Prediction of a landslide and analysis of slide motion with reference to the 2004 Ohto slide in Nara, Japan

A slope 120 m wide and 100 m high collapsed including the roadbed of a national highway of Route 168 at Ohto, Nara, Japan on August 10, 2004. The precursory phenomena of abnormal features were found as cracks growing on the road-side slope 7 months before the catastrophe. The movements of the slope were monitored by extensometers. The data of the extensometers showed that creep mode turned from the secondary into the tertiary due to the heavy rainstorm of Typhoon Namtheun. The slide claimed no victims because the highway was closed 43 h before the catastrophe, anticipating a possible hazard when the creep velocity reached 4 mm/2 h. Comparison of rupture time predictions suggested that precision of the prediction using the reciprocal of creep velocity is higher than that by tertiary creep analysis, although leaving a problem that the prediction of the time zone of failure erred on the dangerous side. The slide generated ground vibration which was observed by seismometers deployed around the slide. Duration of the seismic signals corresponded well with the slide motion deciphered from video records. We found the fact that the seismic energy radiation from a landslide consisted of four stages. This had not been reported in any previous study, and may be important in understanding the dynamics of a rock-slide avalanche.