Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 7 1 2007 10.5194/nhess-7-89-2007 http://www.nat-hazards-earth-syst-sci.net/7/89/2007/ http://www.nat-hazards-earth-syst-sci.net/7/89/2007/nhess-7-89-2007.html http://www.nat-hazards-earth-syst-sci.net/7/89/2007/nhess-7-89-2007.pdf 89 101 2007-01-24 The disastrous 17 February 2006 rockslide-debris avalanche on Leyte Island, Philippines: a catastrophic landslide in tropical mountain terrain S. G. Evans sgevans@sciborg.uwaterloo.ca R. H. Guthrie N. J. Roberts N. F. Bishop Landslide Research Program, Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada 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 450 m 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 Mm³, including significant entrainment from its path, and ran out a horizontal distance of 3800 m over a vertical distance of 810 m, equivalent to a fahrböschung of 12°. 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. Allen, C. R.: Circum-Pacific faulting in the Philippines-Taiwan region, J. Geophys. Res., 67, 4795–4812, 1962. Aurelio, M. A.: Tectonique du segment central de al faille Philippine (etude structurale, cinèmatique et evolution geodynamique), These de Doctorat, Universite Pierre et Marie Curie, Paris, France, 500 p., 1992. Aurelio, M. A.: Shear partitioning in the Philippines: constraints from Philippine Fault and global positioning system data, The Island Arc, 9, 584–597, 2000. Aurelio, M. A., Barrier, E., Gaulon, R., and Rangin, C.: Deformation and stress states along the central segment of the Philippine Fault: implications to wrench fault tectonics, J. Asian Earth Sci., 15, 107–119, 1997. Barrier, E., Huchon, P., and Aurelio, M. A.: Philippine fault: a key for Philippine kinematics, Geology, 19, 32–35, 1991. Bautista, M. L. P. and Oike, K.: Estimation of the magnitudes and epicenters of Philippine historical earthquakes, Tectonophysics, 317, 137–169, 2000. Corominas, J.: The angle of reach as a mobility index for small and landslides, Can. Geotech. J., 33, 260–271, 1996. Duquesnoy, T., Barrier, E., Kasser, M. Aurelio, M. A., Gaulon, R., Punongbayan, Rangin, C., and the French-Philippine Cooperation team: Detection of creep along the Philippine Fault: first results of geodetic measurements on Leyte Island, central Philippine, Geophys. Res. Lett., 21, 975–978, 1994. Evans, S. G.: Single-event landslides resulting from massive rock slope failure: characterizing their frequency and impact on society, in: Landslides from massive rock slope failure, edited by: Evans, S. G., Scarascia-Mugnozza, G., Strom, A. L., and Hermanns, R. L., Springer, Dordrecht, pp. 53–73, 2006. Evans, S. G., Hungr, O., and Clague, J. J.: Dynamics of the 1984 rock avalanche and associated debris flow on Mount Cayley, British Columbia, Canada; implications for landslide hazard assessment on dissected volcanoes, Eng. Geol., 61, 29–51, 2001. Hart, J., Hearn, G., and Chant, C.: Engineering on the precipice: mountain road rehabilitation in the Philippines, Quart. J. Eng. Geol., 35, 223–231, 2002. Hungr, O.: A model for the runout analysis of rapid flow slides, debris flows and avalanches, Can. Geotech. J., 32, 610–623, 1995. Hungr, O. and Evans, S. G.: Rock avalanche runout prediction using a dynamic model, Proc. 7th Int. Symp. on Landslides, 1, 233–238, 1996. Hungr, O. and Evans, S. G.: Entrainment of debris in rock avalanches: an analysis of a long run-out mechanism, Geological Society of America Bulletin, 116, 1240–1252, 2004. Hungr, O., Evans, S. G., Bovis, M. J., and Hutchinson, J. N.: A review of the classification of landslides of the flow type, Environ. and Eng. Geosci., 7, 221–238, 2001. Inter-Agency Committee on the Guinsaugon, Southern Leyte, Landslide Tragedy: The 17 February 2006 Bgy. Guinsaugon, Southern Leyte, Landslide, February 28, 2006, Government of the Philippines, 18 p., 2006. Keefer, D. K.: Landslides caused by earthquakes, Geol. Soc. Am. Bull., 95, 406–421, 1984. Kim, Y.-S., Peacock, D. C. P., and Sanderson, D. J.: Fault damage zones, J. Structural Geol., 26, 503–517, 2004. King, J., Loveday, I., and Schuster, R. L.: The 1985 Bairaman landslide dam and resulting debris flow, Papua New Guinea, Quart. J. Eng. Geol., 22, 257–270, 1989. Korup, O.: Geomorphic implications of fault zone weakening: slope instability along the Alpine Fault, South Westland to Fiordland, New Zealand J. Geol. Geophys., 47, 257–267, 2004. Lagmay, A. M. F., Tengonciang, A. M. P., Marcos, H. V., and Pascua, C. S.: A structural model guide for geothermal exploration in Ancestral Mount Bao, Leyte, Philippines, J. Volcanol. Geotherm. Res., 122, 133–141, 2000. Lagmay, A. M. F., Ong, J. B. T., Fernandez, D. F. D., Lapus, M. R., Rodolfo, R. S., Tengonciang, A. M. P., Soria, J. L. A., Baliatan, E. G., Quimba, Z. L., Uichianco, C. L., Pagluican, E. M. R., Remedio, A. R. C., Lorenzo, G. R. H., Valdivia, W., and Avila, F. B.: Scientists investigate recent Philippine landslide, Eos, 87, p. 121, 124, 2006. Leynes, R. D., Pioquinto, W. P. C., and Caranto, J. A.: Landslide hazard assessment and mitigation measures in Philippine geothermal fields, Geothermics, 34, 209–221, 2005. Nadim, F., Kjekstad, O., Peduzzi, P., Herold, C., and Jaedicke, C.: Global landslide and avalanche hotspots, Landslides, 3, 159–173, 2006. Peart, M.: The Kaiapit landslide: events and mechanisms, Quart. J. Eng. Geol., 24, 399–411, 1991. Philippine Bureau of Mines: Geological Map of the Philippines, Scale 1:1 000 000, 1963. Pierson, T. C.: An empirical method for estimating travel times for wet volcanic mass flows, Bull. Volcanol., 60, 98–109, 1998. Punongbayan, R. S., Arboleda, R. A., Bornas, M. V., and Abigania, M. T.: The 3 August 1999 Cherry Hills Landslide in Antipolo City, Philippines, Landslide News, 13, 12–15, 2000. Sajona, F. G., Bellon, H., Maury, R. C., Pubellier, M., Quebral, R. D., Cotten, J., Bayon, F. E., Pagado, E., and Pamatian, P.: Tertiary and Quaternary magmatism in Mindanao and Leyte (Philippines): geochronology, geochemistry and tectonic setting, J. Asian Earth Sci. 15, 121–153, 1997. Scott, K. M., Vallance, J. W., Kerle, N., Macias, J. L., Strauch, W., and Devoli, G.: Catastrophic precipitation-triggered lahar at Casita volcano, Nicaragua: occurrence, bulking and transformation, Earth Surf. Proc. Landforms., 30, 59–79, 2005. Velasco, A. A., Ammon, C. J., Lay, T., and Hagerty, M.: Rupture process of the 1990 Luzon, Philippines (M$_w$=7.7), earthquake, J. Geophys. Res., 101(B10), 22 419–22 434, 1996.