Catastrophic Rockslide-Debris Avalanche at St. Bernard, Southern Leyte, Philippines

Sandra G. Catane1, Hillel B. Cabria1, Ricarido M. Saturay, Jr.1, Cristituto P. Tomarong, Jr.1, Mark Albert H. Zarco2, Eddie L. Listanco1, Winston C. Pioquinto3, Aileen S. Mirasol1

1National Institute of Geological Sciences, University of the Philippines, Diliman,
Quezon City, Philippines
2Department of Engineering Sciences, College of Engineering, University of the Philippines,
Diliman, Quezon City, Philippines
3Philippine National Oil Company Energy Development Corporation,
Fort Bonifacio Makati City, Philippines


Shortly after the 10:36 a.m. earthquake (Ms 2.6) on 17 February 2006, an extremely rapid rockslide-debris avalanche cascaded down the steep slope of Mt. Can-abag, destroying the entire village of Guinsaugon, St. Bernard in Southern Leyte, Philippines. The landslide killed 139 with 980 people still missing and presumed dead, including 248 pupils trapped inside the primary school building. The landslide was initiated at the ridge line along a fault plane associated with the active Philippine Fault Zone. It started as a block slide that transformed into an avalanche. Initiation to final deposition took only about five minutes. The landslide left behind a deep, wedge-shaped scarp. Estimated maximum velocity of the avalanche ranges 120-130 m/sec.

Based on eyewitness accounts, the failed mass consists of three rigid blocks that descended successively and progressively disaggregated forming a series of waves of debris. Explosions occurred prior to the collapse of the first block and when the coalesced blocks collided with a ridge at the base of the slope. The collision caused the debris to breakup into two separate parts. One part followed the course of the stream, Sapa Aliho, while bulk of the moving mass followed a straight trajectory, inundating the valley where Guinsaugon village was located. The debris avalanche destabilized saturated soil-rich landslide deposits at the base of the ridge, initiating a secondary debris flow. Air blast generated by the explosions knocked down trees along the margins of the deposit. The strongest explosion was heard within a 7-km radius.

The failed mass involved fresh volcaniclastic rocks variably mixed with soil. The landslide has a total area of 3.2 km2, an estimated volume of 20 M m3 and a runout distance of 4.1 km. The surface of the deposit exhibits the hummocky topography typical for avalanches. The landslide debris dammed at least four rivers.

Earthquakes and high precipitation preceding the landslide are potential triggers. Cumulative rainfall of 751 mm from February 01 to 16 was recorded by a rain gauge 7 km west of Guinsaugon, which is 2.5 times higher than the average monthly rainfall for February. Excessive rainfall led to elevated hydraulic pressure in gouge-filled discontinuities, making the slope marginally stable. The earthquake at 10:36 a.m., which immediately preceded the landslide, may have been the ultimate trigger.



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