HR: 16:00h
AN: S54A-03    [Abstracts]
TI: Urban Landslides Induced by the 2004 Niigata-Chuetsu Earthquake
AU: * Kamai, T
EM: kamai@slope.dpri.kyoto-u.ac.jp
AF: Slope Conservation Section, Geohazards Division, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Trandafir, A C
EM: atrandafir@yahoo.com
AF: Slope Conservation Section, Geohazards Division, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AU: Sidle, R C
EM: sidle@slope.dpri.kyoto-u.ac.jp
AF: Slope Conservation Section, Geohazards Division, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji, Kyoto, 611-0011 Japan
AB: 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.
DE: 5470 Surface materials and properties
DE: 7212 Earthquake ground motions and engineering
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2005 Joint Assembly