17 February 2006
A rockslide-debris avalanche occurred at 10:26 am on 17 February 2006 in Southern Leyte, Philippines (Lagmay et al., 2006). This large-scale landslide devastated the village of Guinsaugon, with a population of 1,857 (NDCC, 2006). Twenty survivors were rescued from the front edge of the debris field within hours of the disaster, but the majority of the village’s population was believed to have been buried by the avalanche.
The landslide has a planform area of 3,324,400 million square meters, a maximum width of 1.52 kilometers, and a distance of 4.1 kilometers from crown to toe. Independent reports place the volume of the deposit at 15–20 million cubic meters (Lagmay et al., 2006) and 21 million cubic meters (Araiba, in Suwa, 2006). Based on the duration of the telephone calls and the extent of the landslide deposit, the avalanche is estimated to have traveled with a velocity of about 100 kilometers per hour.
Three factors are suspected of having triggered the avalanche. First is the excessive amount of rain that fell in the days before the disaster, even though the landslide happened on a sunny morning. Multi-satellite precipitation analysis from the NASA-JAXA Tropical Rainfall Measuring Mission reported that 500 millimeters of rain fell on Southern Leyte between 4 and 7 February. The Philippine Atmospheric, Geophysical, and Astronomical Services Administration (PAGASA) rainfall station at Otikon, located seven kilometers southwest of the landslide head, recorded 683.6 millimeters from 8–14 February. The highest amount of rainfall over this duration was 171 millimeters, recorded on 12 February (Lagmay et al., 2006).
Second is an earthquake that occurred in Southern Leyte on 17 February 2006. The Philippine Institute of Volcanology and Seismology (PHIVOLCS) recorded a 2.6 magnitude (Ms) earthquake 21 kilometers west of Guinsaugon at 10:35:30.99, using seven of its unmanned satellite-telemetered seismic stations (PHIVOLCS, 2006a; 2006b). An earthquake was also recorded by the United States Geological Survey (USGS) at 10:36:32.59 (PDE-W 2006), about two kilometers north of the landslide, at a depth of 35 kilometers, and with a magnitude (Mb) of 4.3 (USGS, 2006). Records obtained from the F-net broadband seismograph network established and operated by the National Research Institute for Earth Science and Disaster Prevention, Japan, measured the occurrence of ground motion at 10:37. Yamanaka (2006) interprets this record of ground motion as the Rayleigh wave generated by the landslide event.
Third is deformation of the slope prior to the landslide. Accounts described by Suwa (2006) indicate that the slope had been experiencing deformation due to creep. This phase of creep motion may have developed from secondary to tertiary creep at the end of 2005 or in early 2006.
The timing of the landslide is still uncertain, although accounts of the events provided by witnesses converge at around 10:30 am (Lagmay et al., 2006; Suwa, 2006). A telephone call made by Lita Siona, a Department of Agriculture technician and one of the listed fatalities, probably offers the best approximation of the time and duration of the incident. Siona placed a call to the Mayor of St. Bernard, Southern Leyte, as the landslide transpired. According to the Mayor, the conversation started with a query from the terrified technician: ‘Mayora, dakong anas . . . Aruy, asa mi ron dagan?’ (Mayor, a big landslide . . . Oh no, where are we going to run?). The records of the mobile telephone company show that this conversation started at 10:26:00, and that it lasted for 26 seconds. After a brief interruption, it was followed by a second call, which started at 10:27:53 and lasted for 91 seconds before the connection was cut again. These reported times, as verified by the mobile telephone company, are based on the standard atomic clock time used by its computer servers.
By 28 February 2006, 139 bodies had been recovered from the landslide—56 have been identified and 83 remain unidentified. Two victims died in hospital, out of the 33 treated for injuries. The number listed as missing and presumed dead is 980 (NDCC, 2006). The remaining residents of the village were not in the area at the time of the disaster. In total, the official number of fatalities is 1,221 (Lagmay, 2008).
Lagmay, A.M.F., R.S. Rodolfo, A.M.P. Tengonciang, J.L.T. Soria, J.B.T. Ong, D.F.D. Fernandez, M.R. Lapus, E.G. Baliatan, Z.P. Quimba, C.L. Uichanco, E.R. Paguican. (2008). Science guides search and rescue after the 2006 Philippine landslide. Journal of Disasters 32 (3): 416-33.
Lagmay, A.M.F. et al. (2006) ‘Scientists investigate recent Philippine landslide’. Eos. 87(12).pp. 121–124.
NDCC (National Disaster Coordinating Council). (2006). NDCC Update No. 16 re: Landslide at Brgy Guinsaugon, Saint Bernard, Southern Leyte.
PHIVOLCS (Philippine Institute of Volcanology and Seismology). (2006a). Philippine Institute of Volcanology and Seismology, Earthquake Information No. 2, 17 February 2006.
PHIVOLCS. (2006b). Event bulletin for 2006-02-17, Recorded Short-period Waveforms (Vertical Component).
Suwa, H. (2006) ‘Catastrophe caused by the 17 February 2006 Southern Leyte Landslide in the Philippines’. Journal of Japan Society for Natural Disaster Science.25(1).pp. 83–97.
USGS (United States Geological Survey). (2006). NEIC Earthquake Search Catalog.
Yamanaka, Y. (2006). Website EIC Note of Seismology, Earthquake Research Institute, University of Tokyo, February 23, 2006.