Causes of the 17 February 2006 Southern Leyte Landslide

The 17 February 2006 landslide buried a small village: Guinsaugon in Southern Leyte, Philippine and claimed 154 victims and 990 missing. Composite studies based on the field surveys, interviews with the residents, and inspections of various data including seismological ones have been used to clarify the causes of the slide and its movement. Twenty million cubic meters of Pliocene volcanics slid from the fault scarp that has been formed by the movement of Philippine Fault Zone. Although, landsliding of the slope was enabled firstly by the steepness of the slope, which has resulted from the fault motion; secondly, the presence of a major joint system oblique to the direction of the Philippine fault; thirdly, a progressive rock-creep, whose evidence have been noticed by the local residents with occurrence of cracks on the slope, tilting of trees, a slope failure and a dry out of river water prior to the slide, it was probably triggered by the heavy rainfall including 751 mm in the early half of the February. Two seismograph networks detected a small earthquake whose epicenter was just under the landslide slope or near to it. Although it is not completely clear whether the wave signal of the data is from a tectonic earthquake or from this landslide, the conditional evidences suggest the higher probability on the latter case: the tectonic earthquake did not occur, but the landslide generated that ground tremor. The residential areas similar to Guinsaugon are located on the foot of the fault scarp. Although those areas are convenient for the people to live and work, geomorphic evidences indicate those area must have suffered repeatedly from landslides of similar scale.