Introduction

A series of powerful earthquakes (the strongest with a magnitude of 6.8 in JMA-Japan Meteorological Agency; 6.5 on the Richter scale) and aftershocks jolted northern Japan on October 23, 2004, killing at least 30 people and injuring more than 2000 people largely as the result of building collapse. The strongest earthquakes occurred during a period of several hours with the main epicenter located near Yamakoshi village, Niigata Prefecture.

Thousands of landslides were triggered by the earthquakes and aftershocks with much associated damage to roads, agricultural fields, and residential areas in Niigata Prefecture. This preliminary report addresses several types of slope failures induced by the 23 October 2004 earthquakes in Niigata Prefecture, and is accompanied by a suite of photographs taken during a field reconnaissance along the main route between Ojiya city and Yamakoshi village, as well as along the Shinano River and in three residential areas of Niigata Prefecture, i.e., Kamo town, Otoyoshi shuraku, and Takamachi danchi (new town).

The survey was conducted by a joint investigation team led by Prof. Roy C. Sidle and Assoc. Prof. Toshitaka Kamai from the Slope Conservation Section, Geohazards Division, Disaster Prevention Research Institute, Kyoto University, who visited Niigata between October 28 - October 31, 2004. The other members of the investigation team were Prof. Satoshi Goto (Dept. of Civil Engineering, Yamanoshi University), Dr. Eishou Ohta (OhtaGeo Consulting company), and Dr. Aurelian C. Trandafir (Slope Conservation Section, Geohazards Division, Disaster Prevention Research Institute,. Kyoto University).

The earthquake- induced landslides (and related damages to roads, buildings and environment) shown in the photos are divided into three categories as follows: landslides in natural and cut slopes, failures of artificial fill slopes in urban areas, and failures around the perimeter of terraces of rice paddy fields.

1. Landslides in natural and cut slopes

Figures 1-3 show photos of a landslide which dammed Maesawa River in Yamakoshi village. A large block glide type of landslide was also triggered by the earthquake along the road to Yamakoshi village (see Figs. 4-6). The sliding mass traveled undisrupted and reached the opposite bank of Inokawa River, and finally created a dam in the Inokawa River, blocking the confluence of Inokawa and Maesawa Rivers. During the impact of the block glide with the river, the landslide displaced alluvial sediments onto the opposite bank as evidenced by the freshwater fish found on a landslide deposit (Fig. 7) as well as the alluvial cobbles found in the deposits.

Figures 8 and 9 show a school at Yamakoshi village that was struck and partly buried by this large landslide. Examples of other landslides triggered by the earthquake on natural slopes in the area of Yamakoshi village are depicted in Fig. 10 and Fig. 15; the landslide in Fig. 15 formed a natural dam across the Inokawa River. Figures 11-14 and Fig. 16 show damage to roads and retaining walls due to earthquake-induced slope failures in Yamakoshi village. Figure 17 depicts an earthquake- induced cut slope failure along the route connecting the cities of Ojiya and Tokamachi.

Earthquake-induced slope failures along the Shinano River are shown in Figs. 18-21. The large landslide from Fig. 18, which destroyed national road #17 is a typical example of a slope failure caused by the amplification of seismic acceleration at the corner of a mountain ridge. A railway that remained suspended in the air after the landslide occurrence can be noticed in the upper middle portion of Fig. 20.

According to the geological map provided by the Geological Survey of Japan, all of the landslides shown in Figs. 1-21 occurred in the regional geological structure consisting of sandy siltstone and thin-bedded alternations of sandstone and siltstone. It is also important to mention that relatively dry or only slightly wet soil conditions were present along the sliding surface of these landslides.

2. Failure of artificial fill slopes in urban area

Two residential areas seriously affected by earthquake - induced failures of artificial fill slopes were visited during this field investigation. Figures 23-30 depict damages to houses and roads due to development of tension cracks and earthquake-induced failure of valley fills in Otoyoshi shuraku of Nagaoka city. The entire residential area in Otoyoshi shuraku (Fig. 22) was actually built on an old earthflow that was reactivated during the 23 October 2004 earthquakes. The geological structure in this area is Middle Pleistocene sediments, and consists of gravel, sand and mud (with weathered reddish soil).

Figures 31- 33 illustrate damages to houses and roads due to seismically-induced deformations and failures of artificial fill slopes in another residential area, i.e., Takamachi danchi. The entire road surrounding this residential area was constructed on fill material; the road was partly destroyed during the earthquake. Takamachi danchi lies on Pliocene to Middle Pleistocene sediments composed of sand, silt and gravel in this area. Apparently, the fill material in both Otoyoshi and Takamachi regions was largely saturated partly due to the rainstorms during the previous series of typhoons that struck Japan before the earthquake. The poor drainage system within the retaining structures supporting the fill certainly contributed to the accumulation of water in the fill slopes.

We also visited Kamo-town, about __ km from the epicenter of the earthquake, where lesser, but significant, damage was incurred in several homes constructed on poor fill materials along a small channelized and rerouted stream (Figs. 34-36 ). Again, these failures may have been exacerbated by poor drainage of fill material held behind retaining walls (see Fig. 36).

3. Failure of terraces for rice paddy fields

Figures 37, 39, 40 and 41 show typical earthquake?induced failures of terraces and adjacent hillslopes around rice paddy fields. These sites were located near Yamakoshi village and Tokamachi city (near the damaged Shinkansen line). In Fig. 38, water draining from the damaged rice paddy field near Yamakoshi village can be seen to be infiltrating into cracks along the road and saturating the hillslope soils, therefore increasing the landslide hazard in the region.