During November 2008 a BGS scientist took part in a field trip and conference in Japan. The field trip concentrated on landslides in the Tohoku region of Japan, many triggered by seismic activity.
The largest landslide visited during the trip was triggered by the most recent seismic event the 7.2 magnitude Iwate-Miyagi-Nariku earthquake which occurred on 14 June 2008. The landslide at the Aratozawa Dam (Figure 1, Figure 2, Figure 3) in Kurihara measures over 1km in length and is 0.8 km wide. The landslide body mass is around 6700 million m3 and the amount of displacement was around 300 m in the main section. The landslide was translational in nature and the geology comprises hard volcanic rocks overlying soft Tertiary sediments.
Figure 1 Aratozawa Dam landslide, Kurihara.
Other areas visited during the field trip included those damaged in the 2004 Mid Niigata earthquake (Figure 4 and Figure 5). The quake struck on 23 October with a magnitude of 6.8 on the Richter Scale, triggering over 360 landslides. This part of the field trip allowed the fieldwork delegates to see how these types of earthquake triggered landslides are mitigated against in Japan (Figure 6). The effects of this earthquake were amplified because of the landslide-prone geological and topographic setting of the area as well as the occurrence of a typhoon shortly before the quake. Most of the landslides were large scale block slides, small slumps and debris flows as well as rock avalanches. Damming of rivers by landslides was a common occurrence following this event. The largest block slide was at Kajikane, Yamakoshi village some 5 km north of the epicentre. The landslide was 450 m wide by 650 m long with a depth of around 70 m and the landslide body occurred within previously slipped material. The estimated volume of this landslide is around 7.5 million m3.