Geotechnical Aspects of the Sumatra Earthquake of September 30, 2009, Indonesia

This paper reports and discusses the results of a field survey conducted by a joint scientific group from Japan and Indonesia to assess the geotechnical aspects of the Sumatra earthquake (M_w=7.6) of September 30, 2009. The studied area included the Padang and Pariaman cities, where a number of buildings collapsed as a result of strong shaking, and a mountainous part of the Pariaman district, a place where massive landslides buried several villages, claiming more than 400 human lives. The main objective of the survey was to investigate the causes and mechanisms of catastrophic landslides; however, other geotechnical problems such as lateral spread and liquefaction were also addressed. Field observations indicated that the catastrophic landslides occurred on relatively gentle slopes, then mobilized into debris flows, and traveled several hundred meters from their points of origin. The failure surfaces developed along the boundary of highly weathered pumice tuff with more intact and less weathered bedrock. Data from a portable cone penetration test showed that the sliding material was rather weak, having SPT N-values in the range of 5-10. The results of the field survey suggested that the main cause of slope instability was high pore-water pressures that generated in the soil mass during the earthquake.