The disastrous 17 February 2006 rockslide-debris avalanche on Leyte Island, Philippines: a catastrophic landslide in tropical mountain terrain

In February 2006, a disastrous rockslide-debris avalanche occurred in tropical mountain terrain, on Leyte Island, Central Philippines. Over 1100 people perished when the village of Guinsaugon was overwhelmed directly in the path of the landslide. The landslide was initiated by the failure of a 450m high rock slope within the damage zone of the Philippine Fault where the rock mass consisted of sheared and brecciated volcanic, sedimentary and volcaniclastic rocks. Tectonic weakening of the failed rock mass had resulted from active strike-slip movements along the Philippine Fault which have been estimated by other workers at 2.5 cm/year. The landslide involved a total volume of 15 Mm(3), including significant entrainment from its path, and ran out a horizontal distance of 3800 m over a vertical distance of 8 10 m, equivalent to a fahrboschung of 12 degrees. Run-out distance was enhanced by friction reduction due to undrained loading when the debris encountered flooded paddy fields in the valley bottom at a path distance of 2600 m. A simulation of the event using the dynamic analysis model DAN indicated a mean velocity of 35 m/s and demonstrated the contribution of the paddy field effect to total run-out distance. There was no direct trigger for the landslide but the landslide did follow a period of very heavy rainfall with a lag time of four days. The rockslide-debris avalanche is one of several disastrous landslides to have occurred in the Philippines in the last twenty years. In terms of loss of life, the Guinsaugon event is the most devastating single-event landslide to have occurred worldwide since the Casita Volcano rock avalanche-debris flow which was triggered by Hurricane Mitch in Nicaragua in 1998.