases of some tunnels will be presented. It is concluded that even small soil deformations can be large enough to cause serious cracking of tunnel linings.
S54A-05 16:30h
Geomorphic Features on the Movement of Reactivated Landslides due to Earthquakes
Strong earthquakes with M6.8 (JMA at maximum) in the Chuetsu region, Niigata prefecture, Japan on October 23, 2004, generated a number of landslides around the epicenter. This region consists of Tertiary mudstone and sandstone, and was originally a landslide-prone area. Some of the earthquake-induced landslides formed natural dams and merged houses under water. These landslides derived from the reactivation of old landslide masses, according to the landslide maps (NIED). Topographic analysis focusing on the landslide movement was carried out with landslide maps before and after the earthquakes. Slope of source areas in the reactivated landslides ranged from 7 to 27 degrees. Equivalent coefficient of friction H/L, where H is runout height and L is horizontal runout distance, was proportional to the slope of the source area in the reactivated landslides. The regression line of the above relationship agreed well with that in original landslides. The width of landslide mass expanded 1.2-1.3 times of the width before slide on average. This means that both original and reactivated landslides had equality of motion. The results allow estimation of the runout distance and deposition area of future landslides in this region.
S54A-06 16:45h
Mitigation of Earthquake-Induced Catastrophic Landslide Hazard on Gentle Slopes by Surface Loading
Catastrophic landslides have occurred repeatedly on gentle slopes during past earthquakes in Japan, sometimes causing great loss of life and significant environmental damage. Reconnaissance reports on the October 23, 2004, Chuetsu earthquake in Niigata Prefecture, Japan, also include collapse of gentle slopes associated with damage to roads and railways which lost foundation support. Additionally, investigations of the spatial distribution and features of landslides triggered by the Chuetsu earthquake revealed that 23% of the landslides mapped within a 2.9 km radius around the major earthquake epicenter occurred on slopes with gradients between 10 and 20 degrees. Past experience demonstrated that such earthquake-induced catastrophic landslides occurred along shear surfaces in saturated cohesionless materials, and the main factor controlling the high mobility of the slide mass after failure was the gradual loss in shear strength with progressive shear displacement. Given the high rainfall prior the earthquake, it is likely that the same failure mechanism characterizes the catastrophic landslides triggered on gentle slopes during the October 23, 2004, Chuetsu event in Niigata Prefecture. Thus, we introduce a procedure based on application of an additional confining stress to the surface to increase the stability of gentle slopes in saturated cohesionless soils subject to catastrophic failure during earthquakes. This surface pressure is achieved by concrete plates tied back with prestressed steel anchors which penetrate through the soil well below the potential sliding surface. Results of a dynamic analysis of undrained seismic performance conducted for a gentle infinite slope that experienced different levels of increase in effective confining stress due to a uniform load applied normal to the surface, illustrate the effectiveness of this measure in mitigating the earthquake-induced catastrophic landslide hazards.