Catastrophic rockslide-debris avalanche at St. Bernard, Southern Leyte, Philippines

Abstract

On 17 February 2006, a rockslide-debris avalanche cascaded down the steep slope of Mt. Can-abag, burying the entire village of Guinsaugon in St. Bernard, Southern Leyte, Philippines.Casualties include 139 dead with 980 still missing and presumed dead, making it perhaps the most catastrophic landslide in Philippine history. The landslide started at the ridge top along a fault plane associated with the active Philippine Fault Zone. It started as a block slide that transformed into an avalanche. The entire event lasted for only a few minutes. Estimated maximum landslide velocity is 120-130 m/s. The landslide left behind a deep, wedge-shaped scarp. The central part of the deposit exhibits a hummocky topography typical of avalanches, in contrast to the flatter surface of the debris-flow-type marginal deposit. High amounts of soil in the matrix were derived from the scouring of ancient landslide deposits and rice fields in the valley. The landslide has a total area of 3.2 km 2 and a runout distance of 4.1 km. Estimated volume of debris is approximately 20 Mm 3 . At least four streams were dammed by the landslide debris. Intense precipitation and earthquakes preceding the landslide are the potential triggers. Preliminary back analyses assuming a planar and wedge slip surface yielded very low factors of safety even under dry conditions. A more rigorous analysis of the failure mechanism of the landslide is needed.

Gravity basement of the Guinsaugon landslide along the Philippine Fault Zone

“…Figure 5 shows the relationship between the landslide surface features and the subsurface fault structure. Field observations by Catane et al (2006) confirmed the presence of an active fault-related zone of highly fractured and sheared materials from the top of the ridge down to its base. These structures are important preparatory factors for the landslide.…”

“…Witnesses had also observed several minor events before 17 February, such as the occurrence of small rock falls a few days before the major landslide and the distinct siltation of a stream leading to Guinsaugon one day before the catastrophic event. Catane et al (2006) (Fig. 2).…”

“…The intensity of the earthquake was measured at II-III on the Philippine Institute of Volcanology and Seismology (PHIVOLCS) Earthquake Intensity Scale (PEIS) (Daag et al, 2006) (III-IV on the Rossi-Forel Scale; Catane et al, 2006). The USGS-NEIC and PHIVOLCS recorded these earthquakes, apparently of tectonic origin, on the basis of their foci.…”

Hazard mapping related to structurally controlled landslides in Southern Leyte, Philippines

“…The study area is part of the Southern Leyte segment of the sinistral PFZ, which has an average movement of 0.55 cm yr −1 (Cole et al, 1989) up to 3.5 cm yr −1 (Duquesnoy, 1997, as cited by Catane et al, 2006). This has led to formation of regional discontinuities and development of thick clay-rich gouge zones in the surface prominent in the en- tire province and other locations along the PFZ (Hart et al, 2002).…”

“…The topmost contributing slope of the Guinsaugon landslide is 50 to 60 • (Catane et al, 2006) with a post-landslide configuration of the crown exhibiting a distinct four-slip-plane failure. Based on the observed orientation of discontinuities and immense pore pressure developed from accumulated rainfall, a combined wedge, toppling, and planar slide is believed to have caused gravitational failure at Guinsaugon (Catane et al, 2008).…”